HAROLD MEISLER U. S. Geological Survey, 100 North Cameron St., Harrisburg, Pa.
Origin of Erosional Surfaces
in the Lebanon Valley, Pennsylvania
Abstract: Summit elevations in the Lebanon creeks—in which streams and interfluvial areas Valley, part of the Great Valley, range from 440 were in a state of erosional equilibrium. The land to 720 feet above msl (mean sea level). This range surface in equilibrium with the ancestral Quit- cannot be accounted for adequately by the pene- tapahilla Creek lies at a higher elevation than plain concept. Although accordant summits, the adjacent land surfaces that were in equilibrium chief evidence for peneplains, occur over large with Swatara Creek. areas, summits are not accordant between adjacent The land surface on the carbonate rocks, which areas within the valley. is in the ancestral Quittapahilla Creek system, lies The Lebanon Valley is underlain in the south by at a lower elevation than shale within the same carbonate rocks and in the north by shale. The system, but it commonly lies at a higher elevation major stream valley in the carbonate area is now than shale in adjacent parts of the Swatara Creek partly occupied by segments of two streams, but system. at one time it was the location of one major stream Accordance of summits is the result of uniform —the ancestral Quittapahilla Creek—which was erosion of uniform rocks in basins whose discharge beheaded by a tributary to Swatara Creek. points are at the same elevation. Lack of accordant Landforms of the Lebanon Valley are probably summits on uniform rocks is the result of erosion the result of erosion within two separate stream in basins whose discharge points differ in elevation. systems—Swatara and ancestral Quittapahilla
CONTENTS Introduction . . . 1071 Geomorphic history 1080 Purpose and scope . . . 1071 Conclusions 1082 Area location . . . 1072 References cited 1082 Historical background . . . 1072 Methods of investigation . . . 1073 Figure Acknowledgments . . . 1073 1. Map of southeastern Pennsylvania showing lo- Geologic features . . . 1073 cation of Lebanon and Great valleys . . 1072 Erosional features . . . 1073 2. Map of Lebanon Valley showing geomorphic Description . . . 1073 subdivisions and drainage pattern .... 1074 General statement . . . 1073 3. Topographic profiles across Lebanon Valley 1075 Shale areas 1, 2, and 3 . . . 1075 4. Hypsometric curves of areas within Lebanon Shale area 4 . . . 1076 Valley 1077 Carbonate area . . . 1076 5. Stages in development of drainage pattern in Hypsometric curves . . . 1076 Lebanon Valley 1081 Origin . . . 1076 General statement . . . 1076 Table Peneplain concept . . . 1077 1. Elevations of shale areas 1075 Erosional-equilibrium concepts ...... 1078 2. Elevations of carbonate area 1076
INTRODUCTION bonate rocks on the south and the shale on the north sides of the valley. The difference in ele- Purpose and Scope vation is commonly attributed to one of two Classical interpretations of the geomorphol- causes: (1) the existence of two peneplains—the ogy of the Lebanon Valley have been concerned Harrisburg peneplain, which developed on chiefly with describing real or supposed pene- shale, and the younger and lower Somerville plain levels within the valley and explaining peneplain, which developed on limestone, or the difference in elevation between the car- (2) the existence of only one peneplain, the
Geological Society of America Bulletin, v. 73, p. 1071-1082, 5 figs., September 1962 1071
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Harrisburg, which was lowered differentially on physiographic province in Pennsylvania, few the soluble limestone. detailed studies have been made of the Lebanon The author believes that peneplains never Valley. Papers on the Lebanon Valley have developed in the Lebanon Valley. The major been concerned chiefly with the identification geomorphic features of the valley are inherited of peneplains and the levels of accordant ele- from a time before the capture by Swatara vations. Creek of Quittapahilla Creek when the streams Campbell (1903, p. 287) thought that the and interfluves were in equilibrium in each of meanders of Swatara Creek indicated a wide- these two basins. There is no evidence that the spread peneplain developed on the shale in the carbonate rocks have been eroded significantly Great Valley near Harrisburg. Presumably this lower than the shale. surface was younger than the Schooley pene- plain and more extensive than the Somerville Area Location peneplain. He named it the Harrisburg pene- The Lebanon Valley is defined in this report plain. In the Lebanon Valley it ranges from 500 as the part of the Great Valley in southeastern to 600 feet above msl. Campbell (p. 283-28
