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1967 Coastal and Fluvial Landforms, Horry and Marion Counties, South Carolina. Bruce Graham Thom Louisiana State University and Agricultural & Mechanical College

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Recommended Citation Thom, rB uce Graham, "Coastal and Fluvial Landforms, Horry and Marion Counties, South Carolina." (1967). LSU Historical Dissertations and Theses. 1270. https://digitalcommons.lsu.edu/gradschool_disstheses/1270

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THOM, Bruce Graham, 1939- COASTAL AND FLUVIAL LANDFORMS, HORRY AND MARION COUNTIES, SOUTH CAROLINA.

Louisiana State University and Agricultural and Mechanical College, Ph.D., 1967 G eography

University Microfilms, Inc., Ann Arbor, Michigan Coastal and Fluvial Landforms, Horry and Marion Counties, South Carolina

A Dissertation

Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of the requirements for the degree of Doctor of Philosophy

m

The Department of Geography and Anthropology

by Bruce Graham Thom B. A, , University of Sydney, 1961 January, 1967 ACKNOWLEDGMENTS

This study was sponsored by the Geography Branch, Office of Naval

Research, Contract Nonr 1575(03), Task Order NR 388 002. The Division of Geology, State Development Board, South Carolina, H. S. Johnson Jr. , State Geologist, provided financial and technical assistance during the summers of 1965 and 1966. The writer is indebted to Dr. R. J. Russell who initiated this project in 1964, and for constructive criticism of the subsequent research. Mr. P. B. Larimore made available unpublished data obtained over the period 1956-1958. The w riter wishes to acknowledge the assistance of C. N. Raphael, and R. D. Adams in the field. Dr. D. Whitehead assisted with plant identifi­ cations, and B. Blackwelder and Dr. J. DuBar made the mollusk identifica­ tions reported in this study. Mr. J. N. Jennings and Dr. J. T. Hack visited the w riter in the field and made many helpful suggestions. Facilities for analyses of samples, drafting and photography were provided by the Coastal

Studies Institute, Louisiana State University. The w riter is thankful to his

various colleagues at Coastal Studies Institute for the opportunity to discuss the problems of this project. The preparation of this dissertation was under

the direction of Dr. Wm. G. Mclntire to whom the w riter is very grateful for

encouragement and advice during the past four years. CONTENTS

PAGE

ACKNOWLEDGMENTS...... 11 CONTENTS ...... iU TA B LES ...... v FIGURES ...... vi PLATES ...... v iii ABSTRACT ...... ix INTRODUCTION ...... 1 REGIONAL DESCRIPTION ...... 5 Geographic setting...... 5 Geologic background ...... 8 LANDFORM TY PES ...... 12 Depositional surface--definition ...... 12

Coastal facies ...... 12 Fluvial facies ...... 14 Methods of m apping...... 14 SEQUENCE OF DEPOSITIONAL SU RFA CES ...... 17 Previous literature ...... 17 Surfaces of late Recent age ...... 19 Coastal facies--barrier ...... 19 Coastal facies--backbarrier fla t ...... 24 Fluvial fa c ie s ...... 25 Surfaces of pre-Recent age ...... 27 Terrace I and valley d u n e s ...... 27 Waccamaw V alley ...... 27 Little Pee Dee and Lumber v alley s ...... 29 Great Pee Dee V alley ...... 22 Valley du nes ...... 24 Myrtle surface ...... 25 Coastal facies--barrier ...... 25 Coastal facies--backbarrier f l a t ...... 39 Fluvial facies ...... 42 Jaluco surface ...... 46 Coastal facies--barrier ...... 46 Coastal facies--backbarrier f la t ...... 20 iii PAGE

Conway su rface ...... 50 Coastal facies--barrier ...... 50 Coastal facies--backbarrier flat ...... 51 Fluvial facies ...... 52 Horry surface ...... 53 Coastal facies--barrier ...... 53 Backbarrier flat and fluvial fa c ie s ...... 58 GEOMORPHIC HISTORY ...... 60 Evolution of depositional sequence ...... 60 Geomorphic processes--past and present...... 66 CONCLUSION ...... 70 BIBLIOGRAPHY ...... 72 APPENDIX l a ...... 77 APPENDIX lb ...... 78 A PPE N D IX 2 ...... 87 A PPE N D IX 3 ...... 88 A PPE N D IX 4 ...... 89 A PPE N D IX 5 ...... 90 A PPE N D IX 6 ...... 91 APPENDIX 7 a ...... 93 APPENDIX 7 b ...... 94 APPENDIX 8 a ...... 95 APPENDIX 8 b ...... 97 APPENDIX 9 a ...... 98 APPENDIX 9 b ...... 100 VITA ...... 102

iv TABLES

PAGE

Table I. Generalized geologic column, Horry County, S. C. .... 10 Table n. Suggested age and environment of morphostratigraphic units in Horry and Marion counties, S. C ...... 62 Table III. Sequence of environmental change and development of landscape, Horry and Marion counties, S.C ...... 69

v FIGURES

Map showing the Coastal Plain portion of the Pee Dee drainage basin and the location of the study a rea ...... 3 Index map of Horry and Marion counties, S. C ., showing location of topographic (A to J) and geologic sections (X and Y ) ...... 4 Distribution of coastal depositional surfaces and their division into barrier and backbarrier flat facies, Horry and Marion counties, S. C. Fluvial surfaces adjacent to the major rivers are shown in Fig. 4 ...... 13 Distribution of fluvial depositional surfaces and valley dunes, Horry and Marion counties, S. C ...... 15 Generalized geologic cross-sections transverse to coast showing the relationship between major stratigraphic units and depositional surfaces. For location of sections see F ig . 2 ...... 18 Topographic sections across barrier surfaces, Horry County, S.C. For location of sections see Fig. 2. . . . 21 Stream patterns in the Waccamaw Valley on the flood plain and Terrace I. Note contrasting size of meander scars on Terrace I compared with the modern river. The location of Bucksville and Longs is shown on Fig. 2. 26 Generalized longitudinal profiles of fluvial depositional surfaces within Horry and Marion counties but excluding Georgetown County. Profiles represent airline distances 28 Stream and dune patterns near the junction of Little and Great Pee Dee rivers ...... 30 Stream patterns in the Little Pee Dee Valley immediately below the confluence with the Lumber River ...... 31 Stream and dune patterns in the Great Pee Dee Valley. Note that the channels and point bars of Terrace I truncate Dune Sheet 2 but are partially buried by the younger Dime S heet 1...... 33 Beach ridge patterns, Horry County, S.C ...... vi Dissection pattern of pre-Recent depositional surfaces, Horry and Marion counties, S.C. The location of drainage basins discussed in text is shown by letters A-K ...... 40 Drainage net for two basins on the Myrtle B arrier (traced from air photos). For location of basins see Fig. 13. . • . 41 Drainage nets for four basins dissecting backbarrier flat surfaces, Horry County, traced from air photos. H: Myrtle surface; I: Conway surface; J-K: Horry surface. For location of basins see Fig. 13 ...... 43 Topographic sections across Waccamaw (H-H*), Little Pee Dee (I-I’) and Great Pee Dee (J-JT) valleys. FP denotes modern flood plain while I, II, HI, represent the numbered terrace surfaces. For location of sections se e F ig . 2 ...... 45 Drainage net for basins dissecting the Jaluco barrier, traced from air photos. For location see Fig. 13. . . . . 49 Drainage net for basins dissecting the Horry barrier, traced from air photos. For location see Fig. 13. . . . • 57 Waccamaw-Pee Dee deltaic plain and adjacent Recent and pre-Recent coastal surfaces, Georgetown County, S.C. . 79 PLATES

PAGE

Plate la. Modern beach and foredune at Waiter Island, near Little River Inlet, Horry County (see also section A-A’, Fig. 6). . 22 Plate lb. Freshwater peat approx. 3,000 years old (C14 site No. 1, Fig. 2, Appendix 4) exposed during a storm on May 1, 1964 ...... 22 Plate 2a. Bluff exposure on Bull Creek showing two dune sheets over- lying peat at river level. (C14 site No. 6-7, see Fig. 2, Appendix 4)...... 36 Plate 2b. Close-up of lower dune sheet (Dune Sheet 2) containing cross-bedded sand and buried paleosol ...... 36 Plate 3. Wet slough in hardwood swamp typical of highly vegetated drainage channels in Horry and Marion counties. Photo taken in winter ...... 55

v iii ABSTRACT

Horry and Marion counties have many geomorphic characteristics typical of the outer portion of the Atlantic Coastal Plain. The sequence of coastal and fluvial deposits reflects five phases of coastal progradation, de­ signated Horry, Conway, Jaluco, Myrtle, and Recent. During each phase a depositional surface has been built composed of a barrier island behind which has accumulated quiet-water (backbarrier flat) and fluvial sediments. A sixth depositional surface composed entirely of fluvial sediments (Terrace I) has also been mapped. Differences in elevation, soil development, dissection, degree of preservation of primary morphology, and stratigraphic relations indicate that four major transgressions and possibly two minor transgressions are responsible for the depositional morphology, In the periods between Horry and Conway-Jaluco, Conway-Jaluco and Myrtle, and Myrtle and Recent major regressions of the sea occurred during which time the surfaces were exposed to fluvial dissection, soil development and partial wind deflation. During the intervals which separate Conway-Jaluco from Myrtle and Myrtle from Recent, two minor regressions-transgressions may have taken place leading in the first instance to the development of the Jaluco surface, and in the second instance to Terrace I. The changing nature of geomorphic pro­ cesses accompanying the fluctuations of sea level in late Cenozoic times has lead to a complex pattern of landforms which have been superimposed upon the prim ary depositional morphology. These include entrenched drainage basins, benched valley sideslopes, localized fluvial terraces, sinkholes, Carolina Bays and valley dunes. The older the surface the greater the variation in morphology. Modification of relict topography is clearly a continuing process, and relict topography associated with past phases of higher or lower sea level show varying degrees of preservation. The chronology of these environmental changes remains at present a relative one, and only improvements in absolute dating techniques together with studies in adjacent areas will show how com­ plete a Quaternary record is present in Horry and Marion counties.

ix INTRODUCTION

The northeastern portion of South Carolina (Fig. 1) is an excellent area to study landforms characteristic of the Atlantic Coastal Plain. Seaward of the Orangeburg Scarp (Fig. 1) landforms are predominantly depositional in origin. They are composed of marine and fluvial deposits of late Cenozoic age (late Miocene to Recent), the surface of which has been modified in part by subsequent erosional processes. This study is an historical analysis of erosional and depositional landforms within a small portion of this terrain, in Horry [pron. or-ree] and Marion counties (see Figs. 1 and 2 for location). Here the landforms reflect a pattern of geomorphic evolution involving the pro­ gradation of a coast and changes in the type, magnitude and relative importance of processes over time. By endeavoring to discern the nature of forces which produced the observed geomorphic pattern, it is the object of this localized study to contribute to the understanding of the growth and continuous develop­ ment of the Coastal Plain landscape. It was apparent after an initial reconnaissance in the spring of 1964, together with a study of aerial photographs, that this landscape has had an episodic history. Several barrier islands or barrier spits were recognized at different elevations with apparent differences in degree of dissection and weathering. Broad clay flats were found behind each of these sand ridges at a slightly lower elevation, and were interpreted as relict lagoon or salt marsh areas. Along the Waccamaw, Little Pee Dee and Great Pee Dee rivers, fluvial terraces were observed, which could be traced downstream to where they merge with the clay flats. Therefore, during the spring and fall of 1964 an attempt was made to reconstruct the geomorphic history of these coastal and fluvial deposits. In the summers of 1965 and 1966 the geomorphic infor­ mation was supplemented by a drilling program to examine the stratigraphic relations between and within coastal and fluvial landforms. The analysis of Horry and Marion county landforms involved the fol­ lowing six steps: 1 2

(1) comparison of present processes, environments and landforms with their relict equivalents at higher levels; (2) mapping the major geomorphic elements within Horry and Marion

c o u n tie s; (3) description of the topographic units that are significant in under­ standing the sequential development of this landscape, and examination of their relationship to each other; (4) relating the stratigraphy to the overlying morphology and to modern sedimentary environments; (5) testing the usefulness of criteria such as degree of dissection and soil profiles as means for distinguishing age differences between deposits; and (6) making inferences from these data about the sequence of geomorphic events and process change during the late Cenozoic interval. This thesis is part of a more general study of the geology of Horry County which involves analysis by other workers of the paleontology and bio­ stratigraphy, sedimentology of Recent and pre-Recent surfaces, and palyno- logy of peats and organic clays. In the present report only those aspects of the geomorphology pertaining to the region's history will be discussed. The problem of Carolina Bays, a significant feature in the area, will be considered e lse w h e re . ribninfli

-Conway;

Figure 1. Map showing the Coastal Plain portion of the Pee Dee drainage basin and the location of the study area. R i i t r 701 2 7 /*«*( taWlacarka* Sit*

370 Ft^trtl taaO C r« r« }!•!• ft««4 0«fiM

it

'7* COUN 30 501 C*llr**l* ft II* M arl** C a a majj; SOI

MARION

5 lata far! COUNTY

Murnlls Inlti 701 tO Mllat 3 7 * 13 Ka

Figure 2. Index map of Horry and Marion counties, S. C ., showing location of topographic (A to J) and geologic sections (X and Y). REGIONAL DESCRIPTION

Geographic setting Horry and Marion counties include about 1,600 square miles in the northeastern corner of South Carolina (Fig. 1). The area is bordered on the southeast by the Atlantic Ocean, the Northeast by the North Carolina state line, the northwest by the Dillon County line, and the south and southwest by the Great Pee Dee River and its deltaic distributaries. The study area displays elevations ranging from sea level to 115 feet. The Waccamaw, Little Pee Dee-Lumber and Great Pee Dee rivers are bor­ dered by swampy bottomland or flood plains. Rising in Lake Waccamaw in North Carolina at about 40 feet above sea level, the Waccamaw River flows in a tortuous course to the southwest before joining the Great Pee Dee. Little Pee Dee and Lumber rivers which join in the study area, also have their head­ waters in the Coastal Plain, and in common with the Waccamaw discharge black, organic-charged water into the Great Pee Dee. The latter rises on the Blue Ridge of North Carolina at about 4, 000 feet above sea level. In the upper 200 miles this river is known as the Yadkin. Erosion of Piedmont and Blue Ridge rocks results in this river carrying suspended silt and clay in compara­ tively fast-flowing turbulent water. Appendix la summarizes some of the hydrologic properties of these three rivers. The uplands bordering the flood plains are laced with drainage courses graded to river level. Invariably they are non-eroding, flat-bottomed valleys densely covered with water-tolerant trees and shrubs. They are locally known

as "swamps" (e.g. Lake Swamp, Polly Swamp, Fig. 2). Typically, these water-courses possess a dendritic pattern which tends to truncate relict depo­ sitional features of the upland surface. Although alluviating today, their entrenchment has imparted much of the local relief encountered on the uplands.

Near the coast this may only be about 10 feet, but near the flood plains, par­ ticularly toward the inland, local relief may be as much as 40 feet. Away from drainage courses, the uplands are generally quite flat. Sideslopes leading to flood plains and dissecting "swamps" are of low decliv­ ity, rarely exceeding 5° and averaging between 1 and 3°. Dunal topography is 5 6 subdued on the sandy barriers of pre-Recent age. Relief seldom exceeds 10 feet although relief in modem dune topography may attain 20 feet or more.

Swamp depressions pock-mark the upland surfaces. In sandy terrain these may have an elliptical or oval shape and be fringed by a rim of sparsely vege­ tated sand. In the geologic literature these features are called Carolina Bays

(see Johnson, 1942). Locally, however, any poorly drained portion of the up­ land may be called a "bay. " This includes those undissected areas of barriers and flats underlain by clay where the water table is at or above the surface for most of the year. The climate of the study area is typical of the southern Atlantic Coastal Plain (Kbppen's Cfa). Exceedingly cold weather is rare, and although the ground may infrequently freeze to a depth of one inch, soil frost and snow usually melt within a few hours. The mean annual temperature at Conway is 64°F. The mean for the summer is 79°F. and for the winter 48°F. Rainfall is distributed throughout the year and is heaviest in summer. Annual precipita­ tion averages 51 inches. Infrequent hurricanes in the autumn season may result in 10 inches of rain in as many hours. These storms may lead to severe flooding in the bottomlands and along the coast. Generally, flooding takes place in the river valleys each winter making agriculture in such areas an ex­ pensive operation. Precipitation and temperature data for selected stations within the study area are summarized in Appendix 2.

