The Alaska }Earthquake March 27, 1964 Regional Effects

rosion and Deposition on a Raised Beach, .Montague Island

G E 0 L 0 G I CAL SURVEY P R 0 FE S S I 0 N A L PAPER 5 43 - H

3 1819 00086457 5

THE ALASKA EARTHQUAKE, MARCH 27, 1964: REGIONAL EFFECTS Erosion and Deposition on a Beach Raised by The 1964 Earthquake Montague Island, Alaska

By M. J. KIRKBY and ANNE V. KIRKBY

. A quantitative study of geomorphic modifications of uplifted coastal features

GEOLOGICAL SURVEY PROFESSIONAL PAPER 543-H UNITED STATES DEPARTMENT OF THE INTERIOR

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY

William T. Pecora, Director

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1969

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402- Price 60 rents (paper cover) THE ALASKA EARTHQUAKE SERIES

The U.S. Geological Survey is publishing there­ sults of investigations of the Alaska earthquake of March 27, 1964, in a series of six professional papers. Professional Paper 543 describes the regional effects of the earthquake. Other professional papers describe the history of the field investigations and recon­ struction effort; the effects of the earthquake on com­ munities; the effects on the hydrologic regimen; and the effects on transportation, communications, and utilities.

CONTENTS

page Page Page Abstract------H 1 Erosion and deposit,ion of bay- Erosion and deposition on up- Introduction------2 head deposits in MacLeod lifted beaches and rock plat- Physical setting------2 Harbor-Continued forms-Continued Methods------4 Stream erosion-Continued MacLeod Harbor beaches- Geology------4 Influence of underlying ma Continued Bedrock------4 terial on stream erosion- - Regrading of the beaches-- - Recent unconsolidated deposits Streams in silt------Patton Bay beaches------in MacLeod Haxbor--- .-- Streams in sand ------Stratigraphic evidence for Sand------Stream in gravel------sea-level changes------Silt------Lateral and vertical erosion- Elevations of beach features- Gravel------__------Model of erosion------Tectonic deformation of Beach deposits in Patton Recession of knickpoints- - - beaches------Other erosive agents in regrad- Obliteration of break in Erosion and deposition of bay- ing of deposits -_------slope at top of beach- - - - head deposits in MacLeod Regrading of rock platforms- Harbor ------Erosion and deposition on up- Formation of bay-head de- lifted beaches and rock plat+ Regrading of sand and posits------forms------gravel------Stream erosion ------MacLeod Harbor beaches- - - - Conclusions------Data collected------Beach profiles------

ILLUSTRATIONS

FIGURES

Page 1. Index map of Montague Island------_------H3 2. Map of bay-head deposits in MacLeod Harbor- ...... -- 6 3. Panoramic photograph of eroded bay-head deposits in MacLeod Harbor------9 4. Diagram showing relationship of volumes removed by fluvial erosion to elapsed time----- 9 5. Graph of variations in erosion rates of streams in MacLeod Harbor.. ------10 6. Photograph of typical stream with silt banks in MacLeod Harbor------11 7. Map, long profile, and cross sections of typical stream in silt in bay-head deposits of MacLeod Harbor------8. Planetable map, long profile, and cross sections of stream in sand, MacLeod Harbor- - - - - 9. Cross profile of gravel sheet and lateral migration of streams, MacLeod Harbor------10. Graph showing inferred rates of lateral and of vertical erosion for streams, MacLeod Harbor------.------11. Graph showing postearthquake degradation of silt, by rainwash and marine erosion, along a line of pilings.------_------...... 12. Sketch map of shoreline lithology and location of measured beach profiles------13. Graph of beach profiles around MacLeod Harbor ...... 14. Graph of beach profiles and stream profile through beach ...... 15. Photograph of rock platform showing differential erosion- ...... CONTENTS

16. Photograph of rock platform, MacLeod Harbor- ...... 17. Map of Patton Bay showing coastal features and deformation ...... 18, 19. Stratigraphic sections and beach profiles, Patton Bay - --_------20. Photograph showing cobbles of raised beach on truncated bedrock platform, Patton Bay-- 21. Stratigraphic section showing old raised beach, Patton Bay .-_------22. Stratigraphic sections and profiles of Nellie Martin River- - - ..---..------23. Graph showing distribution of elevations of beach features------_- - - _ ------_------24. Sketch of beach profiles in Patton Bay in relation to the preearthquake mean high water- 25. Graph showing variation in differences of elevation among coastal features in Patton Bay- 26. Sketch map of differences in elevation of raised beaches at the north end of Main Sandy Bay------27. Sketch map and graph of differences in elevation of raised beaches at the south end of Main Sandy Bay------28. Sketch map of cliff degradation and accumulation of talus, Patton Bay ---- _------29. Sketch map of cliff degradation by landslides and gullying in North Sandy Bay ------30. Idealized profile through two raised beaches- - - ...... 31. Graph showing sequence of platform cutting in Patton Bay ...... 32. Graph showing relationship between change of eleration of sea level and cross-sectional area- 33. Graph showing position of preearthquake storm beaches-- - .. - _------

TABLES - Page 1. Summary of data for streams in MacLeod Harbor ------_...... H10 2. Relationship of width of platform regraded, cross-sectional area of material removed, and change of relative sea level- -- ...... 37 THE ALASKA EARTHQUAKE, MARCH 27, 1964: REGIONAL EFFECTS

EROSION AND DEPOSITION ON A BEACH RAISED BY THE 1964 EARTHQUAKE,MONTAGUEISLAND,ALASKA

By M. J. Kirkby 1 and Anne V. Kirkby 2

ABSTRACT

During the 1964 Alaska earthquake, was very rapid, most of the downcut­ Evidence of two relative sea-level tectonic deformation uplifted the south­ ting in unconsolidated materials occur­ changes before 1964 was found in Pat­ ern end of Montague Island as much ring within 48 hours of the uplift for ton Bay. At a high stand of sea level as 33 feet or more. The uplifted shore­ streams with low flows greater than 10 lasting until about 2000 B.P. (before line is rapidly being modified by sub­ cubic feet per second. Since then, ero­ present), an older raised beach was aerial and marine processes. The new sion by these streams has been predom­ formed which, over a distance of 5 miles, raised beach is formed in bedrock, sand, inantly lateral. Streams with lower dis­ shows 40 feet of deformation relative gravel, and deltaic bay-head deposits, charges, in unconsolidated materials, to the present sea level. Peat deposits and the effect of each erosional process still had knickpoints after 15 months. exposed by the 1964 uplift also record was merumred in each material. Field­ No response to uplift could be detected a low sea level that lasted until at least work was concentrated in two areas­ in stream courses above the former pre­ 600 B.P. MacLeod Harbor on the northwest side earthquake sea level. The 1964 raised beach was used to and Patton Bay on the southeast side 'Vhere the raised beach is in bedrock, test the accuracy of identification of of Montague Island. In the unconsoli­ it is being destroyed principally by former sea-level elevations from raised dated deltaic deposits of MacLeod Har­ marine action but at such a low rate beach features. The P·re-1964 sea level bor, 97 percent of the erosion up to June that no appreciable erosion of bedrock could be accurately determined from 1965, 15 months after the earthquake, was found 15 months after the earth­ the height of the former barnacle line, was fluvial, 2.2 percent was by rain­ quake. A dated rock platform vaised so an independent check ·on high-water wash, and only 0.8 percent was marine; earlier has eroded at a mean rate of level was available. The most reliable 52 percent of the total available raised· 0.49 foot per year. In this area the fac­ topographic indicator was the eleva­ beach material had already been re­ tor limiting the rate of erosion was rock tion of the break in slope at the top of moved. The volume removed by stream resistance rather than the transporting a beach between a bedrock platform -and erosion was proportional to low-flow dis­ eapacity of the waves. a cliff. Even here, the former sea level charge raised to the power of 0.75 to The break in slope between the top of could only be identified within 5 feet. 0.95, and this volume increased as the the raised beach and the former seacliff The breaks in slope at the top of gravel beaches were found to be poor indicators bed material became finer. Stream re­ is being obliterated by debris which is accumulating at the base of the cliffs of former sea level. sponse to the relative fall in base level and which is no longer being removed On Montague Island, evidence of by the sea. Current cliff retreat by rock­ former high sea levels appeared to be l Formerly Research Associate, Dept. of Geography, The Johns Hopkins University fall, mudflows, and landslides was esti­ best preserved ( 1) as raised bedrock and now LPcturer in Geography, Univer­ mated at 0.7 to 2.0 feet per year, and in platforms on rocks of moderate resist­ sity of BJ"istol, England, parts of Patton Bay the accumulation of ance in slightly sheltered locations and • Graduate student, Dept. of Geography, The Johns Hopkins University, Baltimore, debris has obliterated 78 percent of the (2) as raised storm beaches where the Md. original break in slope in 15 months. relief immediately inland was very low.

H1 H2 ALASKA EARTHQUAKE, MARCH 27, 19'64 INTRODUCTION

On March 27, 1964, arr earth­ On both sides of the island the PHYSICAL SETTING quake shook parts of Alaska and amount of subaerial modification Montague Island, one of the caused great devastation in inhab­ of the 1964 raised beach could be outermost islands of Prince Wil­ ited areas. However, maximum up­ measured. Especially important liam Sound, lies off the southern lift took pla;ce, and large bedrock was the opportunity to study the coast of Alaska at lat 147.5° W. faults were active, on Montague area only 15 months after the and long 60° N. It is about 50 miles Island, an uninhabited island off earthquake. In the MacLeod Har­ long, and has a maximum width of the southern coast of Alaska. bor area, rivers in soft sediments 15 miles and a minimum width The southern end of Montague have adjusted so swiftly that evi­ of less than 5 miles. The island Island was uplifted as much as 33 dence of the postearthquake his­ feet or more during the earth­ tory of the area will soon be re­ is rugged and mountainous, and qua,ke (fig. 1), and two active moved by erosion. Subaerial a chain of peaks forms its back­ faults several miles long caused degradation of the raised beaches bone at an average altitude of many landslides and broke or dis­ and abandoned seadiffs is so rapid about 2,500 feet. Cliffs which placed many hundreds of trees that within a few years little of the occur almost all around the island (Plafker, 1967). Around the coast, original marine form will be left make the coast dangerous. The marine deposits and a marine­ unchanged, and the growth of veg­ 1964 earthquake elevated and ex­ abraded bedrock platform were etation, which has already begun posed a flat bedrock platform lifted above sea level and exposed on the beach, will hasten its which encircles the island; the to subaerial processes. obliteration. former marine cliffs, now beyond The uplift associated with the Fieldwork was done in 1965 at the reach of the sea, were broken earthquake has provided physio­ two locations, MacLeod Harbor by a few inlets and sandy beaches graphic conditions on a scale much and Patton Bay (fig. 1). The proj­ that allowed access to the island. larger than could be simulated in a ect was a reconnaissance and, al­ laboratory and with the added ad­ though the study was concerned Travel across Montague Island vantage that the features and with postuplift changes in raised is difficult. It is uninhabited except for seasonal hunters and loggers; changes were natural. The bay­ beaches, differences in emphasis re­ there are neither roads nor even head deposits of MacLeod Harbor sult from physiographic and tec­ paths, except those made by bears. (figs. 1, 2) on the northwest coast tonic differences between the two The upper parts of the mountains of the island provided an undis­ bays. The work was supported by are rocky and are snow-covered sected surface, about 1 square mile a grant from the National Science Foundation, administered by The all year. Below the peaks the land in area and with a slope of only 20', Johns Hopkins University. We is either steep and covered with which had been suddenly uplifted would like to thank Professor M. alders or stunted conifers, or the 33 feet. On this surface, conse­ G. Wolman of The Isaiah Bow­ slope becomes so gentle that water­ quent drainage channels were ini­ man Department of Geography, logging prevents tree growth, and t.iated and were rapidly eroded The Johns Hopkins University, patches of wet peat interspersed vertically and laterally; thus, a for his continuous scientific and with pools of wa,ter form muskeg. unique opportuni,ty was provided administrative help; and A. T. to study the effects of rapid uplift Hohl, Department of Geology, At altitudes below about 1,000 on the mode and rate of fluvial Princeton University, and Thomas feet, the coniferous forest forms a processes and the resultant valley Dunne, Department of Geog­ dense cover down to sea level forms and long profiles. raphy, The Johns Hopkins Uni­ Only where the slope is gentle The earthquake also brought to versity, for their assistance in enough for muskeg to form or view a bedrock platform and asso­ carrying out the fieldwork. Mr. C. where it is so steep that it is totally ciated deposits, which allowed LaBounty, a longtime seasonal unstable is there any break in the observation and measurement of resident of Montague Island, pro­ forest. Most of the island has not features that are usually under vided valuable data on pre- and been logged and is covered with water and therefore difficult to postearthquake conditions in the apparently virgin forest contain­ study. bay-head deposits. ing conifers more than 6 feet in EROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND H3

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/ UPLIFT. IN FEET / I I \ \ ..>30

20-30 CJ 10-20 0 D

/ < 10 / ------~u~------Fault active in 1964 earthquake Dashed where probable. U, upthrown side 0 5 10 MILES

1.-Index map of Montague Island showing amount of uplift (after Plafker and Mayo, 1965). A. , area of figure 12; B, area of figure 17.