EXPLANATION
Great Valley Lebanon valley Figure 1. Map of southeastern Pennsylvania showing location of Lebanon and Great valleys
Pennsylvania that lies within the drainage area also thought a lower peneplain, the Somerville, of Swatara Creek. It includes most of the drain- was developed on limestone near the major age area of the Susquehanna River in the Great streams at an elevation of 400 feet and stated Valley east of the Susquehanna River (Fig. 1). that in the Lebanon Valley the Somerville plain The Lebanon Valley is bounded on the north was slightly developed along Swatara Creek. by Blue Mountain, a ridge formed by the re- Knopf (1924, p. 658) recognized remnants of sistant Tuscarora Sandstone of Silurian age, and two erosion surfaces on the Martinsburg Shale on the south by a series of hills formed by of Ordovician age north and east of Harrisburg. Triassic conglomerate and diabase. The valley One of these, at 520 feet above msl, is the Har- is approximately 30 miles long. Its width ranges risburg peneplain. The other, consisting of from 14 miles near Lebanon to 9 miles near isolated monadnocks at 660 feet, she called the Hummelstown. Sunbury peneplain. The lowlands below the Harrisburg peneplain were called the Lancaster Historical Background subcycle rather than the Somerville peneplain, Although a great deal has been written about because the Somerville includes several different the geomorphology of the Valley and Ridge surfaces (Knopf, 1924).
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Hickok (1933, p. 114) believed that the the topographic maps and the elevations of over Martinsburg Shale is slightly more resistant to 2000 points were interpolated, from 20-foot erosion than the carbonate rocks so that contour lines, to the nearest 10 feet. erosion surfaces generally occur at lower eleva- tions on the carbonate rocks than on the shale. ACKNOWLEDGMENTS Three extensively developed erosion surfaces Appreciation is expressed to Mr. John T. are preserved on the Martinsburg Shale in the Hack of the U. S. Geological Survey and Mr. Lebanon Valley, at 500, 550, and 600 feet Alan R. Geyer of the Pennsylvania Geological above msl. A few monadnocks at 660 and 700 Survey for critical reading of the manuscript. feet suggest additional older erosion surfaces. Other surfaces developed on the Martinsburg GEOLOGIC FEATURES (Hickok, p. 115) are terraces at 360, 400, and The southern part of the Lebanon Valley is 450 feet above msl. underlain by alternating beds of Cambrian and Ashley (1935, p. 1398) maintained that the Ordovician limestone and dolomite, and the present topographic surface in the Appalachian northern part is underlain by the Martinsburg region reflects a single old peneplain (Schooley Shale of Ordovician age. The Martinsburg con- peneplain) that has been differentially lowered. sists chiefly of shale, but it contains minor Ver Steeg (1942), in answer to Ashley, sug- amounts of limestone, sandstone, andesite lava, gested that the widespread, broad, flatfish and diabase. remnants of the Harrisburg peneplain could not The rocks in the valley strike generally east- have been so excellently preserved if the sur- northeastward, and progressively younger for- face had been greatly lowered as Ashley claimed. mations crop out northward. The oldest and Bethune (1948, p. 17) stated that the summits southernmost band of rocks consists of lime- underlain by Martinsburg Shale represent stone and dolomite of the Conococheaque lowered traces of the Schooley peneplain. Limestone of Late Cambrian age. The Beek- Fenneman (1938, p. 234) thought that in the mantown Group of Ordovician age, consisting Lebanon Valley the surface on the Martinsburg of limestone and dolomite, crops out north of Shale was 100 feet above that of the limestone. the Conococheaque. Cropping out northward He believed there was at least one peneplain in from the Beekmantown Group, in order of de- the valley, as indicated by the level of the shale creasing age, are the Annville, Myerstown, and hilltops, but acknowledged that the lower sur- Hershcy Limestones of Prouty (1959) of face on the limestone was commonly consid- Middle Ordovician age, and the Martinsburg ered by some writers to be a later (Somerville) Shale of Late Ordovician age. peneplain. Because the sequence of formations is over- Macar (1955, p. 259) conceded that the for- turned, individual beds generally dip to the mation of several different levels on the shale south, and the older formations overlie the may possibly have been caused by differential younger ones. These rocks appear to be part of lowering of a single surface. However, he indi- the overturned south limb of a recumbent cated that this hypothesis would not be ap- synclinorium (Gray and others, 1958). plicable on a large scale and was convinced that there were more levels than the Somerville and EROSIONAL FEATURES Harrisburg peneplains. He cited levels at 400- 440, 500-520, and 600 feet above msl as ex- Description amples. General statement. The Lebanon Valley has been divided for the purpose of study and Methods of Investigation analysis into six areas called shale areas 1, 2, 3, The geology of the area was determined 4a, and 4b, and the carbonate area (Fig. 2). principally from published geologic maps, but Shale area 1 consists of the drainage basins some field checking of the geology was done by north of Swatara Creek and west of Little the author. Topography and stream patterns Swatara Creek. Shale area 2 consists of the were determined from 73^-minute quadrangle drainage basins north of Little Swatara Creek, maps (scale—1:24,000). east of Swatara Creek, and downstream from Elevations of summits were obtained from the junction of Little Swatara and Crcsskill the highest contour line on the summits. To creeks. Shale area 3 consists of the Little determine mean elevations of topographic sur- Swatara Creek basin upstream from the junc- faces, a 1-inch grid system was constructed on tion of Little Swatara and Crosskill creeks.
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Shale areas 1, 2, and 3 form a continuous longi- TABLE 1. ELEVATIONS OF SHALE AREAS, IN tudinal strip of the Lebanon Valley in which FEET ABOVE MEAN SEA LEVEL area 1 is farthest downstream and area 3 is farthest upstream. Shale area 4 is south of Predominant Mean summit Lowest Swatara and Little Swatara creeks, below the Area elevation elevations elevation junction of Little Swatara and Crosskill creeks. It has been divided into areas 4a and 4b. Shale Shale area 1 : 460 500 520 540 320 area 4a lies south of shale area 1, shale area 4b Manada Creek basin 460 500 520 540 320 lies south of shale area 2. Bow Creek basin 450 520 540 330 Webster School Shale areas I, 2, and 3. Table 1 gives mean, basin 460 500 520 540 340 predominant summit and lowest elevations of Vesle Run basin 460 520 540 360 shale areas 1, 2, and 3 and of the basins which Reeds Creek basin 450 500 520 540 370 compose these areas. Mean and summit eleva- Swatara Creek basin (western half) 460 520 540 390 tions of the basins within shale area 1 are almost Shale area 2: 500 560 580 390 identical despite the fact that basin-discharge Swatara Creek basin points (lowest elevations) are progressively (eastern half) 490 560 580 390 lower westward. Elizabeth Run basin 500 560 580 400 Earlakill Run basin 500 560 420 In shale area 2, predominant summit eleva- Crosskill Creek basin 520 560 580 440 tions and mean basin elevations do not vary Shale area 3: 550 580-720 440 greatly from basin to basin. There is only a 30- Shale area 4a: 490 560-620 320 foot difference in mean elevations between the Shale area 4b: — 600-720 390 easternmost and westernmost basins. Further- more there is no westward lowering of summits. Shale area 3 has a more varied topography The difference in elevation of the discharge than areas 1 and 2. Shale summits are 580 feet points of the easternmost and westernmost above msl along the northwestern boundary, basins is 50 feet. There is an abrupt 40-foot in- 620 to 720 feet above msl along the southern crease in both mean basin elevation and summit and southwestern boundaries, and 620 feet elevation from shale area 1 to shale area 2. above msl along the eastern boundary. Within