Several communities of plants can be recognized in relation to the broad topographic units described above. The flora of the poorly drained flood plains, drainage "swamps" and "bays" has been little disturbed in comparison with the densely settled uplands. Flood plains are covered by a mixed hardwood forest in which bald cypress (Taxodium sp.), tupelo gum (Nyssa aquatica) and black gum (Nyssa sylratica) form the dominant species. Less water-tolerant trees including loblolly pine (Pinus taeda), occur on micro-topographic highs within flood plains (levees, point bars). On the flood plain of the Great Pee Dee valley segregation of trees according to topographic site is very pronounced. Other hardwoods are important in the swamp forests of drainage courses that 7

dissect the uplands. These include red maple (Acer rubrum), ash (Fraxinus americana), water oak (Quercus nigra), elm (Ulmus sp.), and sweet gum (Liquidambar styraciflua). An understory of scrub gradually becomes more significant toward the head of these water-courses. On flat poorly drained interfluves occurs the "bay" community which consists of dense scrub, 10-15 feet high, penetrated by the odd pond pine (Pinus serotina), sweetbay (Magnolia virginiana), and bay magnolia (Qordonia sp.) Growing under very low pH conditions with a sandy substrate covered by 1- 4 feet of woody peat, the "bay" vegetation is dominated by such shrubs as Persea sp., Myrica sp. , and Vaccinium sp. Other plants of prominence include the fern, Woodwardia, the briar, Smilax, and Sphagnum moss. The better-drained upland areas are largely under pine, most of which has been planted this century. Loblolly (Pinus taeda) and longleaf pine (P. palustris) are indigenous to the area, but slash pine (Pinus elliottii) has been introduced. Many species of oaks are present, including the live oak (Quercus virginiana) which is very common on little-disturbed sand ridges in the river valleys, or where preserved near old h o m es. Upland areas are the sites for agricultural settlement in Horry and Marion counties. This area was first settled in 1734 on the uplands bordering the valleys and now has a population of just over 100, 000. In the 18th and 19th centuries the tidewater portion of the Great Pee Dee flood plain was used for rice cultivation. Large plantation homes were constructed on adjacent bluffs. Today tobacco grown only on the uplands is the most important crop. It is cultivated on small farms, 4-20 acres in extent, along with corn, soya-bean sweet potatoes, and some cotton. Less than 50 per cent of the landscape is farmland. Small barns in which the tobacco crop is cured are prominent fea­ tures of the area. There are numerous roads including six U. S. highways: 17, 701, 501, 378, 301, and 76 (Fig. 2). Only the flood plains are difficult of access. The coastal area is a popular summer tourist resort and is highly urbanized. Except for areas such as the Myrtle Beach State Park, most of the land behind the beach has been disturbed for the construction of homes, motels, amusement parks, golf courses, etc. Conway is the county seat. 8

Geologic background Previous geologic research in this northeast portion of South Carolina has been of two types: generalized and paleontological. The generalized accounts have involved the whole state (see Tourney, 1848; Sloan, 1908; Cooke, 1936). This type of study has been limited to the description of prom­ inent exposures, the recording of fossil localities, and broad generalizations of the geologic history largely based on assumed relations between depositional units, including the so-called Pleistocene "terraces." These works have been superseded elsewhere by more detailed accounts of particular areas (e. g. Malde, 1959, near Charleston; Pooser, 1965, and Colquhoun, 1965, in the Congaree and Santee valleys and adjacent uplands). A sim ilar trend is ap­ parent in other Coastal Plain States. The work of Hack (1955) in Maryland, Jordan (1962) in Delaware, Oaks and Coch (1963), Oaks (1965)1 and Coch (1965) in southeast Virginia, Hoyt, W eimer and Henry (1966) in the vicinity of Sapelo Island, Georgia, and Alt and Brooks (1965) in Florida indicate the advantages of detailed fieldwork, which in some instances were supplemented by drilling programs. The second type of geologic research in Horry and Marion counties has been paleontological. Fossil-rich exposures along the Intracoastal Waterway (Fig. 2) have attracted considerable attention (Cooke, 1937 ; Mansfield and MacNeil, 1937; Frey, 1952; DuBar and Chaplin, 1963; DuBar and Howard, 1963; DuBar and Furbunch, 1965). DuBar is at present continuing paleo- ecological studies on Neogene formations from this area in association with a program of geologic and geomorphic mapping also involving H. S. Johnson, Jr. (State Geologist), and the w riter.

*Oaks (1965), in an excellent summary of the post-Miocene geologic literature of the Atlantic Coastal Plain has divided research here into four periods: (a) an initial period of regional stratigraphic studies (1835-1902); (b) a period of chiefly morphologic studies (1902-1940); (c) a transitional period (1940-1951) and (d) the present period of detailed stratigraphic studies. 9

The general character of the geology of northeastern South Carolina is summarized in Table I, a stratigraphic column for Horry County. A wedge of Cretaceous and younger sediments 1,200 feet thick overlies Piedmont crystal­ line and metamorphic rocks {Richards, 1945). This thickness increases to north and south reaching a maximum of over 10,000 feet near Cape Hatteras. Cretaceous formations (Black Creek and Pee Dee) outcrop close to sea level in the study area. They constitute the sedimentary "basement complex" which is easily recognized in drilling with a power auger. The Pee Dee formation, for instance, is a very tight, dark gray marl. An important geologic and topographic contact on the Coastal Plain is the Orangeburg Scarp (see Pooser, 1965, and Fig. 1). Miocene formations (e. g. Duplin) occur to the east of this scarp as do marine and brackish-water deposits of Quaternary age. To the west, Eocene, Paleocene (?) and Cretaceous rocks rise to 600 feet above sea level. Between the Fall Line, shown by the outer lim it of the Piedmont in Figure 1, and the toe of the Orangeburg Scarp at +200 feet lies the Upper Coastal Plain which displays as much as 140 feet of local relief. This is in strong contrast to the relief of the Lower Coastal Plain which seldom exceeds 40-50 feet. Cenozoic strata in the study area in the Lower Coastal Plain ranges in age from the Late Miocene (Duplin) to Recent. They consist of unconsolidated sand together with beds of clay, m arl, limestone, semi-indurated sandstone, and gravel. There is no known occurrence in Horry County of Eocene or Oligocene formations sim ilar to the Black Mingo, Santee Limestone, McBean, or Cooper Marl formations described by Malde (1959) and Pooser (1965) from central South Carolina. The Duplin formation in this area is a soft, shell-rich m arl, containing coarse sand and small granules in places. It is believed to represent a shelf environment

T ab le I

Generalized Geologic Column, Horry County, S.

Depth (feet)

0- 50 Sands, silts, clays, unconsolidated, R ecen t to interbedded and cross-bedded Pleistocene 50- 80 Shell-bearing sand or m arl, uncon­ Pleistocene to solidated; and calcareous limestone, P lio cen e indurated. "Waccamaw formation" 80- 110 Soft, cream-colored marl or calcareous Late Miocene limestone, rich in Ostrea and Pec ten species. "Duplin formation" 110- 500? Argillaceous sandstone or marl, dark Upper Cretaceous gray, micaceous, calcareous, with discontinuous hard limestone ledges. "Pee Dee formation" ("Black Creek formation," see Cooke, 1936, p. 36, outcrops in Marlon County in place of the Pee Dee formation). 500-1200 Sand and gravel, micaceous, non- Upper Cretaceous calcareous, compact "Tuscaloosa or Middendorf formation"

1200 Crystalline basement Pre-Cambrian to P aleo zo ic 11

Younger Cenozoic sediments, presumably of Pleistocene or Recent 2 age, are clearly separated in time from Waccamaw and equivalent deposits. The present study will attempt to demonstrate that a comparison of the geo­ morphology and soils of the depositional surfaces in northeastern South Carolina supplemented by a study of subsurface relations, shows that the dep­ ositional history of this region has been episodic since Waccimaw times. There have been three major transgressions and two minor ones which have left their mark on the Horry-Marion landscape in the post-Waccamaw (Quaternary ?) interval.

2 In this paper the period designated as Recent refers to the inter­ val since the sea level commenced its last major rise to the present (i. e. from approximately 18,000 BP, Curray, 1965). LANDFORM TYPES Depositional surface -- definition The present study employs the term "depositional surface" in reference to a sedimentary deposit exposed at the surface at a particular range of elevation. More specifically a depositional surface in this area comprises the nearshore, barrier, lagoonal (including m arsh and estuarine), deltaic and fluvial deposits developed during a transgression of the sea and exposed during a regression. These various deposits may be grouped into two facies divisions: coastal and fluvial. Coastal facies Sedimentary environments directly influenced by coastal processes such as waves, littoral currents and tides form coastal depositional surfaces. As illustrated in Figure 3, five barriers (barrier islands or barrier spits), trending parallel to the present coast, have been recognized in Horry County. They are referred to as the Recent, Myrtle, Jaluco, Conway and Horry bar­ riers in the following description. Any particular barrier possesses a highly varied topography because of the irregular accumulation of dune sand or the development of beach ridges along ancient shorelines. Sheltered from direct wave action by the barriers are broad flats com­ posed of fine-textured sediments, usually silty clays. These flats constitute the tidal marsh, tidal flats, tidal channels and lagoonal environments charac­ teristic of areas back of coastal barriers. On Figure 3 they are shown as "backbarrier flats," a term which has less genetic implication than "lagoon," "sound" or "estuarine" flats. Backbarrier flats with their tidal creek drainage pattern merge with the deltaic area of the large rivers. A coastal depositional surface is therefore divisible into two major units: a barrier and a backbarrier flat. Within Horry County one such surface is forming today; this is designated the Recent surface on Figure 3. Coastal surfaces of pre-Recent age occur inland of the Recent surface and are named after their barrier ridges (Fig. 3). The primary depositional landforms of these relict features are, of course, now being modified by weathering and

sub-aerial processes of erosion and deposition. 12 Ocean

Figure 3. Distribution of coastal depositional surfaces and their division into barrier and backbarrier flat facies, Horry and Marion counties, S.C. 14

Fluvial facies While coastal deposits are being laid down near the shoreline, fluvial sediment is accumulating as a flood plain under fresh water conditions in the river valleys. A given fluvial surface contains several distinct sedimentary environments associated with the development of levees, channels, point bars, and flood basins. The abandonment and incision of flood plains, or fluvial sur­ faces, to form river terraces has been repeated several times in the study area, so that a flight of three terraces is common along the major valleys. Rather than assign local names to each surface, the abandoned features will be identified by Roman numerals commencing with the lowest terrace. The dis­ tribution of the modern flood plain and river terraces is shown in Figure 4. Also shown on this map are aeolian deposits bordering the modern flood plain in the Little and Great Pee Dee valleys. As will be discussed below, these dunes are related to the phases of valley entrenchment. Toward the head of Winyah Bay in Georgetown County, the fluvial sur­ face is more deltaic in character and is influenced by lunar tides. Here the flood plain merges with the modern backbarrier flat. Similarly, river terraces merge with equivalent backbarrier flats, and this fact permits, in some cases, reliable correlation between fluvial and coastal surfaces.

Methods of mapping The aerial photo mosaics (1M = 1 mile) were first examined for indica­ tion of contacts between depositional surfaces. The boundaries of barrier, backbarrier flat and fluvial facies within a surface were also delineated. Points of detail were further checked on topographic maps and large scale aerial photo­ graphs (1:20,000). Contacts were checked in the field by traverses. Sediments were examined in road cuts, in shallow auger holes or trenches when there were problems of identification. Hand and aneroid levelling was undertaken at selected points to check details of relief not clearly shown on the topog­ raphic maps. All boundaries were finally plotted on a base map at a scale of 1" = 1 mile. These boundaries are reproduced in this paper in Figures 3 and 4. f-.ioj Flaod Miia T t r r i d I LITTLE PEE DEE - llllim T a r ro c a II Tarrac* III, w >- LUMBER VALLEY d ]v«n ., o ... Ova* Slip fata

Tarraca Scorp

******** Upland Starp

W A C C A M A W „ VALL EY

G R E A T PEE DEE V A L L E Y

Figure 4. Distribution of fluvial depositional surfaces and valley dunes, Horry and Marion counties, S.C. 16

The criteria used in the recognition of surface and facies units include: (1) general pattern and type of primary depositional landforms such as beach ridges, tidal drainage networks, etc., (2) scarp contacts between surfaces reflecting erosional unconformi­ ties and differences in elevation of the surface units; (3) unconformities in the subsurface; lithological and paleontological variations were used to distinguish facies contacts in the subsurface (this work utilized a power auger drill operated by H. S. Johnson, J r . , State Geologist, S. C.); (4) differences in the degree of weathering of sim ilar lithologies in deposits at different elevations; this criterion involved the comparison of soil profiles; and (G) variations in the nature of dissection of pre-Recent surfaces and facies according to geometry of basin, relative relief and sideslope development. SEQUENCE OF DEPOSITIONAL SURFACES

The stratigraphic sequence of deposits which comprise the landscape of Horry and Marion counties is shown in two generalized cross-sections trans­ verse to the coast (Fig. 5). This figure illustrates the descending nature of the depositional sequence with older surfaces located farthest from the coast and attaining highest elevations. Also emphasized are the scarp contacts between coastal surfaces as well as between fluvial terraces within river val­ leys. The remnants of older facies can be traced beneath younger deposits at lower levels, a phenomenon most clearly displayed by the outcrop of the "Waccamaw formation” along the Intracoastal Waterway. These data, along with other criteria noted in the previous section have made possible a more detailed description and a more accurate interpretation of the geomorphic sequence than has hitherto been undertaken.

Previous literature In common with most of the Atlantic Coastal Plain, Horry and Marion counties have not been the subject of detailed geomorphic research in compari­ son with studies of Recent and Pleistocene depositional surfaces along the Gulf Coast (e.g. Russell, 1936; Fisk, 1939, 1940, 1944; Price, 1958; LeBlanc and Hodgson, 1959; Bernard and LeBlanc, 1965). Cooke (1936) in his map of the "terrace formations" of South Carolina recognized six post-Pliocene units in Horry and Marion Counties: R ecen t P a m lic o (25 ft.) T alb o t (42 ft.) Penholoway (70 ft.) W icom ico (100 ft.) Sunderland (170 ft.) Doering (1960) recognized only five "terrace formations" in this area:

17 — n i - f H - U *

• M iU «

VC 105.4

COASTA1 SUlFACfS tU IFA C tl f AC It f

1 k.t.ii M**J Ff«J* □ la rrla r taarf ftmvim I 7 •rrtc* I H N««r*h*r« immd - > h* J/ *V* « c • • *w - 0«p P i« M«r M Mj. lit r* r r i i J l«ilb«rrl*r>fl«f '■ rE l 7 « r r« «• hi fCr.l...... C C*«wmf fifitriai vfJI>ifc« fI H Httff

Figure 5. Generalized geologic cross-sections transverse to coast showing the relationship between major stratigraphic units and depositional surfaces. For location of sections see F ig. 2. 19

S h o relin e E lev atio n Intracoastal Flat Elevation R ecen t 0-20 ft. Pleistocene P a m lic o 25-30 ft. 20 ft. T a lb o t 50 ft. 40 ft. P enholow ay 80-90 ft. f 70 ft. W icom ico 100-110 ft 90 ft. Doering’s generalized map (ibid., Fig. 12) seems to place the Sunderland formation (shoreline elevation 120 feet, intracoastal flat elevation 120 feet) inland from Marion county with the contact passing through the town of Dillon. Colquhoun (1965) in an equally generalized map of the physiography of South Carolina, recognizes the Pam lico-Princess Anne, Talbot and Wicomico "terrace sediment complexes" in this area. As previously mentioned, DuBar and others have undertaken faunal studies of the fossil-rich Waccamaw and Pamlico formations. In these studies, attempts have been made to infer environments and stratigraphic relations. A m ajor contribution of his research has been the recognition of facies within the classic Pamlico "terrace formation" (DuBar and Chaplin, 1963). Re­ vision of this formation is suggested (DuBar and Solliday, 1963) supporting the Virginia research of Oaks and Coch (1963) and work in Florida (Alt and Brooks, 1965). A preliminary study of the "depositional elements" of the area between the Cape Fear and Great Pee Dee rivers was recently undertaken by Johnson and DuBar (1964). They recognized eleven "elements" in this report, but now consider (personal communication) that there were several misinterpretations of depositional environments.

Surface of late Recent age Coastal facies--barrier. The modern barrier islands, located near Cherry Grove to the north and M urrells Inlet to the south (see Fig. 2 for location of all place names), lack well-developed accretionary topography, and are generally characterized by a single dune ridge. As shown in the 20

topographic section Figure 6, A-A\ these islands are bordered on the seaward side by a broad sand beach, and on the inland side by a series of coalescing washover fans. The barriers seldom exceed 1,000 feet in width. At low tide the beach may be as much as 200 feet wide. The beaches of the barrier islands at Cherry Grove, near the North Carolina state line, frequently possess ridge and runnel microtopography, whereas to the south near M urrells Inlet the beach face has steeper slopes (up to 5°) and cusps may be seen frequently. These variations probably reflect differences in beach grain size; coarse sand, rich in shell fragments, is more significant in the southern area as compared with medium-fine sand at Cherry Grove. In both localities the area behind the beach is a semi-stabilized, dune sand complex rising to a maximum of 30 feet above present sea level (Plate la). Enclosed depressions are frequently en­

countered in the dune field. Lobes of sand project into the tidal m arsh from the rear of the dune area. The m icro-relief on these tongues is very slight (Fig. 6, A-A'). They slope from + 8 feet above MSL to + 3 feet, and near to the m arsh contact may be quite bare of vegetation. In his study of barrier and backbarrier sedimentary environments in southeastern Virginia, Oaks (1965, pp. 51-53) referred to these areas as backdune flats. The lobate form of these sand sheets suggests a washover fan origin, an inference supported by the type of sedimentary structures and the relationship to breaches in barrier dunes produced during infrequent hurricanes. The barrier islands seldom exceed 3 miles in length. They are sepa­ rated from each other by narrow inlets characterized by rapid currents, deep channels and sand shoals which spread both landward and seaward. In some areas man has filled the less powerful inlets only to find that during hurricanes the old channels are reopened. Both at Little River Inlet and M urrells Inlet the southward movement of beach drift is resulting in a natural fill on the northern sides of inlets. This is also reflected in the greater width of hum­ mocky dune topography at the southern ends of the barrier islands. In con­ trast, northern portions of the barriers are relatively narrow with frequent breaches in the foredunes. Here foredunes show strong signs of wave F«*t

201 W o lU r It. lacH vn* F I • i c * # • • • I

900 1600

t h Neuit RECENT Pon d i M V ■ T L I

600 i Ft VC 75

Rd. 17

£ 'M i

Rd. 17 M YR T L E 30i » (C ( N T

VC 33.3

J A L U CO

Mil at

50 Swomp CONWAY

30 VC 50 Rd. 45 lOOi HORRY

80 VC 100

Figure 6. Topographic sections across barrier surfaces, Horry County, S.C. For location of sections see Fig. 2. Plate la. Modem beach and foredune at Waiter Island, near Little River Inlet, Horry County, See also section A-A', Fig. 6.

Plate lb. Freshwater peat approx. 3,000 years old (C14 site No. 1, Fig. 2, Appendix 4) exposed during a storm on May 1, 1964. P la te la .