280-547 0 - 69 - 2 H4 ALASKA EARTHQUAKE, MARCH 27, 1964 diameter and over 400 years old. with a hand level or Abney cli­ locity is approximately twice the The trees are as much as 200 feet nometer, and distances were meas­ mean velocity of the whole cross high, and there is a thick, usually ured by pacing. Preearthquake section. Discharges were measured thorny, undergrowth. The forest mean high water was assumed to at ·a time of relatively low flow floor is covered with shallow ponds be at the top of the abandoned bar­ (see table 1, p. H10) but were cor­ and bogs ; large fallen tree trunks, nacle line. (Plafker, 1965, p. 1-2). rected for daily variations by multiplying by a standardizing many of them rotten, lie across one The level of postearthquake mean high water was obtained from the faotor: another in a chaotic pattern and U.S. Coast and Geodetic Survey Discharge of stream 1 on June provide unstable footing. predicted tide tables. These 20, 1965 Even on the postearthquake agreed, within 1 foot, with the Discharge of stream 1 on day raised beaches, plant growth is position of the new barnacle line of measurement rapidly establishing itself; only 15 where it was observed. To avoid confusion, the expres­ months after the earthquake, con­ Grain sizes of the MacLeod sion "1964 sea level"c has not been ifer seedlings were growing on Harbor bay-head deposits were de­ used in this paper. Instead, the surfaces that were formerly below termined by counts (Wolman, expressions "preearthquake sea sea level ( G D. Hanna, 1965, oral 1954) of 100 pebbles in stream level" and "postearthquake sea commun.). beds and by sieve analyses of fine level" are used. A sea level higher Most of the rivers on Montague materials. On beach profiles we than the post-earthquake sea level Island are narrow, steep torrents measured the size of the largest is called a raised sea level, and a that rush down the slopes in a ser­ stones in a section and estimated sea level lower than the preearth­ quake sea level is called a low sea ies of rapids and waterfalls. Only visually the percentage of material level. However, inasmuch as the the Nellie Martin River has finer than 1 inch in diameter. Per­ earthquake took place in 1964, it achieved any substantial flood­ meabilities were determined ap­ proximately by measuring the is correct, and gives rise to no am­ plain development, and it flows biguity, to refer to the uplifted time for 1 inch of water to infil­ through a wide valley, whose floor beach exposed by the earthquake trate; the water was contained by near the coast is covered with thick as the 1964 raised beach. The a metal ring. peat deposits that form vertical changes in relative sea level are Discharge of streams in McLeod riverbanks as much as 10 feet high. probably caused more by move­ Harbor was calculated as ment of the land than by absolute METHODS Surface velocity in center of change of sea level, but in the ab­ stream X mean depth X width. sence of positive evidence it is cus­ Mapping was done with a plane­ This calculation gives values ap­ tomary (Sparks, 1960, p. 211) to table and telescopic alidade. Beach proximately twice the true value, refer to the land as fixed in rela­ and stream profiles were surveyed because the centerline surface ve- tion to sea-level changes.

GEOLOGY

BEDROCK view along the shore. These strata conglomerate and volcanic rocks. are thought to be of Tertiary age The sedimentary rocks are uni­ Little is known about the bed­ (Plafker and MacNeil, 1966, p. formly bedded and have flat paral­ rock of Montague Island; most ex­ 66-67). lel bedding planes. The coarser posures are covered by forest and A study of the coastal exposures clastic rocks are of two types : ( 1) muskeg or, at higher elevations, by indicates that the bedrock of the conglomerate having coarse round­ scree and snow. The recent uplift island consists of a sequence of in­ ed quartz and rock-fragment clasts has improved accessibility to the terbedded thin layers of black ar­ set in a gray sandstone matrix and coast and has brought nearly con­ gillite, thick beds of graywackelike (2) breccia of black angular argil­ tinuous exposures of bedrock into sandstone, and occasional beds of lite chips, also in a sandstone EROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND H5 matrix. The volcanic rocks were RECENT UNCONSOLIDATED transformation p r o d u c e d by not seen in an outcrop, but hillside DEPOSITS IN MAcLEOD earthquakes prior to the 1964 boulders and beach cobbles indi­ HARBOR earthquake best fits the known cate that they consist of buff and facts. green medium- to fine-grained ag­ At the head of MacLeod Har­ Occasional well-rounded pebbles glomerate. In the Patton Bay bor, uplift has exposed unconsoli­ as much as 3 inches in diameter are region, light-gray volcanic inter­ dated sediments which formerly found within the sand. Through­ beds occur. occupied ·beach, intertidal, and out the deposit there are discon­ Two major faults, both active in subtidal zones. Three main sedi­ tinuous stringers up to 2 inches the 1964 earthquake, have been ment types are recognized : sand, thick of small wood pieces, shells, mapped on the island (Plafker, silt, and gravel. The areal distribu­ seaweed, and pine needles, similar 1967, fig. 2). The faults trend tion of these types is shown in fig­ to present high-tide swash accumu­ roughly parallel to the island axis ure 2. The deposits occupy a lations. These stringers probably and have dips of 50°-85° NW. The roughly rectangular basin that indicate forme.r positions of high Hanning Bay fault is well exposed deepens seaward; they themselves tide. The permeability of the un­ for about 4 miles between Han­ also thicken sea ward, reaching a saturated sand is about 25 inches ning Bay and MacLeod Harbor; it maximum exposed thickness of 18 per hour. has a maximum vertical displace­ feet, but their total thickness is SILT ment of 13 feet in bedrock and 17 probably much greater. feet in unconsolidated beach ma­ The sand occurs mainly on the The silt is black and uniform in terial (Plafker, 1967). Inland, the south side of the bay and has a composition, except for the per­ fault can be traced by landslides, gradational contact with the silt. centage of sand present, which in­ fallen trees, and earth fissures sev­ The silt occupies an area which creases to·ward the contact with the eral feet deep and 1 foot wide. was formerly occupied by quiet sand. The silt shows an apparent The Patton Bay fault can be water in the deepest part of the bay lack of bedding and internal struc­ traced for about 22 miles, but for head. The upper surfaces of the ture, although occasionally thin most of that distance it is con­ silt and sand were formerly partly parallel bedding was suggested by cealed beneath landslide debris, its intertidal and partly below spring differential erosion along stream­ maximum measured vertical dis­ low water level. beds. Clam shells (mostly Macona placement is 20 to 23 feet (Plafker, nasuta) that are found through­ 1967). This fault follows and ac­ SAND out the deposit are concentrated through erosion on the silt surface centuates a preexisting topo­ The sand of MacLeod Harbor is as a lag deposit. The permeability graphic break in slope. Near the dark gray and uniform through­ of the silt is about 0.01 inch per Nellie Martin River, the Patton out the deposit. It stands in ver­ hour. Bay faultline is characterized by tical cliffs as much as 20 feet high; GRAVEL a linear scarp covered with debris, for the most part it is well bedded, including fallen trees, and modi­ the thickness of individual beds Gravel deposits occur in three fied by numerous landslides. The ranging from 6 inches to 2 feet. forms-as beaches around the in­ faultline scarp varies in height The beds are continuous for a min­ land edge of MacLeod Harbor, as from 100 to 300 feet and has an imum of 200 feet and show the in­ fans around the former mouths of average gradient of 40° made up ternal crossbedding that is char­ streams entering the bay, and as of a series of irregular steps. At acteristic of a mainly fluvial sedi­ lenses and discontinuous sheets at the top of the debris slope a clean ment. or near present sea level. exposure of rock 5 to 20 feet high Scour channels and filled chan­ The beach gravel consists mainly slopes at an angle of 65° to 75°. nels about 4 feet wide are common. of local rock types and occasional There is no evidence of mineraliza­ Convolute bedding occurs at all exotic pebbles of granodiorite, tion and no gouge. levels in the sand, and some convo­ granite, or hornblende gneiss. The Surficial deposits comprise re­ lutions are trunca:ted by overlying gravel is very well rounded and cently uplifted shallow-water ma­ uncontorted beds as much as 12 well sorted and has only minor rine silt and sand, coarse beach feet below the present surface. The amounts of sand matrix. The mean sand and gravel, and fluvial gravel. cause of the convolutions is not size of the pebbles varies widely on Extensive peat and some glacial known, but the possibility that the beaches around the bay; on deposits are also present. they were formed by thixotropic some beaches more than 80 percent H6 ALASKA EARTHQUAlKE, MARCH 2 7, 1 9'6 4

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Cam psi

---.po,ste,anthquake MHW (Approximate)

EXPLANATION [ ~~~~.::~ D Gravel Sand Silt

Contact

------15------Contours of land surface after postearth­ quake downcutting, in feet above post­ earthquake MHW Long-dashed where extrapolated ------20---- .... ····· Contours of preearthquake land surface, in feet above postearthquake MHW Dotted where extrapolated

Eroded margin of preearthquake land ------­surface ''''''''I I I I I I I I Preearthquake storm beach

Main stream

1!11 Stream number X SECiiON

Locality discussed in text 0 500 1000 FEET

2.-Bay-head deposits in MacLeod Harbor and June 1965 elevations af their upper (preearthquake) and lower (postearthquake) surfaces. E:ROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND H7

are smaller than an inch in diame­ from one pebble (% inch) to 12 where the streams flow into the ter, but on other beaches 80 per­ inches. Individual pebbles vary sea, gravel lenses occur beneath cent are larger than 4 inches in from ¥s to 5 inches in diameter; the streambeds. These lenses are diameter. On any individual beach many are disk shaped and most are as much as 3 feet thick, and in the size range is much less, al­ well rounded but have low sphe­ places a sand bed lies between though generally pebble size de­ ricity. The gray-sand matrix them and the gravel sheet on the creases from the storm beach to­ makes up 40 percent of an individ­ present surface. The lower gravel ward the sea, and sorting becomes ual gravel bed; wood fragments lenses have a sand matrix and a better in the same direction. Per­ and abraded logs are common. No maximum pebble size smaller than meabilities are about 450 inches graded bedding or cross bedding that of the present stream gravel, per hour. was observed. The upper and low­ Behind the gravel beach around er contacts with adjacent sand BEACH DEPOSITS IN most of the bay is a storm beach beds are uneven, and the gravel .PATTON BAY which is characterized by little or lenses out, the thinnest beds having Bay-head deposits are not pres­ no sand matrix, poorer sorting, the least lateral continuity. The ent in Patton Bay. Instead, uplift coarser grain sizes, steeper gradi­ sand beds are discontinuous and has merely widened the existing ents on the upper surface both from 1 to 10 inches thick; planar beach deposits or bedrock plat­ landward and seaward, and ex­ crossbeds are present in a few forms. The distribution of the trr,mr,lv high permeabilities. beds. The total section at this loca­ coastal forms is discussed on page The river fan gravel deposits tion (fig. 2) consists of 60 to 70 H20 in relation to individual beach also consist of local rock types and percent gravel beds and 30 to 40 profiles. vary in mechanical composition percent sand beds, not counting The coastline around Patton from their apexes in gaps through sand matrix in the gravel beds. Bay consists mainly either ( 1) of the storm beach to their sea ward The gravel lenses and discontin­ wide gently sloping sand beaches edges. At their apexes they are uous sheets occur only at or below with gravel storm beaches sur­ similar in composition to the 1965 mean sea level. They are mounted by large driftwood ac­ beach gravel. The percentage of found beneath the sand and silt cumulations at their inland edges sand matrix increases and the size deposits and have been partly or (2) of cliff-backed bedrock of the pebbles decreases toward eroded and redistributed by post­ platforms with or without narrow the seaward edges of the fans. earthquake strPam action. They sand or gravel beaches and drift­ Near the lower part of the fans form a more or less continuous wood at their inward margins. (section 1, fig. 2) the deposit is of sheet where the streams have been Between zones of these two main interbedded gravel and sand, the eroded down to 1965 sea level. coastal types, small areas of large gravel beds ranging in thickness Near the present high-water level boulders lie on bedrock.