P la te lb . 23

erosion. This tendency for erosion at one end and deposition at the other end results in barrier island migration in the direction of dominant sediment trans­ port, which according to Hoyt (personal communication), is a common phenom­ enon along the east coast being especially well documented at Sapelo Island in G eo rg ia. Between Cherry Grove and Surf side Beach (Fig. 2) the modern beach directly abuts the Myrtle barrier, the lowest barrier of pre-Recent age (see Fig. 5, X-X' for stratigraphic relationship). Pits dug into the beach in several places revealed the truncated soil profile of the Myrtle barrier, 3-7 feet below MSL. Freshwater peat with in situ stumps overlies this soil zone and is generally 2-4 feet in thickness. It is frequently exposed by storm-wave erosion (Plate lb). A radiocarbon date on the peat near the top of the bed at MSL was 2,865±115 BP (locality No. 1, Fig. 2, see Appendix 4 for further details). At another locality (No. 5 in Fig. 2) a stump was dated from near MSL at 6 ,110±145 BP (J. DuBar, personal communication). This wood date is anomalous when compared with a sequence of younger peat dates to be discussed below. However, the discontinuous occurrence of the peat and wood on the modern beach along 26 m iles of coast between Cherry Grove and Surfside Beach (Fig. 2), indicates shoreline retreat in the last 6,000 years. Freshwater swamp and marshes, formed slightly above HWM, have been en­ gulfed by the retreating shore zone as sea level rose in Recent times. At present this type of peat is accumulating in shallow gullies cut into the Myrtle b a r r i e r . Where the scarp of the Myrtle barrier is slightly indented, the beach and dunes enclose small freshwater lakes. Sub-aerial erosion during periods of lower base level has produced these re-entrants which often appear as small dendritic drainage basins. Barrier-beach formation across their mouths during the late Recent has impounded freshwater. As shown in the topographic section, Figure 6, B-B’, the level of the lakes may rise above MSL. In some cases these dissection channels are filled with freshwater peat to thicknesses of 17 feet. A radiocarbon assay of 5,860±115 BP on fresh to brackish-water peat 10 feet below MSL, dates an early stage of organic fill in one of these 24

channels. Appendix 4 lists the dates, peat type and depth for three samples (Nos. 2-4) from this particular channel. Some of the channels are inundated twice each day by tides (tidal range 4-5 feet), and here sedimentary environ­ ments characteristic of backbarrier flats are encountered. Low foredunes (3-8 feet high) occur behind the modem beach between Cherry Grove and Surf side Beach. In places more extensive dunes have formed and these are typically 10-20 feet high with leeward slopes at or near the angle of repose of sand. These dunes are stabilized by dense scrub today, but within living man's memory they have migrated a few hundred feet across the gently sloping Myrtle barrier before being stabilized. The soil surface of the Myrtle barrier is buried beneath the cross-bedded sands of these dunes as indicated in Figure 6, C-C1 and D-D'. Booming tourist activities in the past two decades have resulted in the destruction of the young dunes in most areas. Even where modem barrier sands are vegetated, weathering has not developed to any appreciable degree. Sedimentary structures are preserved in the sediments to within a foot of the surface, the zone of root concentration. Coastal facies—backbarrier flat. The contemporary backbarrier flat occurs in the area sheltered by Recent barrier islands near Cherry Grove and Little River Inlet in the north and M urrells Inlet in the south (Fig. 2). This depositional surface contains a considerable variety of sedimentary environ­ ments and microtopographic forms. Tidal waters cover the flats twice each day. The m aster creeks are aligned parallel to the coast and in both areas are located near the base of the Myrtle barrier (see Fig. 6, A-A'). These chan­

nels are quite sinuous and show signs of lateral cut and fill in many places. Smaller creeks drain the tidal flats and marsh. Unlike the m aster creeks they become dry during low tide. They are seldom three or more feet deep in com­ parison with m aster stream s which locally attain depths of 10 feet below m arsh level. The m aster creeks contain a coarse bedload, usually sand and shell fragments. The smaller tributaries seldom display a predominance of coarse

sediment in their channels. Bare tidal flats flank the channels at low tide. Soft muds, 2-15 feet deep containing thin layers of sand and shell, are characteristic. Oysters 25

(Crassostrea virginica) in vertical growth position form thin reefs within the tidal flats. Between MSL and high-water level, salt marsh covers the flats. The primary colonist is Spartina alterniflora var. glabra, commonly occurring as a pure stand (see Chapman, 1960). Other species become more prominent at higher levels especially Juncus sp. Near the backdune flat the density of m arsh plants diminishes and large areas of bare ground separate the marsh from the freshwater scrub and herbs typical of barrier islands. Borings to depths of 15 feet beneath the m arsh and tidal flats reveal highly variable li- thology and stratification. Fluvial facies. The fluvial equivalent of the Recent depositional surface extends up the three major river valleys: Waccamaw, Little Pee Dee and Great Pee Dee (Fig. 4). Recent fluvial fill within tributary valleys which dissect coastal surfaces was not mapped. Floodplain morphology of Atlantic coastal plain stream s has not been described in detail. Appendix lb contains a de­ tailed morphological description of the flood plains of the three valleys and the deltaic area at the head of Winyah Bay. The following aspects of floodplain topography are relevant in compar­ ing the modem depositional landforms with their pre-Recent equivalents: (1) relief differences between levees and flood basins and between ridges and swales of point bars are of the order of 5-10 feet for modern forms, but are less apparent for older features; however, the landforms of Terrace I, although sometimes difficult to detect on the ground, are readily apparent on aerial photos and can be easily mapped (see Fig. 7); this is not the case for the higher terraces; (2) sedimentary contrasts between depositional environments are most marked in the Recent flood plains; older equivalents are modified by slope and weathering processes; and (3) the differences between the three modem flood plains in type and magnitude of fluvial landforms, slope of flood plain, and mineral and size composition of sediments are paralleled in their flanking terraces. For in­ stance, the presence of coarse quartz sand on the point bars of the Little Pee

Dee compares favorably with the quartz sand bars of Terrace I and the • l*W

?i". >- [V ly] Flood Plain A/^'* # t*A&'»'I '/VV *' r \#\ Levee w'-v'.'M'<* i *i. Terrace I

lr~/- Beck

0 2 Mile* H- -i1 3 Km.

F ig u re 7. Stream patterns in the Waccamaw Valley on the flood plain and TerraceI. Note contrasting size of meander scars on Terrace I compared with the modem river. The location of Bucksville and Longs is shown on Fig. 2. 27 predominantly quartz sand composition of the higher Terrace II and Terrace

III. In contrast, the Great Pee Dee terraces are composed of very fine sand and silt of varied mineral composition. The modern flood plain of this valley has sim ilar mineral and size characteristics. The longitudinal profiles of fluvial surfaces within each valley are shown in Figure 8. This figure clearly demonstrates the parallelism between gradients of surfaces within a given valley, but not between valleys.

Surfaces of p re-Recent age

Terrace I and valley dunes In the three major river valleys a fluvial terrace (Terrace I) exists between the modern flood plain and a higher alluvial deposit (Terrace II). Figure 4 shows the distribution of Terrace I and associated dunes within the study area. There is no coastwise equivalent of Terrace I in the study area.

It is locally known as the second or high bottoms. Waccamaw Valley. Terrace I stands about 10 feet above the levee level of the flood plain of the Waccamaw Valley (Fig. 8). Covered only by the highest floods, Terrace I in this valley is laced by relict meander channel patterns. It is difficult to measure accurately meander properties on this sur­ face, but they are many times the size of those on the present flood plain (Fig.

7). The longitudinal profile is flatter than that of the flood plain, but downstream it steepens forming a convex profile. This is in marked contrast to the concave profile of the flood plain (Fig. 8). In the vicinity of Bucksville (shown on Fig. 2 and Fig. 7a) Terrace I merges with the flood plain. Remnant point-bar deposits project out of the surrounding swamp. Upstream from this locality Terrace I is a better preserved feature, at places often extending on either side of the valley. These features are well-developed near Longs, shown in Figure 7b, with the stratigraphic relations illustrated in Figure 5, Y-Y1. Sediments of Terrace I in the Waccamaw Valley tend to be somewhat coarser than on the flood plain although broad areas of poorly drained surface lacking distinct microtopography are underlain by fine-textured sediments. i i r r t e PEE DEE t U M B E *

— -

G R E A T PEE DEE

0 W Milti I- "T" 10 Km. VE J5E.4

Figure 8. Generalized longitudinal profiles of fluvial depositional surfaces within Horry and Marion counties but excluding Georgetown County. Profiles represent airline distances.

to 00 29

Soil development on this surface is quite minimal; oxidization is confined to the upper 3 feet in sandy silts. Because of the range in textures no attempt was made to characterize the degree of soil development on this fluvial surface. Dissection is also minimal being restricted to the terrace scarp. Short gul­ lies, now filled with alluvium, have followed the swales of point bars in a few cases. These are similar to the ’’point streams" described by Russell (1939) and Fisk (1940) in Louisiana. Little Pee Dee and Lumber valleys. Braided topography characterizes the Terrace I surface in these two valleys (Figs. 9 and 10). W ell-sorted, medium to coarse sand forms prominent elongate ridges or bars between swampy sloughs. The sloughs anastomose and carry organic-stained water during floods. The bars standing as much as 15 feet above river level are in­ undated only by extreme floods. Longitudinal profiles of the flood plain and Terrace I are parallel for most of their lengths. However, Terrace I dips beneath the swamp section of the flood plain farther downstream (Fig. 8). Below the junction of Little and Great Pee Dee rivers there is little braided topography. Above the junction of the Little Pee Dee and Lumber valleys the braided topography continues on Terrace I. Little Pee Dee landforms are much reduced in size, but large bars and sloughs persist up the Lumber Valley. Scarps cut by the present river are quite evident where bars are located adjacent to younger surfaces. Gully development is not apparent on the sand bars. Well-developed soil profiles were noted; they are sim ilar in morphology to those to be described below as characteristic of the M yrtle barrier in well- drained sites. Post-depositional landforms on the braided surface of Terrace I in­ clude fossil dune fields and Carolina Bays. Relative relief in the undulating

dune area does not exceed 10 feet. Dune types are somewhat obscure, but poorly-developed parabolics indicate movement of sand from southwest towards the northeast. Consideration of Carolina Bays is beyond the scope of this paper,

but their relationships to dunes is described briefly by Thom (1965). 30

GP. D. f LOOD Ft At N

I f iS R TERRACE I

'7 '/ 7 ? PARABOLIC DUNES

CAROLINA BAY

2 Miles

Figure 9. Stream and dune patterns near the junction of Little and Great Pee Dee rivers. Figure 10. Stream patterns in the Little Pee Dee Valley immediately below the confluence with the Lumber River. 32

Borings in Terrace I of the Little Pee Dee Valley indicate alluvial fill to depths rarely exceeding 30 feet (see Fig. 5, X-X’). Generally, the fill is composed of medium to coarse sand interbedded with subrounded quartz peb­ bles up to 2 inches long. This coarse alluvium directly overlies Cretaceous or Miocene formations. Great Pee Dee Valley. Large, discontinuous meander scars charac­ terize Terrace I in the Great Pee Dee Valley. Figure 11 shows the relation­

ship of these scars to the modern flood plain, stabilized dunes and higher ter­ races in an area located 10 miles downstream from the town of Pee Dee (Fig. 2). They occur on the eastern side of the valley 5-12 feet below Terrace II

and about 15-20 feet above the flood plain. Point-bar topography is well pre­ served on this surface. Swampy channels are contiguous with the flood plain and are flooded quite regularly. These scars continue inland to the Fall Line,

and are especially well-developed in Marlboro and Darlington counties. Some scars are incompletely filled and remain as large cut-off lakes. The longitu­ dinal profile of Terrace I was reconstructed from elevations of the point bars (see Fig, 8). The profile is flatter than either the present flood plain surface

or higher terraces (about 0.37 feet per mile). It does not show any tendency toward downstream convexity characteristic of sim ilar alluvial surfaces in Waccamaw and Little Pee Dee valleys. In the Great Pee Dee Valley, point-bar sediments of Terrace I consist of silty sands. A series of hand-auger holes drilled across one channel rem ­ nant showed 4 feet of organic muck overlying 6-10 feet of gray, plastic clay. The clay grades downward into coarse white sand of unknown depth. Soil development on point-bar deposits is minimal. An organic horizon has formed over reddish silts and sands. Horizon differentiation is otherwise quite poor. The more varied mineral composition of Great Pee Dee alluvium apparently encourages greater color contrasts in the weathering profile compared with soils in the other two valleys for deposits of sim ilar age. Post-depositional dis­

section was noted at the margins of the relict point bars. Terrace I therefore possesses many distinguishing characteristics from

the modern flood plain. The greater size of the channels which flowed during FLOOD ftASIN

DUNE SHEET t

T E R It ACE I FOt NT OAR

CH ANNE I

TERRACE II

|UFt AND SCARF

I Mi

Figure 11. Stream and dune patterns in the Great Pee Dee Valley. Note that the channels and point bars of Terrace I truncate Dune Sheet 2 but are partially buried by the younger Dune Sheet 1. 34 the period of Terrace I aggradation suggests a greater discharge than is now experienced during bankfull stage (Gagliano and Thom, 1967). Sediments are generally coarser in Terrace I deposits than the flood plain, although this is not as obvious in the Waccamaw and Great Pee Dee valleys as it is in the Little Pee Dee-Lumber valleys. Even though there are differences between valleys, certain features such as the elevation of its surface, size of primary depositional landforms, sediment type, convex "plunge” of longitudinal pro­ file, and lack of a coastwise equivalent make Terrace I a very prominent feature in the landscape of northeastern South Carolina. Moreover the asso­ ciation of Terrace I with valley sand dunes has made the history of deposition relatively easy to establish. Valley dunes. Vegetated sand dunes are scattered throughout the Great Pee Dee flood plain, blocking off the mouths of the remnant channels of Terrace I, and blanketing the fine-textured surface of the higher Terrace II. In the flood plain these stabilized aeolian deposits are areas of relatively high and dry land within the flood basin environment. Dune topography is generally subdued, and relative relief seldom exceeds 10 feet. Dune types are difficult to differentiate on the ground, but aerial photographs clearly display parabolic dunes oriented NE-SW. The pattern of these dunes with open ends facing south­ west is shown in Figure 9. It can be seen that the lateral arm s of parabolics may attain lengths of 0. 3 mile. The height of terminal walls rarely exceeds 30 feet. In the flood plain, dunes occur as low undulating sheets with lobes of sand projecting toward the northeast, but in most cases they have been trun­ cated by fluvial planation on their southwestern sides {Fig. 11). In the Great Pee Dee Valley, dunes have formed across the mouths of some of the Terrace I meander scars (Fig. 11). Here the partial burial of Terrace I channels and point-bars is very striking because of relief differences and contrasting grain size of sediment. These dunes are referred to as Dune Sheet 1. Figure 11 also shows that Terrace I meander scars have cut into an older set of stabilized dunes which blanket Terrace II in several places. This set, or Dune Sheet 2, also trends NE-SW. Soil differences between Dune Sheets 1 and 2 are not obvious. 35

The construction of two dune sheets formed prior to and after the deposition of Terrace I is verified downvalley in the Great Pee Dee by the observation of two dune deposits, one overlying the other and separated by a soil profile. Buried dune deposits have been noted at two localities where the river is actively cutting into the sand sheet: Bull Creek and Thoroughfare Creek (see Appendix lb and Figs. 2 and 19 for location). The section at Bull

Creek shown in Plate 2 was studied in some detail. Here a peat and organic clay bed crops out at a low water stage. This peat dated 36,200+3,600, -2,500 (No. 7 in Fig. 2 and Appendix 4). It is overlain by 14-15 feet of well-sorted sand, which except for the upper 3 feet is characterized by high-angle cross­ bedding (25-28° dips). This is interpreted as the equivalent of Dune Sheet 2, which was mapped upvalley. The upper 3 feet is weakly podzolized. A humic A^ horizon at the surface was fortunately rich enough in charcoal and fine humic matter to be dated. A composite sample taken from several places along the exposure yielded an assay of 16,900+320 (No. 6). This surface is buried by 12-15 feet of well-sorted, cross-bedded sand, or Dune Sheet 1. Therefore aeolian deposition also occurred during the early Recent. The combination of buried dunes and datable m aterial, and the morphologic rela­ tionship between the two dunes sheets upvalley have facilitated the tentative establishment of a chronology within the Great Pee Dee Valley (to be discussed below) which is of importance in understanding the late Quaternary history of the area of study. Myrtle surface Coastal facies--barrier. From the Little River Inlet in the north to the southern edge of the study area, the Myrtle barrier extends for a distance of

34 miles. Its position relative to the other barriers is shown on Figure 3. This barrier continues south into Georgetown County where it is breached by Winyah Bay. A brief discussion of topography in the vicinity of Winyah Bay is contained in Appendix lb and illustrated in Figure 19, The Myrtle barrier is characterized by low relief dunes and shallow enclosed depressions. Sinkholes, some as much as 20 feet deep, occur on the barrier near Crescent Beach and Cherry Grove (Fig. 2). This barrier stands Plate 2a. Bluff exposure on Bull Creek showing two dune sheets overlying peat at river level. (C14 site No. 6-7, see Fig. 2; Appendix 4).

Plate 2b. Close-up of lower dune sheet (Dune Sheet 2) containing cross­ bedded sand and buried paleosol. P la te 2b. 37

at an average elevation of 25-30 feet above MSL, but ranges from 5 to 35 feet.

Local relief seldom exceeds 10 feet. In two localities {near Crescent Beach and south of Myrtle Beach) the single barrier ridge divides into two parts with an intervening belt of low-lying swamp. This is depicted in Figure 12 by the trace of the barrier crest. In Figure 6, D-D', road 17 follows the main ridge.

A low depression about 1, 500 feet in width occurs inland of the main ridge, and is flanked by a short secondary ridge attaining the same elevation as the main ridge. Aerial photographs show that this secondary ridge possesses regularly spaced, cuspate indentations (not shown in Fig. 12). It is suggested that a shallow semi-enclosed water-body once occupied the depression between these two ridges, and that lagoon segmentation features sim ilar to those de­ scribed by Zenkovitch (1959) and others developed on its inland shores. Dunes from the seaward side seem to have partially transgressed this old water body which probably existed when the Myrtle barrier was actively forming.

Commonly, the Myrtle barrier slopes gently seaward as shown in

Figure 6, D-D*. Poorly defined beach ridges can be detected on this slope in some places (Fig. 12). Elsewhere the barrier has been cliffed by more recent erosion (see Fig. 6, A-A', B-B'), and the "scarp slope" is buried by Recent dunes (Fig. 6, C-C1).

The Myrtle barrier impinges against the Jaluco barrier for a distance of 10 miles (Fig. 3). Backbarrier-flat sediments extend to the north and to the south of this section. Accretionary topography on the Jaluco barrier is trun­ cated by the Myrtle barrier at the contact. The Intracoastal Waterway cuts through the backbarrier flat and barrier facies of the Myrtle depositional sur­ face, and shows very clearly the buried paleosol of the Jaluco surface uncon- formably overlain by various facies of Myrtle age. A typical section is described in Appendix 6.

Unlike the Recent depositional surface and to some extent Terrace I and the valley dunes, the Myrtle surface and older deposits have had their pri­ mary depositional morphology and sedimentary characteristics significantly modified by post-depositional processes of sub-aerial dissection and sideslope M yrrJc 1 Reach

10 Mil**

C~i„ Wt„ csr, LSU

F ig u r e 12. Beach ridge patterns, Horry County, S. C. 39

development, and by weathering. Inland and higher surfaces seem to be pro­ gressively more modified. Gullying of the Myrtle barrier, for instance, has been largely concentrated along the seaward-facing scarp. Locally known as "swashes," these drainage channels drain the gentle slope toward the sea, with tributaries branching at high angles because they follow prim ary swales within the barrier. Figure 13 illustrates the general pattern of drainage basins in the study area and locates those basins that were studied more intensively

(basins A-K). Basins A and B are depicted in Figure 14 and Appendix 5 sum­ marizes some of their morphometric properties. These data indicate that drainage basins on the Myrtle barrier are less extensive than those on bar­ riers farther inland (basins C, D, E, F and G). The area of basin, total channel length, relative relief, swamp width and number of first-order streams are noticeably lower in value compared with the other barrier basins. Also a comparison of the area of drainage basins within the Myrtle barrier with the total area of the barrier (shown in Fig. 13 by the dotted line indicating the con­ tact between barrier-backbarrier facies) is much less for this barrier than for those farther inland.

That the floors of the drainage basins have been filled by Recent alluvium (freshwater peat or backbarrier-flat sediments) has already been noted. This alluvium is at least 6,000 years old (see Appendix 4).

The weathering of the Myrtle barrier is limited to the upper 4-7 feet. As described in Appendix 7a, the soil profile is characterized by a shallow leached zone, 0.5-1.0 foot thick, which overlies a yellowish-brown B horizon.

The sand grains in the latter are stained by iron oxide, but there is little tex­ tural difference between A and B horizons. In more poorly drained portions of the barrier, ground-water podzols or spodosols are common. Here organic colloids are collecting in the illuvial zone, to a depth of 10-12 feet. The upper

10 feet of the section described in Appendix 6 is typical of such a soil. Coastal facies—backbarrier flat. The Myrtle backbarrier flat stands

18-20 feet above present sea level. It is divided into two sections (Fig. 3), the smallest in the northeast adjacent to Little River, and the largest to the southwest merging with the fluvial terrace sediments of ancient Waccamaw and X

w , ,

/

15 Km.

Figure 13. Dissection pattern of pre-Recent depositional surfaces, Horry and Marion counties, S. C. The location of drainage basins discussed in text is shown by letters A-K. 05 05

Figure 14. Drainage net for two basins on the Myrtle Barrier (traced from air photos). For location of basins see Fig. 13. 42

Little Pee Dee rivers. This portion of the flai attains a maximum width of 4 miles, and a large portion of its surface is poorly drained. A tight clay 6-10 feet in thickness underlies the flat. The flatness of the surface and fineness of texture provide the two outstanding characteristics of the Myrtle backbarrier flat. Higher and farther inland facies of this type are more dissected, and possess greater variation in grain size near the surface. Dissection is localized following, for the most part, the coastwise scarp between Myrtle and Jaluco surfaces and the fluvial scarp bordering the modern flood plain (Fig. 8). Figure 15 (basin H) illustrates the nature and pattern of dissection on this surface in comparison with basins located on higher backbarrier flats (basins I-K). As with the development of drainage basins on coastal barriers, the degree of development of basin H appears to be slight when compared with those farther inland (see Appendix 5). A characteristic feature of dissection on this and other backbarrier flats is that drainage courses frequently follow relict tidal or estuarine channels. The resulting drainage net is therefore quite distinct from that on barriers. Although not strikingly shown in Figure 15, basin H, there is some tendency for relatively straight gullies to make sudden arcuate bends where the incising watercourse apparently intersected a prim ary channel. Gleyed soils are very common beneath the Myrtle flat. Where better drained, the profile shows podzolic characteristics with a strong textural break between a pale yellow, silty Ag horizon, 4-8 inches thick, and a silty clay B horizon. The latter is 20-30 inches thick and is characterized by a yellowish brown color, blocky structure, small red mottles, and iron nodules up to 0.4 inches in diameter (Appendix 7b). The clay changes to a more pallid gray color with red mottles 4-5 feet below the surface. This profile should be contrasted with those to be described below for the Conway and Horry backbarrier flats. Fluvial facies. In the Waccamaw Valley, Terrace n can be traced from the North Carolina state line downvalley to below the junction with Bull Creek where it merges with both the Myrtle backbarrier flat and fluvial terrace sedi­ ments of the Little Pee Dee River. As shown in Figure 4, it is not a continuous Ml

Figure 15. Drainage nets for four basins dissecting backbarrier flat surfaces, Horry County, traced from air photos. H: Myrtle surface; I: Conway surface; J-K: Horry surface. For location of basins see Fig. 13. 44 surface. Below Conway it broadens into a wide flat, poorly dissected and com­ posed of silts and clays. A constriction of the valley occurs near Conway, but upstream terrace remnants are relatively narrow, standing 20-25 feet above the flood plain. This river terrace converges on the horizontal coastal surfaces up­ stream, and above the state line terrace mapping becomes difficult. Figure 8 shows Terrace n 20 feet above the flood plain at Conway but only 12 feet at Longs. The reconstructed profile of Terrace n parallels the flood plain, but does not display upvalley steepening. Except near the contact with Little Pee Dee deposits, initial morphology is poorly preserved on Terrace II in the Waccamaw Valley. The downstream remnants are dissected by stream s less than one mile long, which are confined to scarp edges and are partially filled by alluvium. Scarps are commonly scalloped in outline reflecting lateral shifting by the meandering Waccamaw River. Sediments at the surface become more sandy upvalley and are highly variable in lithology. Consequently no attempt was made to characterize soil morphology on this alluvial deposit. The thickness of fill rarely exceeds 30 feet over the length of the valley. In Figure 5, Y-Y', (see Fig. 16, H-Hf for detailed topographic section) an interesting problem is displayed. The remnant of Terrace II on the side of the valley near Longs is about 5 feet higher than on the opposite side. In one boring on the latter side the fill extended to a depth of 40 feet below ground surface. Here the Duplin Marl has been removed and the Ctetaceous Pee Dee formation truncated. However, on the Longs side, the Pee Dee formation extends beneath Terrace n at the same elevation as be­ neath the adjoining upland, or Conway backbarrier flat. A sandy clay facies of this flat also extends beneath Terrace II, but the upland clay unit has been removed and replaced by a 10 feet thick deposit of sand, sim ilar in lithology to the fluvial sand of Terrace n on the other side of the valley. It is inferred that

during the process of entrenchment the stream moved from the Longs side to the opposite side of the valley where it cut its deepest channel. This has left a veneered erosional bench on the Longs side truncating but not removing all backbarrier sediments of the Conway surface. In contrast, Terrace H alluvium fills the entrenched channel eroded into the Cretaceous formations on the opposite sid e. ft - ii m TOOt ? I M I ' I FP r3 0

rnTmTvrmrrrillll^^ H H'

c o «* D • FP • *« i i i n — ^ !OOi I I FP

TH 1 1 I'..r nm iM i l ... i i t r n j i r ..1li1 .-lL-JL.Ij_l.liill.iJ. r

Dunes JOOnTN

3 Mi U s

V E 5 2 . a

Figure 16. Topographic sections across Waccamaw (H-H'), Little Pee Dee (I—I*) arid Great Pee Dee (J-J*) valleys. FP denotes modern flood plain while I, II, HI, represent the numbered terrace surfaces. For location of sections see Fig. 2. 46

Terrace n in the Little Pee Dee Valley is not a continuous surface and seldom occurs as paired terraces {Fig. 16, I-I1). A constriction of the valley occurs at Galivants Ferry (Fig. 2). Downstream from this locality fine- textured sediments (sandy silts) predominate. Where the surface of Terrace II of the Little Pee Dee and Waccamaw valleys meet, sinuous and braided pat­ terns of the form er truncate the weakly-defined channels of an ancient Waccamaw River. The depth of alluvial fill in this area varies from 15-45 feet. Upstream from Galivants Ferry, sandy sediments comprise Terrace II. Traces of aggradational patterns of its original flood plain are not distinct, but degraded bars parallel to the present river can be detected in places. This terrace and the next highest, Terrace III, are well covered with Carolina Bays, a post-depositional feature at least as young as Terrace I, and these along with dissecting drainage basins have obliterated most of the channel traces. About 20-25 feet of medium sand, moderately sorted, but heavily impregnated with humic colloids underlies Terrace II. Basal gravel has been encountered on the eroded surface of the Duplin Marl in some borings. Terrace II in the Great Pee Dee Valley, where not blanketed by valley dunes of younger age, is composed of silts and clays at the surface. The tread portion of this terrace is broad (3-5 miles) and flat (Fig. 16, J-J1), and aggra­ dational patterns are not very evident even though Carolina Bays are absent and post-depositional dissection is localized. A Mrim swamp” (Russell, 1939, p. 1224) occurs at the base of the scarp of Terrace III. The scarp has been scalloped by a river of approximately the same dimensions as the present stream judging from the size and spacing of the scallops. Poor drainage is characteristic of this surface, and gleyed soils are common.

Jaluco surface Coastal facies--barrier. The Jaluco barrier elevated 35-55 feet above sea level, is separated from the Myrtle surface by a low scarp, the toe of which is buried beneath sediments of the Myrtle surface. Its morphology is strikingly different from the Recent and Myrtle barriers. Extending southwestward from North Carolina, the barrier or more strictly, barrier spit, is about 1 mile in 47

width at the northern end. This section has the greater elevation (45-55 feet), and isawelklrained sand ridge containing abundant enclosed depressions (Carolina Bays). Toward the southwest the barrier broadens to 7 miles wide at an eleva­ tion of 40-45 feet, and its surface topography is composed of multiple beach ridges (Fig. 12). The beach ridges flare in a southwesterly direction toward the Waccamaw River where they have been truncated by lateral stream erosion. Ridge spacing varies from 7-15 ridges per mile. At its maximum width there is a set of 71 ridges on the Jaluco barrier. The closely spaced nature of this set is shown in Figure 6, E-E' and Figure 12. Jaluco beach ridges seldom rise more than 3 feet above adjacent swales, and are vegetated by pine. A dense scrub thicket grows in the water-logged swales together with bald cypress trees

(Taxodium sp.). Peats 1-2 feet in thickness have accumulated irTthese swales and reduce the initial difference in relief between ridge crest and swale base.

Drainage ditches, cut at right angles to ridge strike, reveal maximum differences in initial relief of about 5 feet. Jaluco barrier sands, containing rounded, flattened quartz pebbles to one inch diameter, grade downward into a fossiliferous unit dominated by the pelecypod, Mulinia lateralis. In outcrops along the Intracoastal Waterway beneath the truncated Jaluco paleosol, and in some borings on the Jaluco sur­ face, a highly mixed fauna was collected. The inferred environment is neritic.

The total thickness of Jaluco sand and shell-rich sand and clay averages 35-45 feet. A small pond, known as Clear Pond (Fig. 3), with a peat fill +20 feet thick, overlies a deep pocket of Duplin (?) Marl, The Pee Dee formation be­ neath this pocket is about 40 feet lower than its average elevation beneath the

Jaluco barrier. The isolated occurrence of small ponds such as Clear Pond on

Jaluco and Myrtle barriers appears to coincide with 30 feet or more Duplin Marl, a highly porous formation. These ponds are believed to be relict sink­ holes and should not be confused with surficial Carolina Bays which may overlie both soluble and insoluble strata.

Jaluco barrier sands over large areas are impregnated in the upper

20-25 feet by organic colloids ("humate" of Swanson and Palacas, 1965). In- terpretated as a prim ary deposit by Johnson and DuBar (1964), it led these 48

w riters to view the Jaluco surface as a deltaic deposit. This view is incom­ patible with (1) the nature of the surface morphology, (2) the type of cross­ bedding within the beach ridges, and (3) the depth, lateral extent and character of the humate. The humate could more easily be explained by secondary trans­ location of colloids from the surface and root zone (Swanson and Palacas, 1964; Thom, 1967). The secondary origin is inferred from the lack of macro-plant remains, the correspondence between the upper surface of the colloid-rich sands and microtopography, and the gradual transition to better preserved shells and less humate with depth. Humate of this type appears to represent the accumulation of colloids at the water table in a manner sim ilar to that postulated by Thom (1965a) in coastal barriers of eastern Australia. The weathering profile of well-drained barrier sands is very sim ilar to that of the Conway barrier and will be described in the next section (Appendix 8a). Post-depositional dissection has developed several rather large drain­ age basins on the Jaluco barrier. These drain both toward the sea and into the Waccamaw River. Figure 17 shows in some detail two Jaluco drainage basins, and Appendix 5 summarizes their morphometry (basins C and D). Com­ paring basins on the Myrtle barrier (A and B) with Jaluco basins (C and D) several points can be noted: (1) basins on both surfaces show little evidence of contemporary ex­ pansion; channels are not eroding headward nor are they incising; instead they both seem to be filling their channels and only organic-stained water is dis­ c h arg ed ; (2) the Jaluco basins are larger and possess a greater number of tri­ b u ta rie s ; (3) the heads of tributaries on the Jaluco barrier are harder to define, especially in the beach ridge section where swales drain into the dissecting ch an n els; (4) the pattern of Jaluco tributaries is strongly influenced by parallel ridge-swale topography; rectangular networks predominate; (5) the area of barrier surface not integrated by sub-aerial drainage is proportionally smaller for the Jaluco barrier than the Myrtle barrier and; Figure 17. Drainage net for basins dissecting the Jaluco barrier, traced from air photos. For location see Fig. 13. 50

(6) Jaluco channels are filled not only by Recent sediments, but also by fluvial sediments of the Myrtle surface which occur downstream as low terraces. Coastal facies--backbarrier flat. The flat behind the Jaluco barrier on the south side of the Waccamaw is quite narrow and the nature of the scarp con­ tact with river terraces along the Waccamaw River suggests that lateral erosion by the river during the Pleistocene has reduced the area of the flat on its land­ ward side. It stands 35-40 feet above sea level, but close proximity to the river and dissection leaves few areas of undissected interfluve which could be used accurately to obtain the maximum sea level elevation at the time of formation. It is a silt-clay deposit about 40 feet thick and interbedded with layers of sand.

To the northeast of Conway a highly oxidized, silty sand surface projects into the Waccamaw Valley at about 35 feet above sea level. Southwest of Conway on the same side of the valley a sim ilar surface occurs at approximately 30 feet above sea level. At each locality this surface rises sharply to the Conway sand ridge on the northwest side and drops 6-10 feet to the Terrace II surface. Ditch exposures suggest that it laps against the Conway barrier, and is in turn truncated by Terrace H. It appears to represent the fluvial-estuarine facies of the Jaluco backbarrier flat on the northwest side of the valley and in Figure 4 has been mapped as Terrace HI (?). This interpretation has been partially confirmed by borings, which have revealed a complex interbedded unit of silty clay, a clay with shell fragments ( unidentifiable ), and a widespread basal organic clay, rich in plant fragments. However, there is some evidence that the lower units are interbedded with the adjacent Conway barrier. This strati- graphic information will be presented in detail in a following study by DuBar, Johnson, and Thom, but it highlights the problem of the time-relationship between Conway and Jaluco. Conway surface Coastal facies—barrier. The Conway barrier is located northwest of the Waccamaw River, and is divided by Kingston Lake near the town of Conway into two sections. The eastern section averages 50-60 feet in elevation, but compared with the Jaluco barrier is quite narrow, having a maximum width of 1 mile. For most of its length the eastern Conway barrier is a single elongate 51

ridge with hummocky topography, but in two localities the ridge fans out towards the west and beach ridges are noted. In several places the Waccamaw River has cut into the barrier, and broad scallops have been produced by a lateral erosion of the river. Fluvial terrace or floodplain deposits abut the river side of the barrier at lower levels. The western portion of the Conway barrier, like the Jaluco is composed of a fanning series of beach ridges. The average spacing of ridge crests is 5-7 ridges per mile (Fig. 6, F-F'). Toward the town of Conway the western barrier narrows to a single ridge with subdued dunal topography. The narrowness of the eastern and western sections of the Conway bar­ rier does not perm it effective comparison of drainage basin morphology. Elongate channels have cut into the swales of the beach ridges near the junction of the Great and Little Pee Dee rivers (e. g. Polly Swamp on Fig. 2 and Fig. 6, F-F1). The local relief in this area ranges from 5 to 20 feet. Borings in the Conway barrier encounter 30-40 feet of well-sorted, non-fossiliferous sand containing humate or a relatively deep, oxidized weath­ ered profile. The degree of soil development in the quartzose sands of the Conway barrier is very sim ilar to that of well-drained areas of the Jaluco barrier. In comparison with the Myrtle profile, soil development on these two barriers is more advanced. Depth of weathering extends to 15 feet. In the upper part of the weathered zone the leached horizon varies from 2-3 feet in thickness, and overlies a brownish-yellow B horizon, 3-4 feet thick (see Appendix 8a). This horizon contains a small amount of clay (1-5 per cent) which im parts to it a weak massive structure. Within the leached zone there is some evidence of a micro-podzol with a bleached sand horizon less than 7 inches thick. This micro-podzol or oisequal feature is characteristic of Jaluco, Conway and Horry barriers, but is noticeably absent from Myrtle barrier pro­ files. Its significance requires further investigation. Coastal facies—backbarrier flat. The Conway backbarrier flat is a well

preserved surface, elevated about 5 feet above the Jaluco flat (40-45 feet above sea level). The flat nature of this surface and the occurrence of undissected 52 interfluves is most apparent on quadrangle maps (e.g. Conway 7 1/21 quad­ rangle) and air photos. Drainage off the higher Horry barrier has produced a comparatively dense drainage network with wide valleys. Many valleys have flat-bottomed channels 10-15 feet below the interfluves. Swamp drainage pat­ terns are again characteristic of these channels with a noticeable lack of present-day corrasion. Figure 15 (basin I) illustrates a drainage net formed mainly on the Conway surface with a small portion on the Horry barrier. The isolated development of incised, arcuate channels is also apparent. Deflec­ tion of Kingston Lake drainage and tributaries to the southwest behind the east­ ern Conway barrier, has also taken place.