EROSION AND DEPOSITION OF BAY -HEAD DEPOSITS IN MACLEOD HARBOR

FORMATION OF BAY-HEAD steeper slope. The evidence for the seaward edge of the silt. A DEPOSITS this statement, although indirect, steeper slope, therefore, very prob­ Both the surface composition is fairly conclusive: (1) the U.S. ably existed rut the seaward edge and the topography of the pre­ Geological Survey 1 :63,360 topo­ of the deposits, although whether earthquake deposits at the head of graphic map and the U.S. Coast its form was erosional or deposi­ MacLeod Harbor (fig. 2) seem and Geodetic Survey charts show tional is unknown. It is also con­ consistent with conditions of nor­ depths close inshore that indicate sidered probable that this slope mal intertidal and shallow-water a steep offshore slope, and ( 2) be­ was partly exposed by the uplift deposition by rivers at the head of fore 'the earthquake local fisher­ associated with the earthquake, be­ an inlet. At their seaward limit men were able to anchor close in­ cause major streams very rapidly the deposits seem to have formed a shore in areas that are still close to trenched the newly uplifted de- H8 ALASKA EARTHQUAKE, MARCH 27, 1964

posits (C. LaBounty, oral com­ the gravel sheets could be traced which rise within the area that was mun., 1965) ; such trenching would down to the 1965 mean sea level, formerly below sea level; the be favored by the exposure o£ a they very probably indicate a for­ coastal perimeter of the uplifted steep slope by uplift above sea mer relative sea level that was at deposits has been trimmed by wave level. least as low as the 1965 one and action. By July 1965, about 50 per­ Figure 2 shows elevations o£ the that predates the finer deposits. cent of the available deposits had bay-head deposits in June 1965. Cores taken in the present inter­ been regraded to a new level, ac­ Surveyed contours o£ the remain­ tidal zone (Barrett, 1966, p. 997) cordant with the present sea level. ing parts of the upper surface (in show a series o£ pebbly layers ex­ Most of the regrading has been interfluve areas) have been inter­ tending down as low as 1 foot done by lateral erosion of the polated in stream-eroded areas to above mean lower low water. These largest rivers (fig. 3). reconstruct the preearthquake sections seem to show deposition o£ Long profiles were made of 20 topography o£ the deposits. Sur­ 25 to 30 feet o£ sand in and close streams (numbered 1-17 and 20- veyed contours o£ areas o£ post­ to the intertidal zone, and imply 22, table 1), particular attention earthquake stream erosion have a fairly steady rise o£ relative sea being paid to the position of knick­ similarly been interpolated to esti­ level before the earthquake. A sim­ points. At least three cross sections mate the final postearthquake to­ ilar steady rise has been reported were made of each stream in order pography o£ the bay-head depos­ for other parts o£ Prince William to estimate the total volume re­ its. The upper surface could still Sound (Plafker and Rubin, 1967). moved from each by erosion. A be fairly well defined in June Some indication o£ the rate of grain-size analysis was made of 1965, but the former seaward limit rise o£ relative sea level has been samples from near the mouth of o£ the deposi,ts is less definite. obtained by the dating o£ two each stream, and a relative value Four independent levels £rom in­ wood samples. C-14 sample W- of low-flow discharge was calcu­ terpolated 1965 mean high water 1768, from 4 feet above postearth­ lated. Additional information to the preearthquake barnacle line quake mean high water (section 2, about the earlier course of erosion give a mean measurement o£ 33 fig. 2), was interbedded with sand was obtained from U.S. Coast and feet £or the uplift. The mean tid­ that probably was deposited above Geodetic Survey aerial photo­ al range in MacLeod Harbor is extreme low water level (14 feet graphs taken 8 weeks and 21 weeks 9.2 feet, and spring low tides were below the 1965 mean high water) . after the earthquake. formerly 19 feet above the 1965 The radiocarbon age o£ this sample Interpretation of the informa­ mean high water. The lowest ele­ is 820+200 B.P. C-14 sample W- tion collected has been in terms of vations, which are still preserved 1764, from a tree in the position o£ the overall course of erosion by the on remnants o£ the preupli£t sur­ growth approximately at the level rivers, including the recession of face, are 13 feet above the post­ o£ the preearthquake barnacle line, knickpoints. This part of the in­ earthquake mean high water on has a radiocarbon age of 380±200 terpretation is essentially a study sand and 9 feet 'U:bove on silt. B.P. and indicates that mean high of fluvial processes in an unusual water was then at least 5 feet lower Thus, the top o£ the seaward steep environment. The data have also than the preearthquake mean high slope apparently was at least 10 been studied in relation to the water. Rates o£ £all of relative sea feet below spring low water modifica:tion of a raised platform. level calculated £rom samples W- The rapidity with which such a ( 1963) on the silt and at least 6 1768 and W-1764 are 0.033±0.008 raised platform is altered, and the feet below on the sand. The top foot per year and more than agents responsible for it, are dis­ o£ the steep slope may be close to 0.014+0.013, respectively. cussed on pages H15-H16. the approximate position o£ the +10-foot contour o£ the upper STREAM EROSION DATA COLLECTED surface shown in figure 2. The banks of streams 1-9, 15, The presence o£ a gravel sheet Bay-head silt, sand, and gravel and 16 (table 1) are silt; the banks or sheets, both below the 1965 were studied only at the head of of streams 10-14 and 20-22 are stream courses and in the outliers MacLeod Harbor. Since the uplift sand; and the banks of stream 17 o£ silt and sand, suggests that a o£ about 33 feet in March 1964, the are gravel. Bed material is not in­ former stream system similar to area of intertidal and shallow­ frequently coarser than bank ma­ the one which is now developing water deposits has been consider­ teria 1. Low-flow discharges produced these gravel sheets. Since ably dissected by rivers, few of ranged from zero for the smallest E-ROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND H9

3.-Panorama of eroded bay-head deposits in MacLeod Harbor.

streams in sand and 0.035 cfs ( cu­ 200 ,------,------~------,------,,------,------~------, bic feet per second) for the small­ EXPLANATION (187.000,000) est stream in silt (stream 6) to 301 Upper.confidence limit cfs (stream 10). The volume of material removed by all streams Estimated total available

from the time of the earthquake ( 170,000,000 cu ft) to late June 1965 (fig. 4) was calculated from direct measure­ (153,000,000) ments. For the larger streams Lower confidence limit (streams 9, 10, 12, 21, and 22), es­ timates of volumes removed at earlier stages w e r e calculated 6 from aerial photographs, which Sand showed the width of erosion, and X Silt from planetable maps made in June 1965, which gave.the depth of 0 erosion. For these large streams, Gravel errors arise mainly from incorrect interpolation of the initial eleva­ tion of the surface before stream erosion (fig. 2). This error in ini­ tial elevation is unlikely to be more than 1 foot near stream mouths where thicknesses removed are 10 to 15 feet; it will be correspond­ ingly smaller upstream where the Confidence limits rivers are narrower and thick­ nesses of material removed are 0 to 5 feet. Measurements of widths of erosion, made from field meas­ urements and from aerial photo­ graphs, are considered to be much 20 more accurate than the depth measurements. For small streams, errors arise mainly from the intri­ cacy of variation of width and bifurcation, but all major tribu­ TIME AFTER EARTHQUAKE, IN WEEKS taries, were included in our calcu­ lations. An overall error of ± 10 percent is considered a generous 4.-Relationship of volumes removed by fluvial erosion to time elapsed since the maximum allowance (95 percent earthquake. HlO ALASKA EARTHQUAKE, MARCH 27, 1964

TABLE !.-Summary of data for streams in MacLeod Harbor [*,no knickpoints apparent in streaml Total volume Total volume of Distance of Relative Cross- of material material removed knickpoint Width at low-flow sectional removed from Size of bed (millions cu It) Type of bank Stream number from mouth mouth discharge area at the time of the material material Remarks (feet) (feet) (cis) mouth earthquake to D., (in.) May 1964 Aug. 1964 (sq It) June 1965 (8 weeks) (21 weeks) (cu It)

1 ______76 25 0. 37 97 5, 025 2. l_ ____ Silt ______1. 06 ______do _____ 2------Many 22 1. 73 180 6, 160 knick points. 3------75 15 . 67 103 6, 250 2. 64 ______do _____ 4 ____ ------66 23 . 051 128 6, 300 Silt______.do _____ 63 18 . 035 78 5,250 ___ do ______do _____ 5------___ do _____ 6------255 32 . 35 198 41, 500 . 49 ____ 1------335 20 . 50 85 22,400 . 74 ___ ------8 ____ ------1, 150 72 5. 71 480 361,000 1. OL ______do _____ g ______0 190 73.0 2, 600 3. 20X 106 . 96 ____ 0. 8L_ 2. 17-- ___ do _____ Gravel bed. 10 ______0 1, 625 301. 0 25, 200 36. 7X 106 1. 56 ____ 3. 54.- 9. 50 __ Sand and Do. silt. ll ______* 70 0 650 164, 000 Sand____ Sand_._____ 12 ______0 665 80.8 9, 650 18. ox 106 1. 27 ____ 3. 18.- 12. 8 __ __ .do. ____ Do. 13 ______130 1. 10 1, 220 710, 000 Sand______do _____ 14 ______* ___ do _____ * 30 0 120 19, 400 ___ do ___ 15 ______50 . 0129 ------Silt ______Data in------complete. 16 ______50 . 0094 ------___ do _____ Do. 17------110 39 1. 09 176 19, 600 2. 9 _____ GraveL ___ 20 ______2. 16 Sand ______21 ______* ------6 ------___ do _____ 22 ______0 490 9.30 6, 860 6. 04X 10 ------. 92 2. 31 Gravel bed. 0 1, 450 52.3 21,700 29. ox 106 ------5.20 24. 7 ___ do _____ Do. TotaL _____ ------94. 3X 106 ------13. 6 51. 5 ------confidence). Streams appear to 100 erode at a more or less uniform (/) z rate until they approach :the limits 0 :::; 10 set by topography or by neighbor­ ..J ~~-- ing streams, at which time their lJJ rate of erosion becomes much less. ~ ~ 1.0 ciu Zero discharge L LJJ_ Sand-banked stream Stream 10, for example, is con­ >m O::J 1 tinuing to erode at a uniform rate, L X 2.6 ~ u 0.1 Silt-banked stream whereas stream 22 appears to have a:LJ.. 0 0 lJJ 2.9 neared its limit. The overall re­ ~ Gravel-banked stream ::J lationship of volume removed to ..J 0.01 (Stream 17) 0 X 2.1 X 2.6 X 1.1 Figures for silt and gravel streams time elapsed since the earthquake, > show median grain size of bed material, in inches for these streams and for all the 0.001 c______L _____ ...J...______.J______...... L ______j streams combined, is shown in 0.01 0.1 1.0 10 100 1000 figure 4. RELATIVE LOW-FLOW DISCHARGE, IN CUBIC FEET PER SECOND Volumes removed by erosion up to June 1965 have been calculated 5.-Variations in erosion rate with low-flow discharge of streams in MacLeod Harbor. Arrow shows direction of increasingly coarse bed material. by summing cross-sectional areas removed along the length of the stream. Figure 5 and table 1 show INFLUENCE OF UNDERLYING face flow commonly appears on the volumes removed, low-flow dis­ MATERIAL ON STREAM EROSION silt immediately below its up­ charge, bed and bank material, and stream contact with a gravel bed, Three major characteristics of dther data. Each point in figure 5 and results from the large differ­ has been distinguished as to bank the underlying material appar­ ence o£ permeability. Differences material, and streams flowing ently control stream erosion. in permeability probably also between silt banks have been fur­ The first is the permeability of cause large differences in the ratios ther distinguished as to bed-ma­ the deposit, which determines the of high-flow to low-flow discharge. terial size on the basis of a pebble drainage area required to produce The second control is the varia­ count. surface runoff. For example, sur- tion in resistance to erosion. This EJROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND Hll

control might be expected to result in marked differences in drainage densities and networks. A dif­ ference in networks is apparent (see fig. 7 for silt and fig. 8 for sand), but differences in drainage density are masked because many streams receive most of their water supply from streams outside the area of uplifted deposits. Their drainage density is therefore partly determined by the number and spacing of the feeder streams. The third control is the effect of bed armoring. Armoring is wide­ spread in larger streams, mainly with gravel deposits that are eroded less readily than the bank materials. Extensive armoring is usually associated wi'th streams 6.-Typical stream with silt banks (stream 8, table 1), MacLeod Harbor. Note the that have almost reached their conspicuous white clam shells on the silt surface. limit of possible downcutting, so it is not immediately clear, from STREAMS IN SILT able) from silt to clam shells to the data in this area, whether the Initial stream courses m silt gravel. The amount of coarse slow rate of downcutting is a seem to have been consequent and armoring material and the ability cause or an effect of the armoring. to follow the directions of greatest to move it thus increase together, Differences in erosion rates of slope, as exemplified by the although figure 5 shows that the streams flowing in silt and in sand streams that have not migrated armor has a net retarding effect banks may be clearly distinguished laterally and that still follow the on erosion. in figure 5. The distinction seems reconstructed consequent direc­ Small streams in silt (fig. 6) to depend mainly on the differ­ tions. The initial directions of generally have steep banks, narrow ences in the ratio of high- to low­ larger streams, even though they valleys, and very marked knick­ flow discharges, but the greater have since moved laterally, also points in their longitudinal pro­ erodibility of sand may be a con­ correspond to reconstructed conse­ files (fig. 7). Streambank gradi­ tributory cause. The silt channels quent directions. Most of the ents locally decline from the usual can be distinguished by the size of streams in the silt are fed by run­ 60°-70° to 10°-20°, where the silt the bed material except for stream off from the hillside along the is saturated. This decline is asso­ 2, in which the small volume of north side of the harbor. ciated with extensive slumping material removed is probably due A sharp division can be made and earthflows. The saturation is to the fact that the stream's knick­ between streams that can transport usually caused by unchanneled point had already reached gravel seepage from the edge of the gravel effectively and those that and it could not cut back farther. gravel beach. Figures 6 and 7 il­ cannot. Those that cannot carry Note that the plot of the gravel­ lustrate typical silt valleys. banked stream 17 falls with silt­ gravel effectively are characterized banked streams with the same by an abrupt ending of the chan­ STREAMS IN SAND bed-material size. The data may nel at its upper end, at the lower The drainage areas of many indicate that armoring of the bed margin of the gravel beach or at smaller streams in sand (fig. 8) lie with coarser material tends to re­ a gravel fan. The great difference entirely within the bay-head sand duce the rate of erosion and that in permeability between silt and area. These streams have no gravel recession of the knickpoint is gravel emphasizes this feature. In or clam shells on their beds. greatly slowed when it reaches the progressively larger streams, the Clearly defined c}1annels are small coarse gravel deposits which form bed material tends to change (pro­ and are confined to the sea ward the preearthquake storm beach. vided suitable material is avail- ends of the valleys. In the smallest

280-547 0 - 69 - 3 H12 ALASKA EARTHQUAKE, MARCH 2 7, 1 9 6 4

A EXPLANATION

Upper silt surface

Lower silt surface • Knickpoint

D ~ w :.: < & 30 :X: I­ ll: ;51- tii t:j 20 - 0"- B 166 SQ ft 0..z w­ > - al ~ 10 <:::!: A z 198 SQ ft 0 i= c < 0 >w 350 300 250 200 150 100 50 0 _j 0 5 10FEET w B DISTANCE. IN FEET NO VERTICAL EXAGGERATION

7.-Map (A.), long profile (B), and cross sections (C) of typical stream in silt (stream 6, table 1) in bay-head deposits of MacLeod Harbor. Letters on profile show position of cross sections. Numbers in cross sections refer to cross-sectional area eroded. EROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND

Upper surface section

Valley bottom

EXPLANATION

Upper surface Damp sand

Wet sand 0 50 100FEET U

Channel bank

UPPER SURFACE

Cross Cross Cross sectton sectlon sectlon B ____---- posltlon fZPZ surFC5 __/-- ? -- - A_---- i1 Do@" --y

0 0 50 100FEEl ------I1--- Postearthquake MHW LONGITUDINAL PROFILE (Follow~ngthe most northerly tr~butaryvalley)

------Position of upper surface

A\ Valley bottom 20

C gp~ppercrface

Valley bottom 0 10 20 FEE1 CROSS SECTIONS 8.-Planetable map, long profile, and crws sections of a typical stream in sand (stream 13, table I), MacLeod Harbor. HI4 ALA,SKA EARTHQUAKE, MARCH 2 7, 1 9 6 4 valleys, no channels at all are pres- sent. The downstream parts of the sand valleys are broad and have flat bottoms. These are joined, by sharp breaks in slope, to side '///A Valley of stream 22 1 slopes that are uniformly at the w- angle of repose. At the top of the >S [&14 Valley of stream 10 side slopes there is a second break in slope, above which is an almost flat upper surface, which roughly corresponds to the original sur- 6 HORIZONTAL DISTANCE, IN FEET face of the sand before and im- W 0 8 weeks 21 weeks 65 weeks 0 8 weeks 21 weeks x--x-C x C Y Y * Y ---* X mediately after uplift. The whole L_J "h 7 flat section of the valley floor may Lateral migration of stream 10 65 weeks be wet, even above the head and Lateral migration of stream 22 outside the banks of the channel; 9.-Cross profile of gravel sheet and lateral migration of streams 10 and !22 (table basal sapping of the side slopes ap- I), MacLeod Harbor. Arrows are drawn to scale, and show distances and direction parently is chiefly responsible for of migration. valley widening. The upstream LATERAL AND VERTICAL era1 migration by the gravel. Fig- parts of the valleys are V-shaped EROSION ure 9 can thus be interpreted as a in cross section and are dry on the Figure 9 shows gravel sheets record of the rate of downcutting, valley bottom. The irregularity of that crop out approximately at which can be dated by reference to the long profile suggests that there the surface of the present major the lateral position of the streams may be periods of infilling by wind streams. One or more gravel sheets in aerial photographs. between periods of fluvial erosion. at a similar level may extend be- The rates of lateral and vertical Figure 8 illustrates a typical small neath the remaining outliers of silt erosion deduced in this way (fig. stream valley in sand. and sand. This preexisting gravel 10) tend to support the generally Large streams, fed by water probably has some influence on the held view that lateral erosion from the hillsides above the for- rate of downcutting of the streams. mainly takes place after most mer sea level, are gravel floored At one extreme, the control may be downcutting is completed. Ob- and have steep vertical banks in so complete that the present servations of Mr. C. LaBounty, sand, or in sand and gravel in their streams are simply being guided in (oral commun., 1965) indicate upper reaches (but still below the their lateral erosion into hollows that the bulk of the vertical ero- preearthquake water level). Clear in the former gravel sheets and are knickpoints are not present in the thus prevented from further down- longitudinal profiles of either cutting. At the opposite extreme, large or small streams flowing over the gravel sheets may only slow sandbeds. downcutting temporarily, without Stream 22 1 STREAM IN GRAVEL tending to shift the streams later- 500 ally. The lateral migration of One small stream (stream 17) 0 flows down the hillside and then stream 10 shown in figure 9 might 0 10 20 30 40 50 60 70 across a steep gravel beach di- support either possibility, but the rectly into the sea. The gravel had reversal of the direction of lateral some finer matrix and was able to migration of stream 22 shows that stand, at least for a short period, the stream does not simply move in vertical banks. The stream had into a hollow and stay there; if a steep course and a knickpoint streams did this, they mould never change their direction of migra- zone less well defined than those $ 15 tion. Instead, it is concluded that 'O0 20 of the streams in sillt (fig. 14, p. 0 10 20 30 40 50 60 70 H18). It is debatable whether lower positions of the gravel sheet TIME AFTER EARTHQUAKE. IN WEEKS represent later times and that the placement of this single example 10.-hferred rates of lateral and of with the streams in silt (fig. 5) is streams are downcutting vertical erosion for streams 10 and 22, normal. without rigid control of their lat- MacLeod Harbor. EROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND H15

sion in the major streams was very For the MacLeod Harbor sedi­ marine, and slope processes. The rapid, having been almost com­ ments, the constants A, r, and k most effective slope processes prob­ pleted within 48 hours, and that have the following approximate ably were the slopewash, caused by the lateral erosion has been pro­ values: raindrop impact, and erosion by

ceeding ever since. Erosion in un­ 6 wind. Sand: A=1.9 X 10 ; r=0.75 consolidated sediments probably k=0.012. In the broad expanse of depos­ is limited by the rate rut which ma­ 5 its in MacLeod Harbor, fluvial ero­ Silt: A=1.7 X 10 ; r=0.95 terial can be transported by the k=0.012. sion seems to be the most important stream. Thus vertical erosion and process. The sea can only act along la;teral erosion compete for avail­ The differences apparently due L~.> the perimeter, but rivers act over able capacity, as figure 10 seems lithology may partly reflect dif­ the whole surface. The volume of to show for the first period of 8 ferences in hydrology rather than material removed by the rivers has weeks. Vertical erosion will result in erodibility (seep. H10-Hll). been estimated from the planetable in changes in stream gradient­ Total volume removed seems to map of the deposits (fig. 2). perhaps the most independent be the only consistent measure of Along the margin of the depos­ control of transport rate in a the course of erosion. Recession its, marine erosion produces cliffs given cross seotion-and gradient of knickpoints (where present) , as high as 18 feet. Below these is itself controlled by the rate of cross-sectional area at the mouth cliffs a narrow beach is being sediment transport from up­ of the stream, and width at the formed, coarse material accumu­ stream. Material eroded from the mouth of the stream all showed lating at its top. Some intertidal bed will be redeposited if the slope much more scatter in their relation sorting of material may also take is lowered too much, and the re­ to other variables. place, but its total effect in Mac­ sult will be very little net vertical RECESSION OF KNICKPOINTS Leod Harbor is thought to be erosion. No such regulatory The knickpoints in the smaller slight. mechanism applies to lateral ero­ streams in silt apparently are con­ On the silt deposits, clam shells sion, whioh may therefore continue trolled mainly by slight variations protect small sloping pillars of unchecked until aU available ma­ in resistance of the material; the material ; the silt around them is terial is out of the possible mean­ streams seem to pick out certain detached by raindrop impact and der belt of the river. Figure 10 bedding planes which are not oth­ washed away by surface runoff. At illustrates this general course of erwise distinctive. The knickpoints the time of measurement, all these events, but gives no information recede until they finally become pillars sloped at about 30° to the about possible mechanisms. fixed at the junction with the horizontal in the same direction, MODEL OF EROSION gravel beach above; this junction which is presumed to be the angle and direction of the driving rain. The results of these erosional has alre,ady occurred for stream 2. Such an angle is probably not studies are valid only within a On the larger streams, the gra­ characteristic of all storms, so the limited framework. It would for dient of the stream above the heights measured probably repre­ example be quite improper to ap­ former high-water mark and the sent only the last major storm. The ply them to conditions in which gradient on the gravel spread be­ median vertical height of 50 of the erosion was limited mainly by low it seem to be more or less inde­ pendent. No identifiable knick­ these pillars was 1.9 inches (0.16 the resistance of the materials. ft). Within the framework, however, points were observed in the long The total effect of this rainbeat the overall course of erosion fol­ profiles, the only recognizable since the time of uplift could be lowing a sudden relative lowering feature being a local steepening, seen as follows: The shipworm of base level may be approximated probably associated with the former beach or storm beach. ( B ankia spp.) bores into wooden by an equation of the form: pilings underwater, but it cannot V=A · Qr(1-e-7ct) OTHER EROSIVE AGENTS live within the silt; pilings become where Q is the low-flow discharge, IN REGRADING OF honeycombed with these boreholes in cubic feet per second; V is the DEPOSITS and eventually break off within an volume removed, in cubic feet; t inch of the surface of the silt; a is the time since the change of The three principal types of ero­ line of old broken-off pilings pro­ base level, in weeks; and A, r, and sional processes that modified the jecting out of the present silt beach k are constants. bay-head deposits were fluvial, therefore records the former sur- H16 ALASKA EARTHQUAKE, MARCH 27, 1964 face level very accurately, and the total erosion of the surface may be measured. The rarity of surface 100 -- Preearthquake MHW runoff on the sand and gravel is 1-­ w w thought to minimize erosion on LL these deposits by rainbe!llt and ~ 90 rainwash. :2: ::J The profile of the silt along the !;;: 80 line of pilings is shown in figure 0 >- 0:: 11. Both the volume of material <{ 0:: 70 - removed since the earthquake by t:: OJ surface wash above the top of the 0:: <{ A low seacliff (an average thickness w 60 ___L ____ ~ ____L_ __ _J ____ _L ____ J_ __ ~ of 0.7 ft) and the cross-sectional 1) 0 300 400 500 600 700 OJ area removed by marine erosion <{ HORIZONTAL DISTANCE, IN FEET z ( 17 5 square ft) can be calculated 0 from figure 11. These computa­ tions can be compared with the volume removed by fluvial erosion. Volumes removed by the three main processes responsible for the regrading of the deposits are:

By rivers (see fig. 4) =87 mil­ HORIZONTAL DISTANCE, IN FEET lion cu ft By rainwash=0.7 ft X area of 11.-Postearthquake degradation of silt, by rainwash and marine erosion, along a line of pilings. A, Profile of entire beach; vertical exaggeration X 10. B, Part of A, silt deposits= 2 million cu without vertical exaggeration, showing retreat of seacliff. ft By the sea= 175 sq ft X coast­ al perimeter=700,000 cu ft. form of the upper surface of the regrading are fluvial processes, Wind U~Ction is locally evident sand is remarkably even and shows rainwash, and marine aotion, in on channel sides in the sand areas no hollows, such as might be ex­ that order. However, a different only. The total effect probably is pected if deflation had occurred. order of importance might be ob­ a rearrangement rather than a net It is concluded that, in MacLeod tained for, say, a narrow gravel removal of material, because the Harbor, the agents responsible for beach.