The silty clay unit which com prises the Conway flat varies in thickness from 5 to 15 feet. It is far more strongly oxidized than the Myrtle equivalent, but is similar to the Jaluco flat. The B horizon is yellowish brown in color with red mottles up to 2 inches in diameter. A textural change is evident between A and B horizons with the clay content (<8 phi) increasing from 35 per cent in the A horizon to 60 per cent in the lower B horizon. The brownish yellow color persists deep into the solum 6-8 feet below the surface, but gleyed colors become important at depths of 4 feet or more (see Appendix 8b for profile description). Fluvial facies. The remnants of Terrace III which relate directly to the

Conway surface are only preserved in two areas: on the east side of the Little Pee Dee Valley, where the river once eroded a huge arcuate bend into the Horry backbarrier near the junction of the Little Pee Dee and Lumber rivers (Fig. 4) ; and on the east side of the Great Pee Dee Valley where Terrace in fo rm s a narrow terrace parallel to the river, on which the town of Marion is located

(Fig. 2). In the Little Pee Dee Valley, Terrace III is composed of humate- rich sands, well-covered by Carolina Bays. A scarp, 10-15 feet high, clearly separates this terrace from Terrace H (Fig. 16, I-I’). A more pronounced scarp, 15-20 feet high, separates Terrace III from the Horry coastal surface.

The stratigraphy beneath Terrace III is sim ilar to Terrace II in this valley. Figure 8 presents the reconstructed longitudinal profiles of all the terraces and shows that Terrace II and Terrace III closely parallel the lowest terrace and 53

flood plain. Terrace HI converges with the Horry surface upstream. In the Great Pee Dee Valley, Terrace HI, about 20 feet above Terrace

H (see Fig. 16, J-J'), is clearly distinguished from lower fluvial surfaces.

Weathering and dissection are far more advanced. Old channel scars similar in size to the present river, are still apparent because they have been occupied by gullies eroding headward. Texture at and near the surface is more variable than on Terrace H ranging from clay to coarse sand and gravel. Individual beds are quite evident in outcrops. Longitudinal profiles of the higher terraces in the Great Pee Dee

Valley are parallel, but dip more steeply than the present flood plain and

Terrace I (Fig. 8). These higher terraces join downvalley with Terrace II and

III of the Little Pee Dee. In Figure 4 the terraces between the two rivers have been mapped as part of Great Pee Dee valley fill. The trend of old scars and sediment type suggest that fluvial deposits in this area are prim arily the pro­ duct of Great Pee Dee sedimentation. The river junction has clearly migrated downvalley during the Quaternary.

Horry surface Coastal facies--barrier. By far the largest barrier or barrier spit in the study area is the Horry barrier (Fig. 3), designated Horry Cape by Johnson and DuBar (1964). This surface extends for 30 miles from the North Carolina state line to the bluffs overlooking the Little Pee Dee flood plain. Its maximum width is 15 miles and minimum 4 miles. Most of this barrier has an elevation of 100-110 feet above sea level. It descends to the Conway backbarrier flat by way of distinct steps, separated by 15-20 feet high scarps with declivities less than 5°. The width of the steps increases to the southwest. The broadest is

shown on Figure 3, the Brown Bay step. Beach ridges dominate the topography of the Horry barrier (see Figs.

3 and 12). The ridges flare toward the Little Pee Dee River from a narrow

ridge to the northeast. The inter-ridge spacing is similar to that of the Conway barrier. In undissected areas the average spacing is 3-5 ridges per mile. Recurvature of beach ridges both landward and seaward is quite marked on the

Horry barrier. 54

The stratigraphy beneath the Horry barrier is complicated compared with younger barriers. Surficial sands arranged into beach ridges blanket the area and extend to a depth of 20-60 feet (Fig. 5). This sand unit contains dis­ continuous layers of non-fossiliferous, plastic clay, 0.5-5 feet in thickness. Beneath this unit may occur a green clay containing oyster fragments or a dark organic clay. Both these clay layers are patchy in distribution. A shell-rich sand or m arl (nearshore fauna) extends beneath the Horry barrier, and rises from 50 feet near the coastal scarp to 80 feet above sea level at the inland boundary of the barrier. This unit appears to be co-extensive with the Wacca- maw-Duplin formation to seaward (Fig. 5). The degree of dissection and local relief of the Horry barrier is much greater than that of younger and lower barriers. Figure 6, G-G1, demon­ strates the differences in local relief encountered on the Horry barrier. This profile crosses an interfluve area of non-dissected beach ridges where local relief does not exceed 6 feet. Toward road 45, relief increases as a result of dissection which follows the trend of the original swales. Local relief on this barrier may amount to 40 feet. Valley width expands with increase in valley depth. Generally, sideslopes are benched where local relief exceeds 10 feet.

The surficial sand cover is removed from the benches. This exposes lenses of clay which are buried beneath the interfluves by beach-ridge sand. These benches slope toward the valley floor but have irregular surface topography. They can be traced downvalley where they often merge with Terrace IH along the Little Pee Dee Valley, or the Conway backbarrier flat seaward of the coast­ wise scarp which separates Conway and Horry surfaces. The floor of dissecting valleys or "swamps" is quite flat and covered by dense scrub and hardwood swamp vegetation (Plate 3). Organic-stained water drains slowly through the trees along anastomosing courses. No lateral or vertical corrasion has been observed on these streams. Low ter­ races of mixed silt and sand occur in the floor of these valleys. Their contin­ uity, sediment type and lack of strong weathering make them distinguishable from

the more irregular benches situated farther upslope. Plate 3. Wet slough in hardwood swamp typical of highly vegetated drainage channels in Horry and Marion counties. Photo taken in winter. 56

Two types of drainage patterns can be recognized. One is elongate in shape, located where the m aster stream s, following swale alignment, drain southwest into the lower Little Pee Dee River (Figs. 13 and 18), basins E, F and G). Appendix 5 summarizes some of the characteristics of the drainage net of the illustrated (Fig. 18) basins. The area, number, and length of stream s are in contrast with stream s on lower barriers, especially basins E and F. The second type of pattern is more rectangular in shape. Here, the m aster stream drains at a right angle to ridge-swale orientation and low-order tributaries follow the swale pattern. These basins drain into Lake Swamp shown in Figure 2. Weathering extends far deeper (20-25 feet) into the Horry barrier than other barriers of the study area. Clay lenses are strongly oxidized and mot­ tling is very prominent. The soils in the surficial sand could be regarded as well-developed red-yellow podzolics or ultisols. Nodular ironstone has been observed infrequently at the contact between the eluvial and illuvial horizons. The form er may extend from 3-5 feet in depth beneath the surface, and contain within it a micro-podzolic or bisequal profile sim ilar to that encountered on the Jaluco and Conway barriers. The illuvial horizon extends from 5 to 9 feet in depth and, in contrast to the loose sands of overlying horizons, generally pos­ sesses a silty clay matrix which becomes quite hard, massive and fractured when dry. It ranges in color from brownish yellow to red brown (see descrip­ tion of profile in Appendix 9a). Silt and clay content of this horizon may be as much as 10-20 per cent. This percentage gradually decreases with depth until a bed of clay, a prim ary deposit, is encountered. Soil profiles with these characteristics are visible beneath ridge crests in many roadcuts through the dissected portions of the Horry barrier. Dissected and benched sideslopes frequently lack the thick upper solum noted above, exposing the red or brown oxidized zone beneath a thin (1-2 feet) organo-mineral topsoil and leached A £ i horizon. Apparently, most of the deep weathering took place prior to the full development of the dissection pattern. In common with the other pre-Recent barriers, ground-water podzols and humate development characterize the poorly- drained, undissected portions of the Horry barrier. :;> i

Figure 18. Drainage net for basins dissecting the Horry barrier, traced from air photos. For location see Fig. 13. 58

Backbarrier and fluvial facies. Beneath the Horry backbarrier flat in Horry County (Fig. 3), facies reflecting deposition behind a barrier attain a maximum thickness of 65 feet. This flat stands 90-100 feet above sea level and in spite of small areas of flat interfluves, is mostly incorporated into an erosional drainage net, which when looked at on a small scale map or photo­ graph, radiates from this area in all directions. Discontinuous benches and low terraces line the flat-bottomed channels. Lake Swamp follows the contact between the Horry barrier and backbarrier flat. Tributary drainage into Lake Swamp from the flat is dendritic in pattern (Fig. 15, basins J and K) with local incised bends being observed in places. Sandy areas are more extensive on this surface than any of the lower backbarrier flats. Local dune fields are quite common. However, silty clay predominates at the surface and is weath­ ered to depths of 20 feet. The soil profile is highly oxidized (Appendix 9b) with gleyed streaks following root burrows. Below depths of 6 feet primary strati­ fication affects textural change. Alternating silt and clay layers, 1-3 inches thick, weather into a variety of colors including purple, a chroma not encount­ ered beneath flats located at lower elevations. The oxidized upper clay of the Horry backbarrier flat is underlain by a very extensive clayey silt facies, 20-30 feet thick, which contains discontin­ uous beds of oyster, Qstrea sculpturata, a species which is now extinct. This clayey silt facies rests directly upon eroded Duplin and Pee Dee formations. North of the Little Pee Dee and Lumber rivers the backbarrier flat of Horry County extends into Marion County (Fig. 3). Similarity in elevation, degree of dissection and weathering leads to the correlation across the valley. Sediments tend to be coarser on the Marion County side. Horizontal laminae typical of quiet-water sedimentation are characteristic of the backbarrier flat in Horry County. In contrast, exposures in Marion County show abundant evi­

dence of stronger currents (cross-stratification including current ripple structures, and scour-and-fill bedding forms). Sharp contacts between beds, increasing occurrence of gravel inland, direction of cross-stratification, and relics of sinuous channels at the surface, all suggest a strong fluvial compo­ nent from the north or northwest during the deposition of this material. The 59 relationship of this surface to the barrier island located near Dillon (Fig. 1), at 135-140 feet in elevation, was not investigated. GEOMORPHIC HISTORY Evolution of depositional sequence The depositional unit which could be assigned formational status is the assemblage of facies associated with the construction of a barrier or barrier spit at a distinct range of elevations. Thus the Recent formation includes the barrier islands at the extremities of the study area, the mainland beach and dunes, tidal flats and marsh, freshwater peat in drowned gullies, deltaic (estuarine) deposits at the head of Winyah Bay, and modem flood plain sedi- t" ments. Similarly, pre-Recent deposits can be grouped into formations on this basis. As used here, the formation has the characteristics of the morpho- stratigraphic unit defined by Frye and Willman (1962), who have applied it to glacial stratigraphy in the Mid-West. It differs from the concept of "terrace formation" as applied on the east coast of U. S. A. (see Cooke, 1936) by re­ cognizing and mapping facies, and by tracing these facies as buried units be­ neath younger sediments (Fig. 5). In common with the mapping by Oaks (1965) and Coch (1965) it recognizes the possibility of several "formational" units beneath one "terrace," each formation reflecting a significant change in the position of the shoreline. Thus the Conway and Jaluco formations are separated and not assigned to one "terrace" (Talbot ?) simply because they have a common or nearly common elevation range. Also the morphostratigraphic technique enables the investigator to include within one formation those sediments which accumulated during a regression of the sea. The Horry barrier, for instance, includes what was previously mapped as Penholoway and Wicomico "terraces. " Subsurface units can be traced without break beneath the two supposed "terraces" (Fig. 5). Thus the system of mapping used in this paper is based on continuity of deposition as sea level dropped and the shoreline prograded leaving within a coastal barrier a series of beach ridges, which step down toward the present coast. Oaks (1965), Coch (1965) and Alt and Brooks (1965) have had sim ilar objections to the use of the "terrace formation" concept as used on the east coast. In the Gulf Coast, however, in the vicinity of the M ississippi River, Fisk

(1939, 1940; see also Russell, 1964) has demonstrated the usefulness of "ter­

race formations" in studies of Quaternary geology. Here the mapping is on the 60 basis of a coastwise (deltaic) deposit and its upvalley or fluvial equivalent. Unlike the east coast work, a distinct elevation (equated to a sea level) is not used as a guide in mapping, but rather a range of surface slopes. This is made possible because of the tectonic environment of the M ississippi Valley (Fisk,

1939; Russell, 1964).

The following summary of the geologic history (Table II) of Horry and Marion counties is based on stratigraphic data collected to August, 1966. This interpretation is a first approximation and will undoubtedly be modified as more data is accumulated. Age assignments are tentative and must await future application of radiometric techniques beyond the range of the present radiocarbon method.

The evidence for the episodic sequence as postulated in this paper and outlined in Table II can be presented in a categorized form: (1) stepped scarps at the seaward side of each barrier; the toes of the scarps are buried by backbarrier-flat sediments of the next lower surface

(F ig . 5); (2) the occurrence of each surface at a distinctive range of elevations

(Appendix 3); (3) buried paleosols and erosional unconformities beneath the strati­ graphic units which form a particular surface (Fig. 5 and Appendix 6); (4) buried peats resting on an erosional unconformity, but are buried in turn by inorganic sediments (e.g. "Horry clay," shown in Fig. 5, X-X');

(5) paleontological hiatus between Duplin-Waccamaw faunas (Horry 3 surface) and later biofacies (see DuBar and Furbunch, 1965); (6) incision of coastal surfaces to depths below present local base level and their subsequent fill;

3 DuBar, at the time of writing, is endeavoring to distinguish Duplin and Waccamaw faunas in samples from beneath the Horry barrier in an attempt to check the view that they represent two distinct transgressions and not simply evolutionary or facies units within the one formation. The latter view is tenta­ tively accepted in this report. 62

T ab le R

Age Morphostratigraphic unit Environments

Quaternary

L. R ecen t R ecen t c o a sta l e stu a rin e - d e lta ic flu v ial

(unconformity)

E . R ec e n t­ Dune Sheet 1 a eo lian ly Wisconsin

(unconformity)

M. Wisconsin T e r r a c e I flu v ial

(unconformity)

E. Wisconsin Dune Sheet 2 a eo lian

(unconformity)

Sangam on M yrtle-Terrace II c o a s ta l e stu a rin e - flu v ial

(unconformity)

Y arm o u th ? Ja lu c o T e r r a c e in c o a s ta l or Aftonian ? Conway e stu a rin e - flu v ial

(unconformity)

E. Pleistocene- H o rry c o a s ta l P lio c e n e - e s tu a rin e - Late Miocene (?) flu v ial 63

(7) episodes of incised valley fill as shown by flanking river terrace d e p o sits; (8) greater degree of dissection of more inland surfaces reflecting dif­ ferences in time, relief, or both; (9) progressive differences in the degree of weathering on Recent,

Myrtle, Jaluco (Conway), and Horry surfaces (Appendices 7-9); (10) topographic discontinuities between surfaces including differences in beach-ridge spacing and orientation of shorelines; (11) sedimentologic contrasts including occurrence on clay lenses within older barrier sands, and the existence of discoidal quartz pebbles in Jaluco, Myrtle and Recent barriers, but not in Horry or Conway deposits; and (12) time discontinuity between radiocarbon dates from Recent deposits and those beyond the range of radiocarbon technique from the Myrtle deposit (see

DuBar, 1962); as noted above, the future application of other radiometric techniques is needed in this area. In southeast Virginia, Oaks (1965) and Coch (1965) have studied clay mineralogy, degree of alternation of heavy m inerals and grain- surface textures as further means for distinguishing tim e-breaks between depositional units.

These techniques have not as yet been employed in Horry and Marion counties. It is suggested that there are significant time breaks between Horry and Conway, Conway-Jaluco and Myrtle, and Myrtle and Recent. The break between Conway and Jaluco is not so obvious for the following reasons:

(1) only slight difference in elevations; (2) lack of separate fluvial equivalents for each surface up-valley; Terrace III is graded to the level of the Conway backbarrier flat (40-45 feet) which is only slightly above the Jaluco backbarrier flat (35-40 feet).