EROSION AND D-EPOSITION ON UPLIFTED BEACHES AND ROCK PLATFORMS

Beach studies in MacLeod Har­ collected from ( 1) profiles perpen­ lifted sea floors were made in the bor and Patton Bay had three ob­ dicular to the shoreline, ( 2) sur­ MacLeod Harbor area (locations jectives. The first was to provide veys of selected geomorphic fea­ shown in fig. 12). Most profiles data on the detailed form of tures and of amounts of erosion were on rock platforms, but some beaches and offshore platforms and deposition, and (3) strati­ were on beaches that were com­ that are difficult to study under graphic sections and their associ­ posed wholly or partly of sand or normal conditions; the second was ated radiocarbon dates. gravel. Interpretation of the pro­ to observe the rates of erosion and files has been made in terms of the deposition on the raised beach as MACLEOD HARBOR relief found on rock platforms, an aid to the general study of BEACHES in terms of marine regrading of raised beaches; and the third was beaches, and in terms of knick­ to clarify the tectonic history of BEACH PROFILES point recession and fluvial re­ the island. These three objectives Eight longitudinal profiles of grading. are closely related and data were preearthquake beaches and up- The profiles seem to have gentler EROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND

12.-Shoreline lithology and location of surveyed beach profiles between MacLeod Harbor and Fault Cove. H18 ALASKA EARTHQUAKE, MARCH 27, 19•64

f­ w w L.L z ~ 0 I ::;: w :<: <( ::J Most exposed beac h ~ 10 f- 0:: <( w w 0:: 0.. 0 f- 20 w > i= <( -' w 0::

~ :: !L ______L______3__ L______.I_ 8______L ______L ______L ______L ___ 5______~ 2------'------' 0 100 200 300 400 500 600 700 800 900 1000 HORIZONTAL DISTANCE, IN FEET

13.--Beach profiles around MacLeod Harbor showing the effect of increasing exposure to waves. Broken lines show effect of local modification by stream action.

overall slopes below the preearth­ 40 ,------, quake mean high water and broad­ er rock platforms where the 0 ••• 0 • • ••• • ••• • •••••••• 0 • •" •• •• • • • •• • beaches are more exposed to waves · · · Preearthquake MHW · 0 " of long fetch. This relationship is " "' 0 " shown by the series of five repre­ ,." 0 sentative profiles in figure 13. How much of the differences in profiles is due to varying wave exposure and how much, for example, to varying initial slopes of the coast is unknown. Postearthquake MHW 0 Sediments from the upper parts of both present and former beaches A show a consistent change in grain 10 size down slope; the finer material 40 80 120 160 200 240 HORIZONTAL DISTANCE. IN FEET is found in deeper water and on 0 .... lower gradients. In comparing "'u. profiles 3 and 8, a given grain size ~'!:: g~ 5 "'-'stack near profile 8, though it presumably predates cal beds (figs. 15, 16). and in the rocky promontory at them. REGRADING OF THE BEACHES profile 7. This break is, at both Bedrock platforms are notably places, about 16 feet below the pre­ smooth in overall outline, but in The uplifted MacLeod Harbor earthquake barnacle line. The detail vary from highly irregular beaches have been modified by break, which presumably was surfaces with as much as 5 feet of both marine and fluvial processes, formed at or near a former high­ local relief to much smoother sur­ but slope processes seem to have tide level since it does not seem to faces with less than a foot of local had little erosional effect. H20 ALASKA EARTHQUAKE, MARCH 2 7, 1 9 6 4

Marine processes act only in the laterally ; hence the area re­ present: postearthquake high-wa­ zone below the storm beach. In the graded here is much smaller than ter mark, postearthquake storm MacLeod Harbor area, storm that regraded by the stream in beach, preearthquake high-water beaches corresponding to the pre­ profile 4. mark (the former barnacle line), earthquake sea level range from 9 Stream 17, cutting into beach preearthquake storm beach, and feet (at the head of MacLeod Har­ profile 3, is a small stream with a breaks in slope at the foot of cliffs bor) to 12 feet (profile 4) above low-flow discharge of 1.09 cfs, and corresponding to both the pre­ mean high water. There is no evi­ a very steep course through uncon­ earthquake sea level and to raised dence of new storm beaches begin­ solidated gravel (fig. 14). This sea levels. Regrading of the up­ ning to form, but, since uplift, the stream illustrates the tendency for lifted beaches was also studied. sea has regraded a zone between a channel of uniform gradient to Regrading of sand and gravel mean high water and 4 feet above develop across a beach. Lateral was similar to that on the other it. In the short period since the erosion is at a minimum here, ap­ side of the island. An estimate was earthquake, all unconsolidated ma­ parently because this stream is still also made of the rate at which rock terial (but no bedrock) in this actively downcutting. The area platforms on the Patton Bay side zone has been regraded. Marine re­ regraded is therefore very small. have been regraded since the grading probably will eventually On the beach, which is both nar­ abandonment of the older uplifted extend up to the full elevation of a rower and steeper than the up­ shoreline. The many seacliffs, some new storm beach. Marine processes lifted bayhea.d, some resorting of several hundred feet high, also have had a much greater effect than beach deposits has already taken provide an opportunity for exam­ fluvial processes in the regrading place, and streams flow at much ining the way in which the break of all beaches studied. steeper gradients than in the bay­ in slope at the foot of a cliff is Fluvial processes are effective head deposits. Thus stream 17 is obliterated by subaerial processes. only where sand or gravel occurs steeper on the beach than it is Most of the coastline of Patton on the beach. Beach profiles 4 and farther upstream, whereas the Bay now exhibits a rock platform 5 (fig. 13) are along streams, and stream in profile 4 has the same several hundred feet wide, the stream 17 (table 1) is very close to gradient above and below the pre­ landward edge of which is gener­ profile 3. Comparison of profile 4 earthquake water level. The stream ally covered by beach deposits (fig. with profiles 1-3 and 8 (fig. 13) in profile 5, however, which falls 17). This platform, which seems to shows the typical almost-straight down a steep gully onto the beach, accord with the preearthquake sea beach profile that results from re­ is less steep in its beach section level, is backed, except in the cen­ grading by a competent stream than immediately inland. The ter of the bay, by a steep seacliff flowing mainly through sand. The gradient of a stream across the about 100 feet high. Above this upper part of the course of the beach, therefore, seems to be un­ sea cliff the land is gently rolling stream in profile 4 on the uplifted related to its gradient above the and rises 200 to 300 feet above sea beach is erosional ; lower down the preearthquake beach; in other level. In the center of the bay stream spreads out into a fan and words, the relative drop in sea level coastline, however, the cliff is effectively regrades a width of sev­ caused by the earthquake will not absent, and instead there is a low eral hundred feet of the beach by necessarily lead to the propaga­ step and a raised beach platform tleposition. The low-flow discharge tion of a knickpoint inland along 80 to 500 feet wide, which is of the stream is 10.0 cfs. every stream course. The forma­ backed inland by slopes of 40° The stream in profile 5 has a low­ tion of a knickpoint appears to be to 50° leading up to the main flow discharge of only 0.03 cfs. least likely in small streams flow­ 2,500-foot backbone of the island. This stream is competent to erode ing from steep hillsides. Sandy beaches occur in three sand from an area underlain by main areas above the coastline cobbles but it cannot carry the cob­ PATTON BAY BEACHES studied. Just south of the center of bles, so the fluvial regarding is Seventeen beach profiles, on all the bay is the most extensive limited to erosion and deposition types of material, were made in stretch of sand, called in this of sand. Not far from the line of Patton Bay, across the island from report "Main Sandy Bay." The profile 5, the stream is divided into MacLeod Harbor. These profiles largest river on the island, the Nel­ several small channels, each 4 to show the relationships, both ver­ lie Martin River, debouches at the 10 feet wide and 1 to 2 feet deep, tically and areally, between the south end of the bay. At this point which do not appear to be cutting following features, if they are the present beach is backed by two E'ROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND

EXPLANATION

PA TTON High ground

Main Sandy Bay Sand

Cobbles and boulders

APPROXIMATE

Illlll~lllll'I Cliff

-:::::+ Storm beach

--- 15- - Contour of amount of 1964 uplift, in feet

-P21 Location number

Beach profile and number

0 92 % 3/4 1 MILE storm beach I I I I I

17.-Coastal features and deformation caused by the 1964 earthquake, Patton Bay. H22 ALASKA EARTHQUAKE, MARCH 27, 1964 storm ridges, behind which is an slwwing the raised beach deposits rounded, has less fine matrix, and extensive flat area drained by the are described below. contains a wider variety of rock Nellie Martin River and its trib­ The position of beach profile types than the angular gravels of utaries. There is a second sandy P23 is shown in figure 17. Figure sections D 1 and D 2• This rounded area called in this report "Bay 18 shows the profile and a strati­ material is interpreted as being a Mouth Bar" at the southern limit graphic section located on it. The true beach deposit, beyond the of the coast studied, where a lagoon material in the bottom 24 inches edge of the alluvial fan. is dammed up behind two storm­ of the section is similar to the At profile P24 (fig. 17), at the beach ridges. The third sandy area modern cobbles in the upper part south end of North Sandy Bay, is to the north of the center of the of the beach and is definitely iden­ the raised beach platform is much bay. This area, called "North tified as a gravel of the former narrower; it is the northernmost Sandy Bay," is backed by a cliff beach. Its topographic situation exposure where beach gravel is about 400 feet high, above which might make it seem to be a part of clearly resting on a nonstructural rise the main mountains of the the preearthquake beach; but the rock platform. The position of the island. There is also a small area of raised gravel is an integral part break of slope at the foot of the sand and gravel in the middle of of the section above it. ·when sea cliff can be located very accurately the central rock platform, where level dropped from its raised level here (figs. 20, 21). two small rivers debouch across to the preearthquake level or low­ LOW SEA LEVELS the platform. In several localities er, vegetation would have covered Possible evidence of a sea level between a sand beach and an area the former beach material, the slightly lower than the preearth­ of bare rock platform, a short sec­ cliff at the back of the beach quake level is shown by tree tion is covered by large boulders, would degrade, and the observed stumps, commonly found in the 2 to 10 feet in diameter, lying on sequence of peat and angular gra­ position of growth between pre­ bedrock; their undersurfaces are vel would be deposited. The date earthquake mean high water and angular but their upper surfaces of the change of sea level must the top of the preearthquake storm are smooth. This contrast bet>veen have been earlier than the deposi­ beach. These stumps all show some smoothed upper surfaces and tion of the lowest peat in the sec­ wave erosion. More substantial evi­ angular undersurfaces suggests tion, which has been dated by ra­ dence of a former low sea level that marine action has not been diocarbon as 2070±200 B.P. is found in peat layers in the banks powerful enough to overturn them (W-1770). of the Nellie Martin River. A during the period required to The position of profile P19 is continuous peat bed is exposed smooth their upper surfaces by shown in figure 17. Sections D and 1 along the banks in the lower course rubrasion and solution. Smaller D (fig. 19) are close to the bank 2 of the river. The top of the bed rounded cobbles, most of them as of a small river, which has exposed slopes down toward the sea; it can much as 6 inches in diameter, a 4-foot-thick bed of angular to be definitely identified as much as between the large boulders indi­ subrounded gravel in a finer ma­ 1.8 feet below the postearthquake cate that 6 inches is approximately trix overlying bedrock. Sections D 1 high-water mark and seems to con­ the maximum size of stone that the and D 2 are close together and have waves were competent to transport. been correlated on the basis of the tinue farther out to sea (fig. 22). The main coastal features of the position of the top of the angular Under present topographic condi­ bay and the positions of the 17 gravel bed. The gravel is thought tions such peat is found behind beach profiles and other key loca­ to be a former alluvial fan of the storm beaches, that is, about 12 tions (indicated by series Nos. P1- river, deposited while the sea \Vas feet above high-water mark. It 25) are shown in figure 17. at a higher level. In section D 1 the therefore seems probable that this level of the raised platform can be peat layer was formed at a time STRATIGRAPHIC EVIDENCE FOR clearly seen in bedrock, and a small when sea level was at least 14 feet SEA-LEVEL CHANGES step in bedrock is exposed at the lower than the postearthquake HIGH SEA LEVELS seaward edge of the vegetated level at this locality, or 25 feet be­ There is a continuous raised platform. Section F (fig. 19) is low the preearthquake level. A beach platform between Main close to this step, about 50 feet sample taken from 2 feet below the Sandy Bay and North Sandy Bay north of the river. In section F the top of the peat in section B (fig. that definitely represents a single material resting directly on bed­ 22) has a radiocarbon date of former beach. Three profiles rock is much more uniformly 600-+-200 B.P. (W-1766). E*ROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND

40 - preearthquake MHW

30 -

L WY Z SECTION 2 20 - P DEPTH, Vegetation: spruce al- forest and grass

Y> W2

o-% in. angular 10 -

-Angular gravel in clay, as above Preearthquake MHW Peat, as above 0 Angular gravel plus 10 percent rounded gravel in gray clay

pebbles as much as % in. diameter Base of exposed section I/ 0 100 200 300 DISTANCE, IN FEET VERTICAL EXAGGERATION X5

18.-Stratigraphic section and beach profile P23, Patton Bay. ALASKA EARTHQUARE, MARCH 2 7, 1964

X 50

C Y L Vertical cliff 5 f 40 I Yr 4 3 D xC 2 30 W W a(L

>W u -g 20 I- Height of preearthquake I storm beach across river W-I Y

10

0 0 100 200 300 HORIZONTAL DISTANCE, IN FEET

SECTION F SECTION DZ DEPTH DEPTH BELOW SURFACE, BELOW SURFACE. S SECTION D, I Vegetation. dry mature spruce forest DEPTH 0 0 egetation: grass, fern. BEL salmonberry, spruce, alder Peaty humus wrth some mrneral I 4 materlal at base 9 12 11 Brown-gray sllty sand with occasional 13 lenses of well-rounded grit 2 1 Iron-stalned organic clay contalnlng thin organic bands 22 Gray. mottled plastic clay with lenses of well-rounded 27 grit and occasional subangular to subrounded siltstone pebbles

Mainly rounded material from sand sue up to 2 in.. and some large (6-50 in.) well. rounded boulders not of local rock, in a matrix of silt and some clay Base of section

61 63

Bedrock

97 Base of sectton

121 Base of sectionzriver level

19.-Stratigraphic sections Dl, Ds, and F, and beach profile P19, Patton Bay. EROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND H25

20.-Cobbles of raised beach resting on truncated bedrock platform, profile P24, Patton Bay. r L

Obscured Fine-grained light-gray sandstone

0 10 / Preearthquake L____ L__~ 0 _ beach gravel NO VERTICAL EXAGGERATION ~ 21.-Stratigraphic section showing old raised beach at P24, Patton Bay. ALASKA EARTHQUAKE, MARCH 2 7, 1 9,64

SECTION B SECTION C DEPTH DEPTH BELOW SURFACE. BELOW SURFACE, IN FEET IN FEET

Interbedded beach sand and gravel lndlvldual beds Storm beach gravel about 10 In th~ck ~lX0ntlnuousstringers of small SECTION A wood fragments (swash debr~s) DEPTH Grayslltysand wlth small woody BELOW SURFACE fragments Gradat~onallower IN FEET contact Sharp upper contact 0 ,, Surface Peat contalntng wood fragments 16 '"' of all slzes, as much as 20 ~n In d~ameter all abraded Present rlver level

Z 8 2 15- 5 1 Preearthquake MHW - ---- Y 10 -

10 I I I I I I I 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 HORIZONTAL DISTANCE, IN FEET

22.-Stratigraphic sections and long proflle of Nellie Martin River.