(3) lack of differences in weathering profiles; (4) no recognizable stratigraphic hiatus between the two units because the Waccamaw River which separates them has removed strata at the contact to the Cretaceous; at present no unit beneath the Jaluco surface can be assigned

to the Conway formation; and 64

(5) although the Conway surface is more dissected, this may simply reflect drainage from higher areas inland. However, the pinching out inland of fossiliferous beds beneath Jaluco barrier sands, the diversion of Waccamaw drainage behind the Jaluco barrier, fluvial-estuarine deposition (Jaluco) on the inland side of the Waccamaw Valley, and the occurrence of abundant discoidal quartz pebbles in Jaluco barrier sands and their complete absence from the Conway barrier, all point to a minor regression of the sea between Conway and Jaluco. It is possible that this episode is of shorter duration and possibly of less magnitude than that which separates the major breaks listed above. The chronology of late Quaternary sedimentation in the Great Pee Dee Valley has been documented with two radiocarbon dates. Although this chrono­ logy should only be considered tentative, it is believed it may have important implications for problems of coastal geomorphology and climatic change in southeastern U. S. A. (see Gagliano and Thom, 1967). It is obvious that several episodes of cut and fill and dune formation have occurred in this valley. In developing any conclusions regarding the sequence and chronology of these de- positional surfaces in the Great Pee Dee Valley several points are pertinent: (1) floodplain top-stratum deposits, and point-bar sediments of Terrace I and Terrace II are composed of fine-textured silts and clays; (2) the gradient of these surfaces is sim ilar (Fig. 8). (3) sands and gravels characterize channel environments of the flood plain, but the lower unit of coarse fill in the entrenched Great Pee Dee Valley extends to 50 feet below MSL, below the depth of the present channel; (4) well-sorted sands of the stabilized dune sheets are enveloped by floodplain silts and clays; dune-forming winds apparently blew from the south­ west; there is no modern source for these sands above present river level; there is no evidence that the southwest valley wall was once a source; (5) there i two episodes of dune formation, recognized upvalley by spatial separation, and downvalley by a buried soil surface on well-sorted, cross­ bedded sands; and (6) the dune sheets are spatially separated upvalley by Terrace I indicat­ ing a drastic change in the nature of sedimentation. 65

Based on these points the following relative sequence is proposed (see also Table II): (1) Terrace III deposition; (2) River incision, soil formation and dissection of Terrace III; (3) Terrace II deposition; (4) River incision, soil formation and dissection of Terrace II; devel­ opment of Dune Sheet 2; (5) Terrace I deposition; soil development on Dune Sheet 2; (6) River incision (slight soil formation and dissection of Terrace I) and development of Dune Sheet 1; and (7) Modern floodplain deposition; soil development on Dune Sheet 1. Dune development is related in this sequence to phases of lower base level when coarser sediments were presumably more extensive as floodplain deposits. A braided river is envisaged, sim ilar in character to Terrace I on the Little Pee Dee where dunes are observed to the leeward, or northeastern side, of relict braided topography. The amount of incision needed to create conditions for dune development in the Great Pee Dee Valley is less than 50 feet. The dates mentioned above give some idea of the time intervals in­ volved in this sequence. Of course, they do not help decipher the ages of Terrace II and Terrace III. However, Dune Sheet 2 based on one radiocarbon date could possibly be younger than 36,000 years old. Dune Sheet 1 on the other hand is suggested to be 17,000 years or younger. Obviously it is not young­ er than 3,000-4,000 years when sea level reached very close to its present po­ sition, and top-stratum alluvial fill was forming. One could go a step further and assume a minimum depth of the source of dune sands a t-20 feet MSL. Accord­ ing to one estimate, sea level was at that level 6,000-7,000 years ago (Coleman and Smith, 1964). Thus the maximum age for Dune Sheet 1 would be 17,000 BP. and the minimum age would be 6, 000 BP. On the basis of this reasoning it is possible to conclude that one phase of dune development occurred within the so-called Wisconsin glacial interval. The youngest phase extends into the Recent period as defined above. These two phases are separated by an episode of alluvial fill associated with giant 66

meanders (Terrace I). This episode is perhaps less than 36,000 years old and greater then 17,000 years old. The significance of this event to regional and extra-regional correlation and chronology is beyond the range of this study (see Gagliano and Thom, 1967, for a discussion of some of the possi­ bilities and problems).

Geomorphic processes - past and present Two sets of processes reflecting two different but interrelated sets of conditions have left their imprint on the Horry-Marion landscape:

(1) processes predominantly of deposition dependent upon a rising or high stand of sea level at or above the present, including: (a) coastal, in the nearshore, beach and washover fan zones; (b) aeolian, in the backshore zone;

(c) backbarrier flat or estuarine, in lagoons, bays, tidal marsh, tidal flat, tidal creek and inlet environments; (d) fluvial, in river valleys and tributary streams; (e) slope wash and soil creep; and (f) pedological, especially the accumulation of humate in water-logged sands; and

(2) processes predominantly of sub-aerial erosion dependent upon a falling or low stand of sea level, including: (a) entrenchment of valley alluvium and the elevation of abandoned flood plains as river terraces; (b) gullying of elevated depositional surfaces with the headward erosion of small creeks absorbing prim ary channels and swales into the dissecting drainage net; and

(c) extensive slope wash and soil creep facilitating the development of valley- side benches by differential erosion. Dune sheets develop in river valleys downwind from trains of sand associated with degrading rivers. Coastal land- forms developed during this period have subsequently been submerged and possibly destroyed by transgressing seas at a later stage.

Agricultural activities of man over the last century should also be con­ sidered. The cultivation of row crops such as tobacco and cotton promotes accelerated run-off. With the present low gradients this has not resulted in gullying but extensive sheet erosion. Reddish-brown B horizons are frequently exposed on mid-slopes between 2-5°. This process has led to accentuated foot- 67

slope deposition. Very little sand finds its way through the swamp forests of the dissecting drainage courses. However, the construction of large drains through these swamps permits the transportation of sand and finer detritus into the major rivers. This is causing siltation problems which will probably worsen with time. Bare fields during fall and winter seasons have also suf­ fered from wind deflation. Loose A horizon sand is often absent from fields which have been tilled for many years. Man is, therefore, initiating another set of geomorphic processes which are having their effect on the present land­ sc a p e .

Horry and Marion counties display many landforms which are the pro­ duct of past processes. The geometry and distribution of these landforms suggest strong adjustment with processes which are no longer operative. This is demonstrated, for example, by the uniform spacing of beach ridges within a surface but strong variation between surfaces, and the regularity of floodplain topography within a given fluvial surface. However, the form and space rela­ tions of relict facets are being modified with time: "the past is, so to speak, effaced" (Von Bertalanffy, 1952, p. 109). The progressive development of new landforms in response to changes in the environment can be documented in many depositional landscapes (cf. Fisk, 1940; Ruhe, 1952). In northeastern South Carolina this is particularly well-illustrated by the continued develop­ ment of drainage basins and associated morphologic phenomena (e.g. side- slopes); and the progressive weathering of land surfaces of different age.

These processes, together with burial by younger sediments, tend to remove the traces of the original aggradational surface so that landforms on older sur­ faces are more evidently modified than sim ilar features on younger surfaces.

In Table III an attempt has been made to depict the number of times each depositional surface has undergone environmental change since the original surface was formed. It is obvious that the older surfaces have under­ gone more phases of weathering, dissection and alluviation than younger surfaces. For instance, the Horry surface has been subjected to valley dis­ section followed by valley fill on at least four occasions. However, this 68 should be regarded as only a minimum number, because it is not know how many phases of erosion and deposition have not or cannot be recognized in this area. In comparison, Terrace I has been entrenched only once and its gullies have been filled with sediment during the latest phase (Recent) of valley alluviation. T able m

Surface Type and Age

Coastal=C Horry Conway-Jaluco Myrtle ------Recent

Fluvial-F Horry Terrace in T e rra c e H - - - Terrace I - - - R ecent

Aeolian=A ------Dune II - - - Dune I - - -

SL+ dC F

SL- eF+ s

SL+ dF+eC+s dCF

SL eF+s eF+s

SL+ dF+s dF+eC+s dCF

SL- eF+s eF+s eF+s dA

SL+ dF+s dF+s dF+eC+s dF+s dF

SL- eF+s eF+s eF+s eF+s eF+s dA SL+ dF+s dF+s dF+eE+s dF+s dF+s dF+s dCF

SL = Sea Level; d = deposition dominant; e = erosion dominant; s = soil development CONCLUSION

A study of the geomorphology of Horry and Marion counties, South Carolina, reveals a variety of depositional and erosional landforms which are post late-Miocene in age. These landforms reflect episodic progradation of the coast and changes in the type and magnitude of geomorphic processes over time. Five phases of coastal progradation have been recognized. Each phase is represented by a barrier island or barrier spit behind which has accumu­ lated quiet-water (backbarrier flat) and fluvial sediments. Collectively these facies form a depositional surface which relates to a distinct elevation range: Horry (60-115 feet); Conway (35-60 feet); Jaluco (30-55 feet); IVlyrtle (5-35 feet) and Recent (0-15 feet). The maximum position of sea as inferred from back­ barrier flat and (or) beach sediments for each surface: Horry 100 ft. Conway 45 ft. Jaluco 40 ft. M yrtle 22 ft. R ec e n t 0 ft. These figures must be regarded with caution for several reasons: first, that no correction has been made for any post-depositional tilting or uplift, if any has taken place; second, accurate figures for sea level are difficult to assess because of the range of variables involved (e. g. tide range, depth of water over relict lagoon surfaces, storm wave deposition in beaches); third, dissection in­ hibits accurate levelling of original surface; and fourth, separation of the Conway and Jaluco surfaces by a distinct time interval is difficult and as sug­ gested above only a minor regression, or no regression at all, may be responsible for these two surfaces. Hence the value of this small area for making inferences concerning world-wide changes in sea level during the Quaternary is relatively small. Surfaces recognized here should be traced north and south into other areas of detailed study. Only when this is done can the geomorphic factors which are purely local in effect be separated from general processes such as those of glacial-eustatic origin. 70 71

A fluvial depositional surface has been mapped in this area which has no coastwise equivalent. This is Terrace I which apparently was graded to a sea level lower than the present. At present it is not known whether alluvia - tion of this terrace was in response to a rise in sea level representing a minor transgression in mid-Wisconsin times (20,000 to 30,000 BP), or developed as the result of a climatic change (see Gagliano and Thom, 1967). The former view is considered the most probable since a major ice retreat occurred dur­ ing that interval (Farmdalian or Paudorf inter-stadial). Curray (1965) has tentatively indicated that sea level rose to within 30 feet of its present position about 29,000 BP. However, this is a problem requiring further investigation.

It is therefore possible to postulate a late Cenozoic history involving four major transgressions of the sea (Horry, Conway, Myrtle and Recent) and two minor transgressions (Jaluco, Terrace I). These six episodes have left a strong imprint on the landscape of Horry and Marion counties. Environ­ ments have changed from coastal and fluvial alluviation to valley entrenchment and expansion of dissecting drainage networks several times in this study area

since late Miocene times. Relict surfaces have been modifiedby the development of post-depositional landforms such as valley sideslopes and benches, den­ dritic and rectangular drainage networks, sinkholes and Carolina Bays. The degree of topographic modification of pre-Recent deposits is greatest on the

Horry surface and least on Terrace I and Dune Sheets 1 and 2. BIBLIOGRAPHY

Alt, D ., and Brooks, H. K ., 1965, Age of the Florida marine terraces. Geol. Soc. Amer. Bull. , v. 76, pp. 406-411. Bernard, H. A ., and LeBlanc, R. J ., 1965, Resume of the Quaternary Geology of the northwestern Gulf of Mexico province. In The Quaternary of the United States, ed. H. E. Wright, Jr. and D. G. Frey. Princeton, New Jersey, pp. 137-185. Chapman, V. J., 1960, Salt M arshes and Salt Deserts of the World. Interscience Publishers, New York, 392 pp. Coch, N. K. , 1965, Post-Miocene Stratigraphy and Morphology. Inner Coastal Plain, Southeastern Virginia. Dept, of Geology, Yale University, Tech. Rept. No. 6, Geography Branch, O. N. R ., 97 pp. Coleman, J. M ., and Smith, W. G ., 1964, Late Recent rise of sea level. Geol. Soc. Amer. Bull. , v. 75, pp. 833-840. Colquhoun, D. J. , 1965, Terrace Sediment Complexes in Central South Carolina. Atlantic Coastal Plain Geol. Assoc. Field Conference, 1965, University of South Carolina, Columbia, South Carolina. 62 pp. Cooke, C. W ., 1936, Geology of the Coastal Plain of South Carolina. IJ. S. Geol. Survey Bull. , 867, 189 pp. Cooke, C. W ., 1937, The Pleistocene Horry clay and Pamlico formation near Myrtle Beach, S. C. Joum . Washington Acad. Sci., v. 27, pp. 1-5 Curray, J. R. , 1965, Late Quaternary history, continental shelves of the United States. In The Quaternary of the United States, ed. H. E. Wright, Jr. and D. G. Frey, Princeton, New Jersey, pp. 723-735.

Doering, J. A ., 1960, Quaternary surface formations of southern part of Atlantic Coastal Plain. Jour. Geol. , v. 68, pp. 182-202. DuBar, J. R ., 1962, New radiocarbon dates for the Pamlico Formation of South Carolina and their stratigraphic significance. Geologic Notes, State Develop. Board, South Carolina, v. 6, pp. 21-24. 73

DuBar, J. R ., and Solliday, J. R. , 1961, Stratigraphy of the Neogene deposits, Lower Neuse Estuary, North Carolina. Southeastern

Geology, v. 4, pp. 213-233. DuBar, J. R ., and Chaplin, J. R ., 1963, Paleoecology of the Pamlico Formation (Late Pleistocene); Nixonville quadrangle, Horry County, South Carolina, Southeastern Geology, v. 4, pp. 127-165. DuBar, J. R ., and Howard, J. H ., 1963, Paleoecology of the type Waccamaw (Pliocene?) outcrops, South Carolina. Southeastern Geology, v. 5, pp. 27 -6 8 . DuBar, J. R ., and Howard, J. H. , 1964, Duplin Formation (Late Miocene) at the Muldrow Place, Sumter County, South Carolina. Geologic Notes, State Develop. Board, South Carolina, v. 8, pp. 25-34. DuBar, J. R ., and Furbunch, H. W. C ., 1965, The Waccamaw Formation (Pliocene?) and its macrofauna, Intracoastal Waterway, Horry County, South Carolina. Geologic Notes, State Develop. Board, South Carolina,

v. 9, pp. 1-24. Fisk, H. N ., 1939, Depositional terrace slopes in Louisiana. J. Geomorph. , v. 2, pp. 181-200. Fisk, H. N ., 194U, Geology of Avoyelles and Rapides Parishes, Louisiana. Louisiana Dept. Conserv. Geol. Bull. , v. 18, 24 Opp. Fisk, H. N ., 1944, Geological Investigation of the Alluvial Valley of the Lower M ississippi River. Vicksburg, Mississippi River Comm., 78 pp. Fisk, H. N ., 1947, Fine-grained Alluvial Deposits and the Effects on M ississippi River Activity. Vicksburg, Miss. , U. S. Army Corps of Engineers, Miss. River Comm., 82 pp. Frey, D. G ., 1952, Pollen analysis of the Horry clay and a seaside peat deposit near Myrtle Beach, S. C. Am. Joum . Sci. , v. 250, pp. 212-225. Frye, J. C. and Willman, H. B. , 1962, Morphostatigraphic units in Pleistocene stratigraphy. Am. Assoc. Petroleum Geol. Bull., v. 46,

pp. 112-113. 74

Gagliano, S. M ., and Thom, B. G ., 1967, Deweyville Terrace, Gulf and Atlantic Coasts. Coastal Studies Bulletin, in press. Hack, J. T ., 1955, Geology of the Brandywine area and the origin of the upland of southern Maryland. JJ. £>. Geol. Survey Prof. Paper, 267-A, pp. 1-43. Hoyt, J. H. , Weimer, R. J., and Henry, V. J. , 1965, Age of late Pleistocene shoreline deposits, coastal Georgia. Abstracts I. N. Q. U. A ., VII Inter­

national Congress, Bouider, Colorado, 1965, p. 228. Johnson, D. W. , 1942, The Origin of the Carolina Bays. Columbia University

Press, New York, 341 pp. Johnson, H. S., Jr. and DuBar, J. R ., 1964, Geomorphic elements of the area between the Cape Fear and Pee Dee rivers, North and South Carolina. Southeastern Geology, v. 6, pp. 37-48. Jordan, R. R ., 1962, Stratigraphy of the sedimentary rocks of Delaware. Delaware Geol. Survey Bull. , 9, 51 pp. LeBlanc, R. J ., and Hodgson, W. D ., 1959, Origin and development of the Texas shoreline. Louisiana State Univ., Coastal Studies Inst., 2nd Coastal Geogr. Conf. , pp. 57-101. Malde, H. E ., 1959, Geology of the Charleston phosphate area, South

Carolina. _U. S. Geol. Survey Bull. , 1079, 105 pp. Mansfield, W. C. and MacNeil, F. S ., 1937, Pliocene and Pleistocene mol- lusks from the Intracoastal Waterway in South Carolina. Joum .

Washington Acad. Sci. , v. 27, pp. 5-10. Matthes, G. H ., 1941, Basic aspects of stream meanders. Am. Geophys.

Union Trans. , v. 22, pp. 632-636. Oaks, R. Q ., Jr. and Coch, N. K. , 1963, Pleistocene sea levels, south­ eastern Virginia. Science, v. 140, pp. 979-983. Oaks, R. Q ., Jr. 1965, Post-Miocene Stratigraphy and Morphology, Outer Coastal Plain, Southeastern Virginia. Dept, of Geology, Yale University, Tech. Rept. No. 5, Geography Branch, O. N. R ., 240 pp. Price, W. A ., 1958, Sedimentology and Quaternary geomorphology of South

Texas. Gulf Coast Assoc. Geol. Soc. Trans, v. 8, pp. 41-75. 75

Pooser, W. K ., 1965, Biostratigraphy of Cenozoic Ostracoda from South Carolina. The University of Kansas Paleontological Contributions, Article 8, 80 pp. Richards, H. G ., 1945, Subsurface stratigraphy of Atlantic Coastal Plain between New Jersey and Georgia. Am. Assoc. Petroleum Geol. Bull. , v. 29, pp. 885-955. Russell, R. J. , 1936, Physiography of the Lower Mississippi delta. Louisiana Dept. Conserv. Geol. Bull. , v. 8, pp. 3-199. Russell, R. J. , 1939, Louisiana stream patterns. Am. Assoc. Petroleum Geol. Bull. , v. 23, pp. 1199-1227. Russell, R. J ., 1964, Duration of the Quaternary and its subdivisions. Proc. National Acad. Sci., v. 52, pp. 790-796. Ruhe, R. V ., 1952, Topographic discontinuities of the Des Moines lobe. Am. Journ. Sci. , v. 250, pp. 46-56. Sloan, E. , 1908, Catalogue of the Mineral Localities of South Carolina. Re­ printed in 1958 by Division of Geology, State Develop. Board, South Carolina, 505 pp. Swanson, V. E. and Palacas, J. G ., 1965, Humate in coastal sands of north­ west Florida. TJ. J3. Geol. Survey Bull. , 1214-B, 29 pp. Thom, B. G ., 1965, Late Quaternary coastal morphology of the Port Stephens- Myall Lakes area, N. S. W. Journ. and Proc., Roy.Soc. JJJ. S. W. ,

v. 98, pp. 23-36. Thom, B. G ., 1965 a, Relation of Carolina Bays to regional geomorphology. Abstracts Geol. Soc. Amer. , 1965 Annual Meeting, Kansas City, pp. 172-173. Thom, B. G ., 1967, Humate and coastal geomorphology. Coastal Studies Bulletin, in press. Tourney, M ., 1848, Report on the Geology of South Carolina. Columbia,

South Carolina, 293 pp. von Bertalanffy, 1952, L ., 1952, Problems of Life. London, Watts and Co. , 216 pp. 76

Zenkovitch, V. P ., 1959, On the genesis of cuspate spits along lagoon shores.

J. Geol. , v. 67, pp. 269-277.

Zeuner, F. E ., 1952, Pleistocene shore-lines. Geol. Rundsch. , v. 40, pp.

3 9 -50. Drainage Flood-plain Channel (a) Discharge River Area Length Slope (a) Width Depth Aver. Max. Min. Yrs. Record

Waccamaw 1,520 148 0.4 2 35- 2-15 1,067 10,300 10 200 Longs

Little Pee 40- Dee 3,110 173 1.16 150 1-20

L um ber 1,760 160 1.33 70- 2-15 3,020 26,800 155 19 la APPENDIX 90 G alivants F e rr y G re at P ee 300- D ee 16.340 (b) 455 0.69 500 3-30 9,049 220,000 700 22 P ee Dee

Units sq. mi. miles ft./mi. feet cfs. Locality

(a) Within area of Fig. 4 (b) Includes Waccamaw and Little Pee Dee-Lumber

Sources: House Document No. 652, 78th Congress, 2d Session, 1944. U.S.G. S. Water Supply Paper, No. 1703, 1960.

Hydrologic Properties of Major Rivers, Horry and Marion Co. APPENDIX lb

The fluvial equivalent of the Recent depositional surface extends up the three major valleys which traverse the area, and includes the fill of the tri­ butary valleys or swamps that dissect the coastal surfaces. In this section only the fluvial deposits of m ajor valleys will be included. Waccamaw. The Waccamaw River flows southwest between the Conway and Jaluco coastal surfaces in Horry County. In comparison with the other two stream s, Little and Great Pee Dee, the Waccamaw River flows with a lower gradient and a smaller discharge (Appendix la). Two periods of peak discharge occur annually: August to October and January to April. The form er is associated with hurricanes and is not as consistant as the latter which results from winter rainfall and causes flood-basin inundation. Fed by non-incising tributaries which flow through swamps, the water in the Wac­ camaw River is highly organic-stained. The flood plain can be divided into four sections. The lower portion of the river between the mouth at the head of Winyah Bay and the confluence with Bull Creek has not been studied in any detail (see Fig. 19). Here the river is a distributary of the Pee Dee. The Waccamaw has a maximum width of 4, 000 feet near its mouth narrowing within 2 miles to 2,000 feet. Its channel follows a relatively straight course behind the extension of the Myrtle barrier in George­ town County (Fig. 19). Tidal m arshes adjacent to the channel were drained for rice cultivation in the 18th century and field patterns still persist today. The occurrence of sand dunes in the lower valley has already been noted. Between Bull Creek and the town of Conway the Waccamaw River narrows from 500 to 175 feet in width with a minimum depth of 12 feet at low water. In this section it flows through wooded swamps at a slope of less than one foot per mile. Levees are low and narrow increasing from a few inches above low water stage near Bull Creek to 3-4 feet at Conway. Vegetation re­ flects the microtopography very closely with less water-tolerant species of oaks and pines on the slightly elevated sites. Tidal variation in water 78 mvMWiA m m m Lnrf Prt-Ree* nl contact W m a m s * Beach Ridgt* r " 1 Valley Dun** H o rry r: : I Recent Bo trier County Salt March Freehva ter Swamp / March * Swamp eontael j O locality No. M. R o a d

M w rra/r. In U9 w TAtrwiMor* / C**ok

C ounty

^ '4 / M

| Ml.

Km.

Figure 19. Waccamaw-Pee Dee deltaic plain and adjacent Recent and pre-Recent coastal surfaces, Georgetown County, S. C. 80 level (less than 2 feet) occurs in this section, but unlike the downstream por­ tion, tidal drainage networks are absent. The main channel splits in several places to form multiple channels, a phenomenon not characteristic of sections farther upstream. Cut-offs are poorly developed and long straight reaches separate short sinuous reaches. Within the sinuous section, meander length varies from 0.25 to 0.35 miles. Broad flood basins are typical of this section of the flood plain. Low islands of sandy point bars, remnants of Terrace I, protrude through the organic clays of the flood basins (see Fig. 7a). From Conway to Red Bluff (Fig. 2) widths range from 90 to 130 feet; the minimum depth in this section is 3 feet at low water. The stream gra­ dient is still quite low, less than one foot per mile, but levee height increases to 10 feet. The backslope of levees is remarkably steep, 3-5°, over a width of 100-150 feet. Levees are composed of silty sand with interbedded lenses of clay 0.5 to 1.0 feet thick. Bank failure and slumping into the river is a common phenomenon. Ridge and swale topography on point bars indicates channel migration. Cut-offs and traces of old channels with dimensions sim ilar to the present stream occur within the flood plain. The river flows in a single channel in this section. It tends to meander in mid-valley, but where it impinges against a terrace scarp the channel is "deformed” (see Matthes, 1941) and a long straight reach develops downstream. Meander length averages between 0.2 and 0.3 miles. Flood basins flank the levees, but tend to be nar­ rower and occupy less area than basins farther downstream. During flood stage the water level rises 5-6 feet in the basins. The low terrace which bor­ ders the flood plain is more continuous in this section and rises from an elevation of 5 feet above levee level at Conway to 10 feet at Red Bluff. Upstream from Red Bluff the Waccamaw River changes markedly: stream gradient increases to about one foot per mile, channel width decreases to less than 50 feet, and meander length decreases to less than 0. 2 miles (Fig. 7b). In this section the river is obstructed by sunken logs and debris, and the minimum depth is 2 feet or less. The gaging station is located in this section, near Longs(Figs. 2 and 7b) and details on discharge at this site are 81

tabulated (Appendix la). Levee height decreases to 5 feet at low stage. The channel pattern is highly sinuous in this section, and long, straight reaches are less common. Cut-off lakes and traces of abandoned channels are noted in the flood plain indicating a capacity for rapid change in channel position. Channel dimensions within the modern flood plain contrast sharply with the scars on Terrace I (Fig. 7b). The stream during low stage appears to have little competence, but the presence of bare areas of quartz sand on point bars indicates effective transportation during flood stage. Bore holes in the Waccamaw flood plain have revealed 30-40 feet of poorly sorted sand and interbedded lenses of clay. This fill rests on eroded Pee Dee or Waccamaw formations (see generalized sections in Fig. 5). Little Pee Dee. The Little Pee Dee River and its major tributary the Lumber River, rise in dissected hills of the upper Coastal Plain of North Carolina at approximately 250 feet above sea level (Fig. 1). After joining the southwest flowing Lumber River, the Little Pee Dee turns 90°, then skirts the westward edge of a coastal barrier, changing its direction toward the SSE near Galivants Ferry before joining the Great Pee Dee (Fig. 2). Points of similarity with the Waccamaw River include discharge of organic- stained water, sim ilar seasonal changes in river stage, and a bedload of predominantly quartz sand and fine gravel. However, Figure 8 and Appendix la respectively show the steeper gradient, and higher discharge of the Little Pee Dee in comparison with the Waccamaw. The gaging station is at Galivants Ferry (Fig. 2) on the Little Pee Dee. The Little Pee Dee flood plain can be divided into three sections. The lower section extends 6 miles upstream from the confluence with the Great Pee Dee. This is a broad flood basin with poorly-developed channels. The river anastomoses into many small channels in this section (Fig. 9 ). Sub­ aerial levees are absent as the stream makes its way through a swamp forest. Judging from aerial photographs, it appears that the sediment-laden Great Pee Dee has built a low cone of alluvium across the mouth of the Little Pee Dee, and that the intricate maze of anastomosing channels has been caused by damming the flow of the Little Pee Dee. 82

Downstream from its confluence with the Lumber River, the Little Pee Dee flows in a flood plain 1-1.5 miles wide, with a constant but rela­ tively high floodplain gradient of 1.16 feet per mile. In this section it has an average width of 150 feet, a minimum depth of 4 feet and is fairly clear of obstructions. Levee height above low water stage increases to 5 feet. Levees of the Little Pee Dee are composed of poorly sorted, silty sands. However, well-sorted point bar deposits are common. As in the Waccamaw Valley, flood basins are areas of dominantly organic accumulation beneath tall hardwood forests. Stream patterns within this section of the Little Pee Dee are those of a single channel which is actively meandering. Cut-off lakes and abandoned channels are prominant. Meanders in this section average 0. 3-0. 4 miles in length. Deformed reaches, although present, are not as striking as on the Waccamaw or Great Pee Dee.

The Little Pee Dee above the confluence with the Lumber is a much smaller stream with an average discharge at Dillon, 20 miles from the Lumber, of 535 cfs averaged over a 21 year period, compared with 3,020 cfs at Galivants Ferry. The flood plain is less than 1 mile wide, and the channel narrowing in width to 40-50 feet is badly choked with debris; numerous sand bars, formed around obstructions, lim it the depth in places to 1 foot or less. Meander length decreases to 0.1 mile. Tight sinuous reaches are separated by long straight reaches which impinge against the valley wall. The Lumber River flows from a northeasterly direction above the con­ fluence with the Little Pee Dee (Fig. 1). Flood plain size, channel width and depth, meander length and discharge decrease slowly upstream, and it is not until Lumberton in North Carolina, 37 m iles above the confluence (Fig. 1), that channel and flood plain dimensions approximate those of the Little Pee Dee above the junction. It is clear that the Lumber contributes the greater amount (estimated at 60 per cent) of the discharge of the Little Pee Dee below the confluence. The Little Pee Dee and Lumber valleys are fed by organic-stained waters from two large tributary "swamps": Gapway Swamp near the North 83

Carolina state line and Lake Swamp (Fig. 2). Both tributaries are heavily vegetated with hardwoods, and drainage is along multiple channels flowing around islands of trees. Bedload transportation is restricted to the move­ ment of organic remains. The streams in these "swamps” are not eroding their beds. An alluvial deposit slightly above modern floodplain level hugs the northwest valley wall upstream from Galivants Ferry (shown on Fig. 10 as "older flood plain"). This flood plain extends up the Lumber River, but is represented in the Little Pee Dee Valley above the confluence by isolated meander scars which have cut into the next highest terrace. This surface parallels the profile of the modern flood plain and is less than 5 feet above levee elevation. It is frequently inundated, and relict channels function as drainage sloughs during periods of overbank flow. The present flood plain truncates the "older flood plain" in several places (Fig. 10). Below the con­ fluence with the Lumber River, braided patterns dominate the surface topo­ graphy. Ridge and slough relief seldom exceeds 3 feet, and the spacing between bars and channels is strikingly sm aller than the braided channels of Terrace I (Fig. 10). In three areas, one in the upper Lumber Valley, another at the junction of the two rivers, and a third below Galivants Ferry where Terrace I is truncated by the modern flood plain, meander scars are prom­ inent on this surface. The meander loops are slightly larger than on the present flood plain (lengths approximate 0,4-0. 5 miles). Soil profile devel­ opment is meager on this surface. It is composed predominantly of fine sands and silts with organics accumulating in the sloughs under present conditions. The "older flood plain" truncates higher river terraces, but is not dissected by gullies entering the valley from the northwest, thus suggesting a late Recent age. Great Pee Dee. The Great Pee Dee River rises on the eastern slope of the Blue Ridge mountains of western North Carolina at elevations reaching 4,000 feet above sea level, and flows across the Piedmont region to the coast, a distance of 455 miles. The Great Pee Dee together with Cape Fear, Santee, Savannah and other stream s which cross the "Fall Line," are referred to as 84

"through" stream s and, in contrast to rivers which rise on the Coastal Plain, carry considerable quantities of suspended load in relatively fast-flowing, turbulent water. Through the study area the Great Pee Dee Valley trends southeast. It is diverted towards the southwest behind the extension of the lowest pre-Recent barrier (Myrtle) in Georgetown County (Fig. 19). Discharge of the Great Pee Dee is partly controlled by a series of dams upstream. The average discharge of 9,049 cfs (22 years of record) is three times as great as the Little Pee Dee, and amounts to nine times that of the Waccamaw (Appendix la). The gaging station at Pee Dee, South Carolina, is located on the northwestern boundary of the study area (Fig. 2). For pur­ poses of description the Great Pee Dee flood plain will be divided into two sections: delta and alluvial. The deltaic section commences at the bifurcation of the Great Pee Dee into two channels: Bull Creek which joins the Waccamaw, and the Pee Dee River which splits into many sm aller channels (e. g. Thoroughfare Creek) farther downstream (Fig. 19). The deepest channel in the delta is the Waccamaw River which has already been described. Other distributaries are relatively shallow (< 10 feet). Channel fill of soft muds is characteristic of the distributaries. In the upper portion of the delta above the junction of the Waccamaw and Bull Creek and the Pee Dee and Thoroughfare Creek, the channels wind through hardwood forest growing in the flood basin of these rivers. Here, levee height is less than 2 feet above low water stage. Relict channel forms such as cut-off lakes and meander scars are poorly developed in this section. At a distance of about 4 m iles below the head of the delta these forms are replaced by a tidal drainage pattern within the swamp. This pattern becomes more pronounced farther downstream in the tidal marsh section of the lower delta. The capture or imminent capture of the Pee Dee channel by headward- eroding tidal stream s is quite evident in this section (Fig. 19). Pee Dee, Waccamaw and Black rivers have not built their delta into the open sea in Recent time. They are filling the head of the crescent-shaped estuary, Winyah Bay. Borings across the mouth of the estuary for a new 85 highway bridge (Route U.S. 17) showed 30 feet of silt and sand with some basal gravel overlying a grey, hard marl, probably pre-Recent in age, at depths of 45-50 feet below MSL. The alluvial plain section of the Great Pee Dee Valley is dominated by a single channel ranging in width from 300-500 feet. Depths decrease from a low water minimum of 15 feet at Bull Creek to 2 feet near Pee Dee (Fig. 2). However, scour pools exceeding 20 feet deep are quite frequent. Levee height increases from 3 feet to 6 feet above low water stage over the same distance. The gradient of the flood plain is 0.69 feet per mile, which is less than the Little Pee Dee but greater than the Waccamaw (Fig. 8). The present river flows prim arily on the southwestern side of the flood plain. However, up­ stream from the Pee Dee gaging station, the river is not confined to one side of its valley, and near the junction with the Little Pee Dee it appears to be migrating eastward into the flood basin of the sm aller river (Fig. 9). Well- developed meanders are prominent in places. Meander length varies between 0.5 and 0.7 miles. Cut-offs are preserved on the eastern side of the flood plain. Where the channel impinges against the valley wall meandering is in­ hibited, and a "deformity" of channel pattern takes place, producing a long, straight reach. Microtopography in the flood basins behind the levees is well- marked, being especially apparent where it is reflected by vegetation patterns on aerial photographs. Water in anastomosing, shallow channels or sloughs, vegetated by water-tolerant trees, flows through the swamp only at flood stage. These channels are frequently less than 2-4 feet deep. Water lines on trees in these sloughs indicate 4-5 feet rise in water level during floods. Sediments are finer and different in composition from those of the Waccamaw or Little Pee Dee valleys. Yellow silt and clay containing abundant mica flakes and heavy m inerals are characteristic, but at depths of 10-20 feet clean sand has been encountered in borings. Organic sedimentation is not important in this valley in floodbasin environments. The bedload of the present stream is coarse sand mixed with pea-sized gravel, but grain size decreases downstream to fine sand and silt at the head of the delta. Logs of drill holes 86

for bridge construction across the Great Pee Dee Valley at U. S. 76 and

U. S. 701 (Fig. 2) bottom in a dark-colored, stiff clay at approximately 35 feet and 50 feet respectively, below floodplain level. It is possible that this is the

Cretaceous "basement” which outcrops at several places in the bluffs to the southwest. No samples were available to verify this interpretation. Conway Precipitation 3.23 3.94 3.86 3.07 3.51 5.21 7.00 6.60 5.38 2.97 2.32 3.26 50.35

Years 70 71 71 72 71 71 71 69 68 70 71 71

M ean Temperature 48.6 49.6 56.2 63.7 71.4 78.0 80.5 79.8 75.3 65.3 55.2 48.4 64.3

Years 63 63 63 63 63 63 63 63 63 63 63 63

M arion

Precipitation 3.08 3.51 3.78 3.06 3.41 4.25 6.29 6.30 5.56 2.77 2.77 3.38 46.76 2 APPENDIX

Years 26 31 31 33 33 32 32 29 30 30 29 28 00 M ean Temperature 47.5 49.9 54.4 62.7 71.3 78.1 80.4 78.8 74.2 63.8 54.9 47.5 64.0

Years 27 28 29 29 29 29 29 28 28 27 27 28

M yrtle B each Precipitation 1.82 2.68 3.88 2.57 3.31 4.41 4.49 5.92 8.20 3.29 2.42 2.66 45. 65

Y e a rs 888888888888

SOURCE: Climatography of the United States, No. 86-25, U.S. Weather Bureau, 1965

Precipitation and Temperature Data, Horry and Marion Counties S urface F a c ie s E levation Length Width T hickness

B a r rie r 0 to 15 34 0 .0 to 0 .4 10 to 35 R ecent Backbarrier flat (a) -4 to +3 5 0.5 to 0.7 5 to 20

B a r r ie r 5 to 30 34 0.8 to 1.5 30 to 70 M yrtle 4 (b ) 0.0 to 0.5 Backbarrier flat 17 to 22 10 to 50 20 (c) 0 .0 to 6 .0

B a r r ie r 30 to 55 28 1.0 to 7.0 30 to 45 Jalu co Backbarrier flat 35 to 40 14 0.0 to 2.0 30 to 60

9 (b) 0.0 to 0. 7 Barrier 40 to 60 20 to 35 20 (c) 0 .0 to 2 .0 Conway Backbarrier flat 40 to 45 31 3.5 to 7.0 10 to 50

B a r r ie r 65 to 115 30 4 .0 to 15 20 to 45 (d) H orry Backbarrier flat (e) 90 to 105 18 11 to 16 40 to 60 (d)

Unit of Measurement ft ± MSL M iles M iles F e e t

(a) At Cherry Grove

Morphometric Properties of Coastal Surfaces, Horry Co., S. C. E levation Age Sample No. Lab. No. Location Sample Type ft. from HWM (Yrs. B .P.)