It is concluded that there was an the peat layer, it is assumed that for this study were the break in earlier raised sea level, higher than relative sea level rose shortly after slope between the foot of a cliff the sea level just before the earth- this time. and a bedrock platform, the break quake, the exact elevation of ELEVATIONS OF BEACH in slope between a cliff and a which is discussed on pages H28 to FEATURES gravel beach, and the top of the H33. By 2070k200 B.P. the sea The sea level at which a raised storm beach ridge. level was low enough relative to beach was formed cannot be mea- The elevation of a break in slope the land for a layer of peat to have sured directly; fossil beach fea- between a cliff and a bedrock plat- grown upon the rock platform tures are among the best available form could be measured in only eroded at the higher sea level. A indicators of the former sea level. three localities. The elevations of period of low sea level followed The range of elevation of several the break in slope above the rele- during which the sea was low beach features, and the relation- vant (preearthquake) mean high enough for peat to form 1.8 feet ship of each to the high-water water, measured as the barnacle below the 1965 high-water mark mark, has been studied in Patton line, were f2.0 (P7), +0.6 (P4), near the Nellie Martin River. The Bay, mainly with reference to fea- and -0.8 (P5) feet. These figures topographic position of the pres- tures formed at the preearthquake suggest that, for the conditions ent peat layer behind storm sea level. From these data the reli- found on Montague Island, the beaches or higher indicates that ability of each feature as an indi- elevation of a break in slope in the sea was locally at least 14 feet cator of a former high-water mark bedrock is within 2 feet of mean lower than at present relative to has been assessed, and some con- high-water level and that this fea- the land. The sea was at this low clusions drawn about the causes ture is the best indicator of a for- level at 600-c-200B.P. Because the of variation in elevation for each mer sea level. dated sample was near the top of feature. The beach features chosen A gravel beach is a temporary E'ROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND H27

deposit whose thickness varies 21 feet above mean high water, only within +8 feet, so this fea- with time. A break in slope ;be- with a mean of 12.0 feet (fig. 23B). ture is also a poor indicator of sea tween a cliff and a gravel beach is These values are consistent with level. However, the difference in therefore likely to show a greater the idea that a gravel beach elevation between two mature par- range in elevation between local- banked against a cliff is in effect allel storm beaches should be an ities than one between a cliff and an incomplete storm beach ; there- accurate measure of -the elevation bedrock platform, and the lower fore, elevations of breaks in slope difference in sea levels when the limit of this range should be close in gravel beaches should be lower two ridges were formed. This dif- to high-water mark. Seven meas- than the elevations of storm ference has been used in calculat- ured elevations of this feature beaches. ing the elevation of the raised sea (fig. 2319) ranged from 0 to 16 feet In several locations a new storm level in Main Sandy Bay. above mean high water, with a beach is probably beginning to In MacLeod Harbor the level of mean value of +9.3 feet. At loca- form in response to the postearth- mean high water could be deter- tion P7 (fig. 17) the elevation of quake sea level. Seven measure- mined from the form of the beach the break in slope increased by as ments show that the ridges formed profile, but no satisfactory corre- much as 8 feet up small reentrants are as much as 6 feet above mean lation was found between beach in the cliffs. It might be expected high water (1965), with a mean form and water level in Patton that the break in slope of a cliff value of 4.1 feet (fig. 23A). The Bay (fig. 24). The gradienk of the with a gravel beach would give the present size and position of the beach, the depth from high-water most reliable evidence of high-wa- ridges illustrates the initial stages mark to the seaward edge of the ter elevation ; however, the pos- in the formation of a mature beach gravel, and grain size of the sible error of +8 feet shows that storm ridge. beach material were all found to this feature is not a reliable indi- The height of the top of a storm be of little use for the precise loca- cator of this altitude. beach is dependent upon wave and tion of sea level. In MacLeod Har- The difference in elevation be- shore environment, but it will ap- bor a consistent relationship was tween the top of the preearthquake proach a maximum elevation found between the preearthquake storm beach and the preearth- which is stable for its own envi- water level and the steep seaward quake mean high-water level was ronment. Mean high-water level edge of the gravel beach (fig. 14), measured at 11 points in Patton can be determined from the deva- but such a step is only to be ex- Bay. The values ranged from 6 to tion of the top of a storm beach pected where the gradient of the

23.-Distribution of elevations of beach features; A, above postearthquake mean high water, of postearthquake storm beaches; B, above preearthquake mean high water, of preemthquake &om beaches; C, above preearthquake mean high water, of the bredlk in slope at heads of beachea

Heights of break In slope near 1 p7 1 es in MacLeod-

NUMBER OF MEASUREMENTS IN EACH HEIGHT RANGE H28 ALASKA EARTHQVAiKE, MARCH 2 7, 19 64

1 FEET 20 1

%.-Beach profiles in Patton Bay in relation to the preearthquake mean high water, showing that the shape of the profile is not a reliable indicator of the po&ion of the mean high water. See figure 17 for location of profiles. seaward-building gravel is ap- shape of the underlying bedrock, the two sea levels at which they preciably lower than the angle of the amount of gravel available, were formed. the rock platform on which it is and random factors probably con- TECTONIC DEFORMATION OF built. The evidence from MacLeod trol the height to which the gravel BEACHES Harbor (p. H19) suggests that is accumulated. At one extreme E1t;vations of beach features these crolnditions are found only there is no gravel, and at the other were measured at many localities in areas well sheltered from wave extreme a full storm beach is over a 10-mile stretch of coastline atback. Because the whole of Pat- formed against the base of the in Patton Bay. The elevations of ton Bay is more exposed than any cliff. The elevation of the break in storm beaches, high-water marks, of the hchesin MacLsod Harbor, slope between the gravel and the and breaks in slope were measured no steps similar to those in Mac- cliff therefore varies from near using the preearthquake high-ma- hod Harbor are to be expected in high-water mark to full storm- ter mark as a standard. In figure Pakton Bay. beach elevations. Sueh breaks in 25, each symbol represents a mea- In summary, where a bedrock slope are not accurate indicators of sured point, and the lines show the platform against the foot of a cliff former water levels. interpolated elevations of each fea- is not covered by unconsolidated Storm-beach elevations increase ture between measured pints, so material, then the elevation of the with time to a maximum, which each line shows the change of ele- wave-cut break in slope is consid- takes several years to reach. This vation of a beach feature along the ered to be the best 'available topo- maximum elevation is only con- coast, in relation to the preearth- graphic indicator of mean high stant for a given wave-exposure water, and the difference in eleva- environment. Storm-beach eleva- quake high-water mark. The figure tion between two breaks in slope tions are therefore poor indicators shows the elevations of pre- and formed at different sea levels is an of the absolute water level, but the postearthquake shoreline features excellent measure of the difference difference in elevation between two and of raised shoreline features between the two sea levels. mature parallel storm beaches and the variation of height of these Where unconsolidated material should provide a good measure of features along the coastline of Pat- lies at the foot of a seacliff, the the difference of elevation between ton Bay. All elevations were mea- BROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND H29

60 EXPLANATION Li .G @ W preearthquake break in slope Crest of postearthquake storm LL between foot of cliff and beach z gravel (G)or bedrock (6) 0 Old raised beach MHW A X 5 Crest of preearthquake storm Postearthquake M HW w beach

0

B Postearthquake storm beach x-x x-x @ 0 -x- -x- x-X-x

Main Sandy Bay North Sandy Bay Nellie Martin W A A m River *ma, W N m 4.-+ hrn m 0-N me V) n anNm 22: n c2 r; 222 22 nN I1 I I I I II I I lllll I II I I 40 1 I II I I I II 0 1 2 3 4 5 6 7 8 9 10 11 SOUTH DISTANCE ALONG SHORELINE. IN MILES NORTH

25.-Variation in differences of elevation among coastal features in Patton Bay. sured relative to the preearthquake earthquake mean high water) and River, close to the southern end of mean high water (which is taken the break in slope between the the storm beach (locs. P9 and P10, as the top of the former barnacle raised beach platform and the fig. 17). The continuity of the zone). Postearthquake mean high steep slope behind it (raised landward edge of the platform and water was estimated from U.S. mean high water). Figure 25 of the raised storm beach shows Coast and Geodetic Survey tide shows the tectonic deformation of that they represent a single sea tables. Where storm-beach crests the shoreline from the time of the level throughout their extent. and breaks in slope between earlier raised beach until just be- At Bay Mouth Bar an earlier beaches or rock platforms and the fore the 1964 earthquake, and also storm beach within 2 feet of the cliffs behind them are present, the deformation caused by the elevation of the younger ridge has their elevations are shown. 1964 earthquake. The deformation been preserved behind the younger Elevations are shown for fea- due to the 1964 earthquake is ridge. This abandoned beach was tures which correspond to pre- plotted in figure 17. These data presumably formed at a sea level and postearthquake sea levels and are in general agreement with the almost at the preearthquake level, also to a raised sea level. The ele- smaller scale map by Plafker but no positive basis mas found for vation of the raised mean high (1965, fig. 7), but they add detail correlating it with the raised beach water has been calculated in one for the Patton Bay area. deposits described above. of the following mays: (1) in The evidence for the raised sea Figure 17 shows not only the Main Sandy Bay, as the differ- level consists of: (1) a continu-ous distance over which the raised ence in elevations between the platform from just north of loca- beach can be detected but also the crest of the preearthquake storm tion P24 (fig. 17) to the north end 15-idth of the platform which has beach and the crest of the raised of Main Sandy Bay; (2) a con- has not yet been destroyed. At the storm beach, or (2) elsewhere in tinuous raised storm-beach ridge north end of the coastline studied, Patton Bay, as the difference in across Main Sandy Bay; and (3) the raised beach platform pad- elevation between the top of the a platform at two points immedi- ually narrows and finally disap- preearthquake barnacle zone (pre- ately south of the Nellie Martin pears in North Sandy Bay where H30 ALASKA EARTHQUAJKE, MARCH 27, 1964 the preearthquake seacliffs are sev­ greater than the amount of error up the inlets (p. H27), so the dif­ eral hundred feet high. The ab­ inherent in the measurements, so ferences in elevation between the sence of the raised beach in this the general pattern of the uplift break in slope on the raised plat­ section can be attributed to the can be accepted. For example, near form and the preearthquake high­ nature of the local rock, which the northern end of the raised water mark will tend to become offers very little resistance to ma­ beach (fig. 25), its elevation above progressively larger than the dif­ rine erosion ; a preearthquake rock the preearthquake mean high wa­ ferences between the two sea levels platform would have been eroded ter rises from 10.2 feet (location in the zone where the steep hillside by marine action and its surface P17) to 45 feet (profile P22) in a is farther from the shore. It is con­ coV'ered with sand and gravel, in distance of 1 mile. This deforma­ eluded that the true change of sea contrast to neighboring more re­ tion is too great to be dismissed as level is greater than that shown by sistant bare-rock pladorms. The an error of measurement, especi­ the storm beaches and less than lack of resistance of the rock is ally inasmuch as the pattern of that shown by the breaks of slope. also indicated by ( 1) the relatively warping in this area is confirmed An interpolated value of the sea­ gentle gradient of the cliffs ( 40°- by the elevations of breaks in slope level change is shown by the 500), (2) the tendency of the cliffs both in gravel and in bedrock. broken lines in figures 25, 26B, and to degrade by landslides, mud­ The relationship between the 27B. flows, and gullies rruther than by two measures of the uplift of the South of theNellie Martin River rockfalls 'and talus accumulrution raised beaches-(1) the preearth­ there are cliffs along the coast; no (this tendency indicat~ a high quake barnacle line to the break in certain evidence of a raised beach clay content in the rock or in its slope of the raised beach and ( 2) was found, but with elevation dif­ primary weathering derivatives), the preearthquake storm beach to ferences as little as 5 feet above and ( 3) the considerable amount the raised storm beach-can be the preearthquake level, evidence of erosion of the cliffs since the studied where the storm beaches would be very difficult to interpret earthquake (fig. 29, p. H35). abut against steep slopes at each in the absence of a continuous fea­ The coastline north of North end of Main Sandy Bay. Figure ture. Possible evidence exists in Sandy Bay was not examined in 26 shows the relationship at the the second storm-beach ridge at any detail. The south side of Box northern end of the bay and figure Bay Mouth Bar and at isolated Point (north of the area of fig. 17) 27 shows the relationship a,t the locations at the mouths of streams has continuous cliffs 50 to 100 feet southern end. Both figures seem to where the cliff is absent. However, high; here low raised platforms show that, in the area of overlap, the primary purpose of this inves­ apparently have not been pre­ the difference in height between tigation was not to locate possible served. The surface of the penin­ the two storm beaches tends to de­ raised beaches, but to use the evi­ sula is relatively low and flat at crease and the difference in height dence provided by a raised beach altitudes of 100 to 200 feet, but between the break in slope and the to support observations of changes there is too much relief for a raised barnacle line tends to increase. to the beaches and platforms up­ platform. It is therefore tenta­ Inasmuch as the storm ridges are lifted by the earthquake. tively assumed that, within the curved, the exposure to waves Comparison of the radiocarbon area studied, only in the central obviously will not be the same on dates for all samples from Pat­ part of Patton Bay was wave at­ each ridge at the points where a ton Bay beaches indicates that tack sufficiently attenuated and section line drawn at right angles observed data can be explained the rock sufficiently resistant to al­ to the ridge intersects them (figs. most simply by two relative sea­ low the preservation of a raised 26, 27). The landward, older ridge level changes before the 1964 beach platform. will have been closer to the hillside, earthquake. Each change has left Since measurements of the rela­ and therefore less exposed to abandoned shoreline features at tive heights of the raised beaches waves. As a result, the landward various elevations (fig. 25). Data are based on the elevations of ridge is lower for a given sea level, from elsewhere in Prince William breaks in slope and of storm and the difference in height Sound suggest that the 1,000 years beaches, it must be recognized, for between the ridges is smaller than before the earthquake was char­ the reasons discussed above, that the difference between the sea lev­ acterized by slow submergence of some measurements may be in els at which the ridges were the land ( Plafker and Rubin, error by 5 to 10 feet. Nevertheless, formed. Elevations of breaks of 1967), and the data from Monta­ the amount of the warping is much slope, on the other hand, increase gue Island are consistent with this EROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND

P14 Ra~sedstorm beach/preearthquake storm beach=15 1 ft

1 PVRaised storm beach/preearthquake storm beach=21.8 ft /

Cab

EXPLANATION f ,oJ r

MHW/break in slope=14.4 ft. Elevation difference between named feotures 0' I -"I I

Profile location and number :\ f x:.. x:.. I

# La U / t Raised storm beach/preearthquake storm beach=15.6 ft

0 200 400 600 800 FEET

30 - EXPLANATION X Sea-level change obtained from difference in elevation between preearthquake MHW and raised sea-level break In J slope o u%iOW ZJ". Sea-level change obtained from difference in elevation between top of preearth- quake storm beach and top of raised storm beach

u,. , sea-level change

B 2 -LO -2 -m b N a> b 3 b 0 ' 1 I1 I I I 1 1 0 1000 2000 3000 NORTH DISTANCE ALONG SHORELINE. IN FEET SOUTH

26.-Relationship between measures of uplift of raised beaches at the north end of Main Sandy Bay. A, Diff-cea in elevation of raised break in slope, raised storm beach, preearthquake storm beach, and pmearthquake mean Mgh watar. B, Measurements; of sea-level change. ALASKA EARTHQUAKE, MARCH 2 7, 19,64

EXPLANATION X Sea-level change obtained from difference In elevation between preearthquake MHW and raised sea-level break in 113 W slope 0 Sea-level change obtained from difference in elevation between top of preearth- Assumed actual sawI Ka10 quake storm beach and top of raised --__ storm beach K n Onw -3

PI 1 P9 I _I 1000 2000 DISTANCE MEASURED ALONG PREEARTHQUAKE STORM BEACH, IN FEET

27.-Relationship between measures of uplift of raised beaches at the south end of Main Sandy Bay. A, Sketch showing difference in elevation of raised break in slope, raised storm beach, preearthquake storm beach, and preearthquake mean high water. B, Measurements of sea-level change. EROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND H33

Sea level Location of seem improbable-then the cliff Date relative to C-14 sample C-14 sample Basis for estimating mean high preearthquake (fig. 17) water obviously will be completely ob­ level (feet) scured by its own talus in no more than a century or two. Sometime before 2070 B.P. +8} W-1770 P23 Raised beach peat. By 2070 B.P ______OBLITERATION BY LANDSLIDES AND 600 B.P ______~-6 GULLYING ~-25 W-1776 P11 Nellie Martin peat. Preearthquake A.D. 0 ------P11 Barnacle line. In North Sandy Bay near pro­ 1964. Postearthquake A.D. -11 ------P11 Estimate from U.S. file P24 (fig. 1'7) , landslides and 1965. Coast and Geodetic gullying seem to have been more Survey tide tables. important than simple talus ac­ cumulation in obliterating the interpretation. If the central value scured 85 percent of the break of break in slope at the foot of the for each radiocarbon date is used, slope at the foot of a vertical cliff. cliff. A horizontal distance of then the sequence of events near The distribution of fallen debris 2,940 feet of cliff was studied in the mouth of the Nellie Martin is shown in figure 28. Some of this ·detail, and the distribution of fall­ River seems to have been as shown material no doubt fell rut the time en material is shown in figure 29. in the table. of the earthquake, but the major In this total distance, 22 percent These dates are consistent with part probably has accumulated of the break in slope is virtually relative sea-level changes at the since the earthquake; the sound of unobscured by debris, 17 percent same time ·and in the same direc­ falling stones was still frequently of the break in slope is obscured by tion on both sides of the island, heard in 1965. alluvial fan material, and 61 per­ although the amount of vertical cent of the break in slope is ob­ movement varies considerably The volume of talus material at scured by landslide material. It from place to place. No clear evi­ profile P3 is estimated to be about was evident that much of the dence of other sea-level changes 210,000 cu ft. The cliff from which movement postdates the earth­ was found in the area studied. it is derived is about 110 feet high and 580 feet long. The talus ac­ quake; the stream in one gully was OBLITERATION OF BREAK IN cumulation therefore represents a very rapidly transporting debris SLOPE AT TOP OF BEACH mean cliff retreat of 2.4 feet in while studies were being made in One immediate effect of the 15 months (to July 1965), assum­ July 1965, and material in some of 1964 earthquake was to initirute ing a 30-percent porosity in the the slides was still very wet and many landslides along the coast­ talus. At this rate, if the total sticky. The extent of postearth­ line, but much mass wasting of the accumulation postdates the earth­ quake erosion is also shown by the former cliffs has taken place since quake and if the observed maxi­ very marked increase between the earthquake. The break in mum angle of rest of 22° is May and August 1964 in fresh slope at the top of the former correct (fig. 28), the cliff will be landslide scars that appeared on beach is being rapidly obliterated completely regraded in 71 years. air photographs taken by the U.S. by several processes-talus •accum• If any of the present accumulation Coast and Geodetic Survey at ulation, landsliding, gullying, and occurred at the time of the earth­ those times. The estimated vol­ vegetation growth. The uplift has quake, then the time required for umes of material transported are prevented the sea from removing the cliff at profile P3 to become 6'70,000 cu ft in alluvial fans and mass waste from the base of the completely covered by its own 370,000 cu ft in landslides. The av­ cliffs, so all the material that has talus will be as follows: erage height of the cliff is 400 feet; been eroded from the cliffs since Accumulation Time required so the mean distance of cliff re­ the earthquake can be measured. at time of for total degrada­ earthquake tion of cliff treat since the earthquake has been From these measurements the (percent) (years) 0.90 foot in 15 months. rates at which the different slope 0 71 Landslides are far more effec­ processes were operating were 40 117 60 176 tive than gullying in degrading determined. 80 350 the cliff, because they act along OBLITERATION BY TALUS Unless almost all of the accumu­ its whole length. If landslides ACCUMULATION lation occurred at the time of the alone are taken into account, and At profile P3 (fig. 17), talus ac­ earthquake-and the continued if all the volume of landslide de­ cumulations have already ob- falling of material makes this bris has accumulated since the ALASKA EARTHQUm, MARCH 27, 196.4

EXPLANATION

21 " 50' Angle of scree slope

0 50 100 FEET L I I

28.-Degrading of vertical cliffs and accumulation of talus at prole P3, Patton Bay. BROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND

EXPLANATION

29.-Sketch showing degrading of cliff by landslides and gullying north of location P24 in North Sandy Bay. earthquake, then the time required slope may help to explain some cliff, but a low step leading to an- for total degradation of the cliff apparent anomalies of raised- other raised beach, the junction be- would be 2,300 years. However, the beach preservation in other areas. tween the two beaches can be gradient of the vegetated steep For example, rapid accumulation hidden by beach deposits and the slope behind the raised beach is of talus may destroy topographic growth of vegetation. It seems rarely less than 38", so it is un- evidence of a raised beach within likely that this obliteration will likely that the process of degrada- a short period, but it is this same occur extensively at the junction tion will proceed very far. At an talus cover which protects the between the preearthquake beach early stage, vegetation will estab- raised beach from subsequent and the raised beach, for example lish itself on the slope, and further weathering and erosion and so at the section shown in figure 19. degradation will be exceedingly over the long term preserves the Where the raised platform is well slow, except perhaps locally along beach in the stratigraphic section. developed, the topographic step streams or gullies. between the raised platform and The striking difference in be- OBLITERATION BY VEGETATION the preearthquake beach is nearly havior between the resistant verti- Where the upper limit of a always low, being nowhere more cal cliff and the nonresistant 50"- raised beach is not a well-defined than a 10-foot step, even though H36 ALASKA EARTHQUAIRE, MARCH 27, 1986.4 The development of the pre- earthquake platform at the ex- pense of the raised platform has been in progress only during the period that relative sea level was rising to preearthquake level, that is, since 6002200 B.P. or earlier. The probable sequence of sea levels and platform cutting is illustrated schematically in figure 31. 30.-Idealized profile through two raised beaches, with soil and vegetation omitted. Figure 3123 shows two of the Step between two rock platforms indicated by A. possible extreme cases if the gra- the difference between the two sea levels may have been 40 feet or more. Figure 30 shows such an idealized profile, with soil and veg- etation layers omitted. Most of the topographic step between the two Raised sea level rock platforms, as visualized in figure 29, is obscured by beach de- posits, so the surface step is very low; a vegetation cover will soon make it extremely difficult to dis- tinguish between the two surfaces by topographic form alone. Two valid criteria may still be used:

(1) there should be a marked sub- Regraded cross section= h*x surface step in the bedrock at A of figure 31; and (2) if datable soil layers (for example peat) t h continue to form on the abandoned Preearthquake sea level beaches of Montague Island as they have in the past, then an age discontinuity should occur in the lowest layers at A.