80th Av. N. Myrtle B each Freshwater peat 1-1893* -2 2,865+115 33°44,30”N, exposed on beach 78°41T45’’W

68th Av. N. Myrtle Freshwater peat B each recovered from cored 1-1894* - 2.6 3,370+145 33°49'15MN, valley fill behind 78°49'10"W beach PEDX 4 APPENDIX 3 1-1895* as for sample 2 as for sample 2 -9 .5 3,710+125 Brackish water peat ■12.3 5,860+115 00 4 1-1896* as for sample 2 CO at base of section

Ocean Forest Hotel, In situ "white pine" J . D uB ar Myrtle Beach stump exposed on 6,100+145 (p. co m m .) S S M S W N , beach 78°50'00"W

Bull Creek, S. of Charcoal and humic US 701 colloids of buried 1-1897* + 18 16,900+320 33°39'00"N, A^ horizon (approx) 79°07,25"W l 1-1898* as for sample 6 Freshwater peat + 3 36,200 buried beneath two (approx) + 3,600 dune sheets -2 ,5 0 0

Radiocarbon Dates, Horry County, South Carolina (*assays provided by Isotopes Inc) C hannel No. 1st Order M axim um B asin A rea length Order No. Relief Swamp Width S tream s Side slope

A 0.22 2 .3 4 3 20 0.1 6 -

B 0 .4 2 4.8 6 3 25 0 .1 16 -

C 7.90 36.20 4 37 0.3 5 97 -

D 7 .40 24.25 4 39 0 .3 60 3

E 23 .1 6 8 .4 5(?) 80 0 .6 178 4

F 12.1 4 8 .4 4 70 0.4 5 134 - PEDX 5 APPENDIX

G 7 .4 22 .5 3 65 0.2 0 51 -

H 2 .3 1 8 .7 3 21 0.25 16 -

I 9 .7 4 39 .1 4 32 0.60 77 3

J 6 .5 9 18.25 3 40 0.55 35 3

K 6.9 6 20 .5 4 44 0 .5 40 3

Unit Sq. Miles Miles (Sum) Enumerative Feet M iles Max. Enumerative Degrees (Total)

Source Air photo Air photo A ir photo Topog. Map A ir photo A ir photo F ield

Morphometrie Properties, Selected Basins, Horry County, S. C. APPENDIX 6

Stratigraphic section on eastern side of Intracoastal Waterway, north of Myrtle Beach, Horry County, S. C. (lat. 33°47'50,IN; long. 78C46,00"W).

0- 1. 0 ft. Sand medium-coarse; quartz; containing abundant humus and roots; pepper and salt appearance; weak crumb structure; very friable; N3/very dark grey. Sharp contact. 1.0- 2.8 ft. Sand medium grained; 10YR5/8 yellowish brown with rusty oxi­ dized stainings along root channels; patches of 10YR2/2 very dark brown; humic rich, massive; soft with vertical fractures. Gradational wavy contact. 2.8- 3.6 ft. Bleached medium-fine sand; single grained, loose; N8/2 white. Sharp wavy contact. 3.6- 6. 7 ft. Organic bound hardpan of varying hardness, massive, soft; can break in fingers; fine-medium sand with organic colloids; banded, sub-horizontal and nearly parallel, some bands anastomosing; cut by widely spaced fracture planes; 10YR2/2 very dark brown, 10YR2/1 black; lighter bands 10YR4/4 dark yellowish brown.

Very sharp contact. 6.7- 9.0 ft. Sandy clay, plastic massive; no shells; N4/0 dark grey. Sharp co n tact. 9. 0-14. 8 ft. Medium-fine sand, single grain; loose with thin discontinuous clay stringers and balls; borings (?); no shell; lower foot con­ taining oxidized band with iron nodules (limonite); 7 .5YR6/8 reddish yellow; 2. 5Y6/2 light brown grey. Sharp contact, very

pronounced. 14.8-16. 7 ft. Buttery clay with shells scattered, somewhat sandy; bluish grey, no color to match chart; closest is N4/dark grey. 16. 7-18.1 ft. Shell rich, clayey sand; slightly sticky, massive, sloppy when wet; contains intact Tagelus plebeius and Crassostrea in position of growth. Same color as above. Wavy sharp contact, bored.

91 92

(Unconformity separating Myrtle deposits above from Jaluco below) 18.1-19. 6 ft. Organic rich medium grain sand; 10YR2/2 very dark brown in top 6", grading down to 10YR6/3 pale brown, massive soft; becoming single grain loose in paler colored area (buried humate paleosol). Gradational contact. 19.6-21.6 ft. Medium grained sand, horizontally bedded with some coarse sand layers; varying in color at top, 7. 5YR5/8 strong brown to 10YR5/4 yellowish brown. Bluish grey in lowest foot. Wet, sloppy; no shell; massive, soft. (Unconformity between Jaluco deposits above and "Waccamaw formation"

below ) 21. 6-23.6 ft. This layer is approximately 1 foot discontinuous sand with shell hash, and whole shell; coarse sand, cross-bedded, poorly sorted, yellow brown; grades into 1 foot thick blue grey coarse sand, angular, shell hash; partly indurated at base: "Wac­ camaw formation" (see DuBar and Furbunch, 1965, Sections 22

and 56). APPENDIX 7a

Myrtle barrier

L o catio n : Near Jet. 67th Ave. N. and Bryan Drive, Myrtle Beach, Horry County, S.C ., Lat. 33°44'46”N; Long. 78°50'10"W. Vegetation: Pine and scrub oak and patchy tussock grass and lichen cover. E lev a tio n : 34 feet above MSL. P ro file : Aoo 0-1/4”: Pine leaf, cone, and oak leaf cover.

Ao: not present Al 1/4-7”: Sand containing charcoal and organic fragments, medium to fine grained; speckled color 2. 5YR4/0 dark grey; weak crumb; very

friable; roots present. Wavy gradational contact with: A2 7-11”: Sand, medium-fine, well-sorted, clean; 5Y8/1, white; single grain, loose; fewer roots. Sharp contact with: B 11-18”: Sand, medium-fine, well-sorted; 2. 5Y6/4 light yellowish brown, single grain, loose, with irregularly shaped mottles up to 2” in diameter, 7. 5YR4/4 brown-dark brown, very friable; some

dark brown in black nodules l/l6th” in diameter firm to friable,

some roots. Grades into: C (B2?) 18-41": Sand, medium to fine, well sorted; 5Y7/4 pale yellow, single grain and loose, with small less 1/2” mottles, 2.5Y6/6, olive yellow, streaky, friable.

93 APPENDIX 7b

Myrtle backbarrier flat

Location; In drainage ditch parallel S. C, 707 near Myrtle Beach, Horry County, S.C. , Lat. 33°41’45"N; Long. 78°55'00"W. Vegetation: Pine, oak and dense grass cover. Elevation: 20 feet above MSL.

P r o file : Aoo and Ao not present Al 0-1”: Fine sandy silt with organic fragments and grass roots, 10YR5/4 yellowish brown; moderate fine crumb structure; friable. Sharp contact with: A2 1-7": Silt, well sorted with some fine sand grains; 2.5Y7/4 pale yellow; fine-medium crumb structure; friable to powder when dry; some iron nodules up to 3/8" diameter, firm; few roots. Sharp smooth contact with: B1 7-13 1/2": Clayey silt; 10YR5/6 yellowish brown with 2.5YR5/6 red mottles up to l/2" in diameter; angular blocky structure, moderate 1" peds, grey clay skins on some of the peds pene­ trating along fractures and following root casts from A2 hori- zon; firm ; some iron nodules up to 3/8" diameter; few roots.

Grades into: B3 30-57": Silty clay, strongly mottled, 5Y7/1 (N/7) light grey clay becoming more dominant with depth; black (5Y2/1 or N/2)

(Mn) mottles and nodules, firm up to 1/4"; and up to 2" wide red (2. 5YR4/6) and strong brown (7. 5YR5/8) mottles red in

center of mottles fading with depth; massive structure; firm when moist, plastic and sticky when wet; few roots.

94 APPENDIX 8a

Jaluco barrier

Location: 1.7 miles N of Nixons Crossroads on U. S. 17 near Little River. Roadside exposure on interfluve. Lat. 33°51'55MN; Long. 74°38’00"W. Elevation: 55 feet above MSL. Vegetation: Pine, scrub oak with ground cover of woody scrub and tussock g r a s s . P ro file : Aoo Leaf litter of pine and oak -- discontinuous. Ao 0-1/4": Decomposed leaf litter. A1 1/4-11/2": Sand, medium-fine, loamy containing some coarse grains; 10YR4/1 dark grey; weak crumb structure; very friable; abundant grass roots. Grades into: A2 1 1/2-5": Bleached sand, medium-fine with some coarse sand grains; 10YR6/1 light grey to grey; single grain; loose; fewer roots. Sharp contact with: Bl 5-30": Sand medium-fine with some coarse sand grains; 10YR7/6 (A21) yellow with faint 10YR6/6 brownish yellow mottles up to 1" in diameter usually 1/4"; some nodules and concretions 1/4-1/2" (iron): 10YR6/6 brownish yellow; weak crumb; very friable except for hard nodules; few roots; isolated humate stain in upper 15" of this horizon. Very wavy gradational contact with: B2 30-42": Clayey sand, irregular, friable pods of reddish brown (7.5YR6/8) (Bll) soft hardpan, grades upwards and downwards into adjacent

h o riz o n s. Cl 42-55": Clayey sand, medium-fine some granules (quartz?); 7.5YR6/8 (B21) reddish yellow; (soft hardpan?) massive; friable; few roots. Color of this horizon fades with depth. Horizon contains wavy bands of irregular thickness which anastomose; slightly silty

95 sand; 7. 5Y R4/4 brow n to dark brow n; m assive; soft; very pronounced at base of profile. APPENDIX 8b

Conway backbarrier flat

Location: On flat backbarrier flat surface along County Rd. 135,1. 8 miles

from Jet. with County Road 24, in drainage ditch at right angles to road. Lat. 35°47,45"N; Long. 79°10'03"W. Elevation: 35 feet above MSL.

Vegetation: Chiefly pine, some hardwoods. Good woody scrub and grass

c o v e r. P r o f ile : no distinct Aoo or Ao Al 0-3": Fine loamy sand containing abundant roots; 7.5YR4/ (N/ ) dark grey; fine crumb structure, very friable. Grades into:

A2 3-12": Fine sandy silt; 2.5Y5/4 light olive brown; crumb structure,

friable; contains roots. Sharp contact with: B1 12-15": Sandy silt; 10YR5/6 yellowish brown, firm crumb structure; intertongues with A2 and B2. B2 15-25": Sandy clay; 10YR5/6 yellowish brown; mottled 2.5YR5/8 red, mottles becoming larger and more distinct towards base, up to 1” wide; moderate blocky structure, plastic when wet, well-

developed grey clay skins on peds. Grades into: B3 25-61": Sandy clay with some coarse sand and granules present; strongly mottled horizon N 5/l (5Y5/1) grey, 10YR6/8 brownish yellow,

to 2. 5YR5/8 red dominating at the center of the mottle; mottles

up to 1-2" in width with no distinct pattern; some red spots firm to hard, friable with difficulty, 1/4-1/2" in size. Yellow

becomes more prominent in mottles at base of horizon. Coarse

blocky to a massive structure near base, plastic when wet;

polygonal structural units of mottles. Becomes greyer and

slightly more sandy below 50".

97 APPENDIX 9a

Horry barrier

L o c a tio n : Roadside exposure along U. S. 501 between Aynor and

Galivants Ferry 1.7 miles from Aynor. Lat. 34°01f10"N;

Long. 79°13’30"W.

E lev a tio n : 89 feet above MSL. Vegetation: Scrub oak, pine with tussock grass, woody scrub and lichen

ground flora. P r o f ile :

Aoo: Patchy leaf litter

Ao 0-1/4": L eaf m old

A l 1 /4 -3 " : Fine loamy sand; 10YR3/1 very dark grey; fine weak crumb structure; very friable abundant roots. Grades into:

A2 3 -8 " : Bleached fine sand; 10YR6/1 light grey to grey; weak crumb to single grain; very friable to loose; rootlets present. Sharp contact with:

B1 8 -1 7 ": Sand, fine, well-sorted; 2. 5Y6/4, light yellowish brown; weak

(A21) crumb; very friable. This horizon is stained by humus, 10YR5/3 brown, becoming less marked with depth. Grades into:

B2 17-54": Fine sand, well-sorted; 2.5Y7/4 pale yellow; single grain to

(A21) weak crumb; loose. Horizon contains bands of fine sand iron stained 10YR6/6 brownish yellow; bands are horizontal and wavy, of variable thickness l/4-l". Some nodules are present

up to 1/2", hard. Sharp wavy contact with:

C 5 4 -6 7 ": Ironstone 2. 5YR4/8 red at center, discontinuous nodules up to

( B ll) 6"x3"x2", hard with knobbly surfaces. Between ironstone is fine sand, 10YR7/6 yellow, single grain, loose. Sharp contact

w ith:

98 root (?) channels, discontinuous, vertical, horizontal and diagonal 3/4-1 1/2" wide up to 30" long, clay 5Y6/2 light grey to N8/ white and 5Y7/1 light grey in sandy areas; massive, friable, no roots; becomes increasingly more sandy towards base of exposure. APPENDIX 9b

Horry fluvial (backbarrier flat equivalent)

L ocation: In roadside ditch on County Road 31 in Marion Co. between State Road 917 and U.S. 76, approx. 1.3 miles from junction with U.S. 76. Lat. 34°11'40”N; Long. 79°11'45”W.

E le v a tio n : Approx. 100 feet above MSL. Vegetation: Cultivated field on slope towards gully; topsoil partially removed by wash and deflation. P ro file :

Aoo and Ao: Absent from this profile. A l 0 -5 ” : Fine sandy loam containing roots; 10YR4/2, dark grey brown; crumb structure, very friable, upper portion truncated?

Sharp contact with: A2 5 -1 3 ' Sandy silt; 7. 5YR5/6, strong brown; crumb structure (adheres to knife) poorly plastic when wet; friable; some fine rootlets abundant pore spaces. Grades into:

B1 13-42' Fine sandy silt with more clay; 10YR5/8, yellowish brown; weak blocky structure, peds 1/4-3/4”, friable; cohesive when wet, but still poorly plastic; weak red mottles, 2. 5YR5/8 (red), usually inside peds, 1/4-1/2” in diameter, slight clay skin of ped face, very few roots. Grades into:

B2 4 2 -6 3 ” Sandy clay with red mottles 2. 5YR5/6 and 10R4/8, also 10YR6/8 brownish yellow; mottles 1-1 1/2” diameter, very pronounced and slightly sandy in texture; discontinuous light grey to grey (5Y6/1) zones l/2 ”-l" wide (irregularly?) distributed between mottles (gleyed); massive, firm to friable, roots rare. Grades

into: B3-C 63-98” Clayey sand, dominantly of red color (10R5/8) with "purple” tones (10R3/4, dusty red?), also weak mottling brownish yellow 10YR6/8 and yellow 10YR7/8; gleyed clay following form er

100 101

C 67-91": Fine sand, slightly silty; 10YR6/8 brownish yellow to 7.5YR5/8

strong brown; massive hard, when dry; hardpan decomposes

in water. Grades into:

C 91-102": Silty sand; weakly mottled 10YR6/6 - 6/8 brownish yellow to

10YR7/2, light grey mottles l/2 -l" apparent increase in size

with depth, some with red centers 2.5YR6/6 - 6/8 light red. VITA

Bruce Graham Thom was born in Sydney, Australia, on October 29, 1939. He attended Bondi Junior Technical School and Scots College graduating from the latter high school in 1956. With the aid of a Commonwealth Scholar­ ship he enrolled in the University of Sydney, Faculty of A rts, in 1956 and graduated with First Class Honors and the University Medal in Geography in

1961. In 1962 he received a Teaching Assistantship with the Department of Geography and Anthropology at Louisiana State University, and in 1963 became a Research Assistant with the Coastal Studies Institute at Louisiana State

University.

102 EXAMINATION AND THESIS REPORT

Candidate: Bruce Graham Thom

Major Field: Geography

Title of Thesis: C oastal and F luvial Landforms, Horry and Marion Counties, South Carolina

Approved:

h r ? *

M ajor Professor and Chairman C-

Dean of the Graduate School

EXAMINING COMMITTEE:

Date of Examination:

January 6, 1967