REGRADING OF ROCK PLATFORMS The rock platforms accordant with the preearthquake sea level appear to have been formed by the regrading of a preexisting plat- form accordant with the raised- beach sea level. Regrading is well marked near location P21 (fig. 17) where many bedrock knobs on the upper part of the preearthquake platform stand as outliers, clearly accordant with the raised-beach platform, and in places sur- 31.-Probable sequence of platform cutting in Patton Bay. A, Cutting of raised platform. B, Cutting of main platform during a time af rising sea level: case 1, rounded by preearthquake beach main platform at same gradient as raised platform; case 2, main platform at lower deposits. gnadient than raised platform. EROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND H37 dients of the raised platform, main TABLE2.-Relationship between change of sea level (h), width of platform regraded (x), and cross-sectional area of material removed (A = h.x) platform, and initial land slope are allowed to vary; it may be seen h, in feet (raised z in feet (base of Site (flg. 17) level to preearth- iaised beach to A-h.z) in square that the cross-sectional areas re- quake level) postearthquake feet mean high water) moved in forming the main plat- form range from a minimum of P24- _ _ ------43 400 17,200 lhhx to a maximum of hx, where P23_------40 420 16,800 P22-----_---_------45 400 18, 000 h is the difference between the P21--_------34 350 11,900 P20------21 320 6,500 elevations of the raised beach PlQ------_------23 500 11,500 notch and the preearthquake notch ------17 550 9, 200 and x is the width of the main P17--_-_------_--- 10 650 6, 500 ------13 450 5,800 platform, measured at right angles P16------_-- 12 550 6,600 to the shoreline. P9----__----_-----_-- 6 750 4,500 In theory, the rate of regrading would depend on the resistance of the rock and on the thickness of moved by regrading (A),to the rock to be removed and would de- depth through which regrading crease as either faotor increases. has had to work. 1.t can be ex- The mean rate of regrading can Rock resistance is an unknown pressed by the best-fit variable, but the fall of relative relationship (fig. 32) be found by dividing these two sea level from the raised level to equations by the period during the preearthquake level is a meas- which relative sea level rose to the ure of the thickness of the mate- preearthquake level. Relative sea rial to be removed ; it is compared Dividing this expression through level was already rising at the time with platform width in table 2. by h, the approximate width of of formation of the low sea-level Figure 32 shows the relation- the rock platform is given by the peat, dated at 60+200 B.P. (p. ship of the cross-sectional area re- relationship H33). The rise of relative sea level had probably begun before the deposition of sample W-1768 in MacLeod Harbor at 800+200 B.P. The period of platform cut- ting has therefore been taken as 1,000 years (T), and the estimated mean rate of removal of material has been A/T=2.0+0.36h sq ft per ymr k30 percent. The mean rate of widening of the platform has similarly been x/T=0.36+2.0/h ft per year +-30 percent. These empirical equations can be compared with two limiting ideal situations, the first a constant rate of removal of material, and the second a constant rate of lateral cutting. The first situation typi- CHANGE OF SEA LEVEL (h).IN FEET cally would exist where the cliff is 32.-Relationship between change of sea level and crass-sectional area of material soft and is easily eroded, or the eroded during regrading of raised bedrock platform, Patton Bay. Points are plotted from data in table 2. cliff is very high; the process is IASKA EARTHQU-, MARCH 27, 19'6 limited by the capacity of the sea The upper limit of the future had been regraded by fluvial ac- to carry away the eroded material. marine erosion and deposition of tion; just above the postearth- In the notation of the equations sand and gravel seems to be limited quake storm beach 33 percent had above, this situation gives: by the final storm-beach height, been regraded. The extent of re- A/T =constant, which should approximate the pre- grading depends on the position x/T a l/h. earthquake storm-beach height, of the streams flowing off the hill- The second situation is applicable that is, 8 to 16 feet above high- sides, and the lateral extent of re- to mistant cliffs where the limit- water level. The horizontal extent grading will increase as the ing factor is the power of the of beach resorting up the bay is streams change course on the waves to undercut the cliffs, and less clear. On a rock platform, the beach. The streams flowing into the transporting capacity of the shape of the platform in cross sec- North Sandy Bay are small and water is more than adequate: tion should determine the position drain very steep slopes. Regrad- of the storm beach. The beach pro- ing consists mainly of erosion of A/T a h files (fig. 24) indicate khat the base the beach materials, followed by x/T =constant. of the storm ridge forms from 4 to deposition of hillside material as Figure 32 shows that the data 6 feet below high-water level, or at alluvial fans, often above the for Patton Bay more closely fits about mean sea level. the second situation. The intercept original beach level. Main Sandy Bay is the best ex- of the area for zero net change of The overall course of regrading ample of a beach of unconsolidated elevation is smaller than the h- is illustrated by the beaches in material, where waves could dependent term and may be inter- MacLeod Harbor (profiles 3, 5, 8, change the position of mean sea preted as the amount of material fig. 13) and North Sandy Bay, and level in a horizontal direction by removed from a coast where the by the raised-beach section in Pat- erosion or deposition. Figure 33 sea returned to its original level shows the presumed form of the ton Bay (profiles P19, P23, fig. and merely formed a steeper off- raised beach before the preearth- 17). After a raised beach or rock shore platform. A sufficiently quake sea level and the position of platform that is backed by a steep great relative lowering of sea level the preearthquake storm beach for hillside has been elevated, zones of presumably would overload the profiles P11, P12, and P13. Wheth- marine and fluvial action are di- transporting capacity of the sys- er the effect of the sea-level change vided by a storm-beach ridge. On tem and thus would approximate was erosional (PI2 and P13) or a beach, the storm ridge is formed the first ideal situation, but SO depositional (Pll), the position of about 300 feet away from the old great a change was not observed in the new storm beach is immediate- storm ridge. On a rock platform, the field. ly at the foot of the old one, the the storm ridge is formed with its In Patton Bay the resistance of horizontal distance between the seaward base at about mean sea the rocks seems to have been the two storm beaches being almost main control of the erosion and re- level. This ridge remains until it is constant at 300 feet despite varia- grading of the rock platform to demolished by wave erosion of the its preearthquake level, and the tions in the amount of net relative rock, after which there will only transporting capacity of the waves sea-level change. The postearth- be a gravel beach banked against has been more than sufficient to quake storm beaches in profiles P1 a bedrock step as at A in figure 31. remove the debris produced. and P24 also are about 300 feet Below the storm-beach ridge a new from the old storm ridges. equilibrium beach or bedrock plat- REGRADING OF SAND AND GRAVEL In North Sandy Bay an esti- form will be formed by marine ac- mate was made of the percentage tion ; the exact profile of this ridge Where the beach is in unconsol- of the area regraded by fluvial ac- cannot be predicted (fig. 24). idated materials, the whole area of tion (even if later covered with Above the storm beach, infilling the beach below the crest of the alluvial fan deposits). This esti- by alluvial fan or colluvial mate- newly forming storm beach has apparently been regraded by ma- mate was made for the same sec- rial alternates with the develop- rine processes. It seems probable tion of beach as the estimates of ment of soil and peat until the that the vertical range of this re- obliteration of the break in slope streams are able to establish semi- grading will increase as storms of by landslides and gullying (p. permanent courses and equilib- greater magnitude build a higher H33). At the top of the beach, close rium gradients through the new storm-beach ridge. to the cliff, 24 percent of the area storm beach. HROSION AND DEPOSITION ON A RAISED BEACH, MONTAGUE ISLAND

Raised

k,nstormPreearthqua beaches ke /(13 Raised MHW for P12 and P13

20 0I 1 I I I I I 100 200 300 400 500 600 700 800 900 1000 HORIZONTAL DISTANCE. IN FEET

33.-Position of preearthquake storm beaches in relation to the presumed original surface of the raised beach. Profiles of original surface were obtained by transposing present profilee to raised high-water marks.

CONCLUSIONS

Although most of the results of long-term adjustment of the long grate upstream following a rela- studies on Montague Island have profile would be aggradation or tive fall in sea level. only local relevance, some conclu- headward recession of a knick- In addition to being an example sions may be drawn which have a point could not be determined, but of a dissected uplifted surface, the wider application. In MacLeod it is here suggested that the long- head of MacLeod Harbor is also Harbor, the 33-foot uplift of the term adjustment may depend on an example of an area where beach deposits and their subse- whether the new course is less fluvial processes are occurring so quent erosion provided a rare op- steep or steeper than the course fast that changes may be readily portunity to observe and measure above the former sea level. Thus, studied. A mean rate of net sedi- the actual effect of a change in sea in mountainous regions, where the ment removal of 1 cfs in a stream level on fluvial erosion. It was streams have relatively steep gra- of low-flow discharge of 300 cfs found that the streams flowing in dients, a fall in sea level might may be unusually rapid, but the sand and silt immediately cut a lead to either aggradation or fact that the erosion rate varies in course of almost uniform gradient erosion; there would not neces- direct proportion to low-flow dis- between the old and the new sea sarily be a relation between charge may have wide application, levels. This phase of the readjust- changes of sea level and the pres- inasmuch as it shows the effect of ment occurred within a matter of ence of knickpoints. In lowland discharge on the rate of erosion days for all streams flowing over regions, however, where the lower and the transport of uniform ma- unconsolidruted materials and hav- courses of rivers have very low terials. ing low-flow discharges greater gradients, it is anticipated that The relationships between the than about 1 cfs. Whether the knickpoints will develop and mi- preearthquake sea level and the H40 ALASKA EARTHQUAKE, MARCH 2 7, 19164 beach features associated with it tion of a platform at the old sea there is a rapid fluvial regrading have been very clearly exposed by level, and (2) a large uplift where of beach material near the mouths the uplift. Measurements over the lowered relative sea level ex- of streams and a general slow in- about 10 miles of coastline show posed a larger area of platform filling by alternate layers of vege- that the height of storm beaches and thus delayed its total destruc- tation and alluvial and colluvial varies from 6 to 22 feet above high- tion. The influence of the slope of d e p o s i t s. Larger streams cut water level, the mean being 12 feet. the former coast could not be through the storm beach and drain Where gravel was banked against clearly seen in the field, but part of the area behind it. Within bedrock cliffs at the top of a beach, broader platform remnants do a period estimated to be about 5 the height of the break in slope seem to be associated with some of years, areas of extensive deposits, between the gravel beach and the the areas of greater uplift. In Pat- such as the bay-head sediments of cliff varied from 2 to 15 feet above ton Bay, therefore, rock platforms MacLeod Harbor, will be regraded high-water level, the mean being seem to be best preserved in ztn area mainly by fluvial action before a 9 feet. Therefore, neither of these (1) slightly protected from wave new storm beach develops. features can be depended upon to attack, (2) where the rock is mod- The break in slope between the indicate the relative sea level with- erately resisbant and (3) where top of the beach and a cliff is al- in less than 5 to 10 feet unless the relatively great uplift has followed most completely obscured after a exposure and configuration of the the formation of the platform. few years unless the cliffs are par- shoreline is known in unusually The 1964 deformation has pro- ticularly resistant. Talus and land- great detail. vided information about the way slides may degrade some or all of The area studied was not large in which a raised beach is modified the cliff, unless the rocks weather enough to serve as the basis for after uplift. Rates of widening of to fine-grained material and are general deductions about where a rock platform by erosion have stabilized at an early stage by vege- raised beaches are best preserved, been calculated and have been tation. Where the top of the beach but some observations were made. shown to be roughly constant at is backed by another older beach, Gently sloping shores in unconsoli- 0.49 feet per year for cliffs of quite large (40 ft) differences in dated materials seem well suited to moderate height. The rate of reces- sea level were found to produce preservation of raised storm beach- sion of 0.7-2.0 feet per year for only low (5 ft) topographic steps. es. On rocky shores, such factors as cliffs on Montague Island should These steps could be so obscured rock resistance and wave exposure indicate the order of magnitude of by unconsolidated beach deposits tend to influence the rates of for- recession in other areas. Storm and vegetation that a series of mation and destruction of rock beaches form on old platforms raised beach levels might appear platforms in the same way, so there and their bases are roughly at on the surface to be one continuous is no reason to assume that these mean sea level ; they are converted slope. factors influence preservation. In to gravel banks against a step as Raised beaches formed at a Patton Bay, however, the raised the platform is eroded by the known date and having their ini- be~achplatform has in fact been waves. tial profiles almost perfectly pre- served are very rare, but they did best preserved where the rock Beaches of sand and gravel (other than bay-head deposits) exist on Montague Island iininedi- seems to be most resistant (al- seem to be regarded by marine ac- ately after the 1964 earthquake. though not highly resistant), and tion within 1 year, except for the Thus an ideal opportunity JYRS pro- the platform narrows dramatically upper part of the beach, where a vided to study the rates and proc- and disappears where the rock be- new storm beach forms about 300 esses of destruction of raised comes less resistant. Other factors feet seaward of the previous one, beaches. Such studies of presently which should theoretically aid provided the sea level drops at forming geomorphic features are beach preservation are (1) a low least 5 feet. The new storm beach an excellent basis for understand- gradient on the earlier shoreline, may take many years to attain its ing and identifying similar fea- which favors more rapid forma- full height. Behind a storm beach, tures formed in the past. E'ROSION AND DGPOSITION ON A RAISED BEACH, MONTAGUE ISLAND REFERENCES CITED

Barrett, P. J., 1966, Effects of the 1964 Plafker, George, and MacNeil, F. S., Plafker, George, and Rubin, Meyer, Alaska earthquake on some shal- 1966, Stratigraphic significance of 1967, Vertical tectonic displace- low-water sediments in Prince Wil- Tertiary fossils from the Orca ments in south-central Alaska dur- liam Sound, southeast Alaska : Group in the Prince William Sound ing and prior to the great 1964 Jour. Sed. Petrology, v. 36, no. 4, p. region, Alaska, in Geological Sur- earthquake : Jour. Geosciences 992-1006. vey research 1966: U.S. Geol. Sur- [Osaka City Univ.], v. 10, art. 1-7, Plafker, George, 1965, Tectonic defor- vey Prof. Paper 55@B, p. B62-B68. p. 1-14. mation associated with the 1964 Plafker, George, and hIayo, L. R., 1965, Sparks, B. W. 1960, Geomorphology: Alaska earthquake : Science, v. 148, London, England, Longmans, Green no. 3678, p. 1675-1687. Tectonic deformation, subaqueous slides, and destructive waves as- and Co., Ltd., 371 p. I'lafker, George, 1967, Surface faults Wolman, hI. G., 1954, A method of sam- on Montague Island associated with sociated with the Alaska March 27, the 1964 Alaska earthquake: U.S. 1964, earthquakean interim geo- ling coarse river-bed material : Am. Geol. Survey Prof. Paper 543-G, logic evaluation : U.S. Geol. Survey Geophys. Union Trans., v. 35, no. 42 P. open file report, 21 p. 6, P. 951-956.

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