a i " 1200- -1200 c £ 1000 - -1000 | 800- -800 » 600- -600 c c 400 - -400 1 200- -200 Figure 3. Topographic profiles across Lebanon Valley, southeastern Pennsylvania; locations of profiles A-A' and B-B' shown in Figure 2 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/9/1071/3442172/i0016-7606-73-9-1071.pdf by guest on 28 September 2021 1076 HAROLD MEISLER—EROSION SURFACES, LEBANON VALLEY, PA. the area, summit elevations are 580-600 feet Hypsometric curves. Hypsometric curves of above msl in the northwest, 600-660 feet above shale areas 1, 2, 3, 4a, and the carbonate area msl in the south, and 620 feet above msl in the are shown in Figure 4. A hypsometric curve east. shows the distribution of land-surface area Shale area 4. The topography of shale area with respect to elevation. Elevation of the land 4 is more complex than that of areas 1, 2, and surface in feet above mean sea level is plotted 3 and is further complicated by the occurrence on the vertical axis. The percentage of the total of such resistant rocks as andesite lava, diabase, area lying at or above a given elevation is and sandstone. The elevations of unusually high plotted on the horizontal axis. ridges developed upon these resistant rocks have been omitted from calculations of mean TABLE 2. ELEVATIONS OF CARBONATE AREA, IN elevations and have not been considered as part FEET ABOVE MEAN SEA LEVEL of shale erosional surfaces. Summits in shale area 4 are higher than sum- Predominant Mean summit Lowest mits in areas 1 and 2, despite the fact that most Area elevation elevations elevation of them are closer to Swatara Creek than those in areas 1 and 2. These relations are shown in Carbonate area: 480 440 - 620 320 the profiles across the Lebanon Valley (Fig. 3). Spring Creek basin 430 440 - 500 320 Although the lithology of the ridges is the Killinger Creek basin 460 500 - 560 360 same in both areas, the summit elevations in Bachman Creek shale area 4a descend westward from 620 feet basin 500 560 580 380 above msl in the east to 560 feet above msl in Beck Creek basin 490 580 600 400 the west; whereas those in shale area 1 range Snitz Creek basin 530 600 420 Veterans Hospital from 500 to 540 feet above msl. The mean ele- basin 540 600 620 450 vation of shale area 4a is 20 feet higher than that of shale area 1. The two shale areas are perhaps compared more accurately if the coun- Hypsometric curves of shale areas 1, 2, and 3 try immediately adjacent to Swatara Creek is are almost identical in shape and for most of omitted from area 4a. If this is done the mean their extent are separated by fairly constant elevation of area 4a is 30 feet higher than that elevation differences. The curve of area 3 is of shale area 1. In shale area 4a the highest approximately 45 feet above that of area 2, elevations occur along the divide between which is about 45 feet above that of area 1. Swatara Creek and the two major streams of These differences correspond closely to the the carbonate area—Quittapahilla and Spring mean-elevation differences of shale areas 1, 2, creeks. This divide is considerably closer to the and 3; the mean elevation of area 3 is 50 feet carbonate area streams than to Swatara Creek higher than that of area 2, which is 40 feet and forms the ridge immediately adjacent to higher than that of area 1. the main carbonate valley. The juxtaposition Although shale area 4a is adjacent to and as of the highest shale ridge and the lowest car- far down-valley as shale area 1, most of its bonate-rock valley leads to the erroneous im- hypsometric curve is well above that of area 1. pression that the carbonate rocks were eroded To the right of the 95-per cent point, the curve below the shale (Fig. 3). of area 4a coincides with that of area 1. In shale area 4b, summit elevations along the divide between Swatara and Quittapahilla Origin creeks descend westward from 720 feet above General statement. A theory of erosional de- msl in the east to 600 feet above msl in the velopment of the Lebanon Valley should ex- west; in shale area 2 they range from 560 to 580 plain the following features of the land surface: feet above msl. In shale area 4b there are many (1) the accordance of summits and uniformity high ridges of resistant rock, but the cited ele- of mean basin elevations in area 1, (2) the ac- vations along the divide are on shale. cordance of summits in shale area 2, (3) the Carbonate area. Table 2 shows elevations in abrupt 40-foot drop in mean and summit eleva- the carbonate area. Accordant summits are tions from area 1 to area 2, (4) the wide varia- absent, and mean and summit elevations of tion in summit elevations (from 580 to 720 feet) drainage basins within the carbonate area in area 3, (5) the fact that the elevations of descend westward from Veterans Hospital basin summits are higher in areas 4a and 4b than in to Spring Creek basin. areas 1 and 2, (6) the westward descent of sum- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/9/1071/3442172/i0016-7606-73-9-1071.pdf by guest on 28 September 2021 EROSIONAL FEATURES 1077 mils and their lack of accordance in shale areas are present—the 540-foot level in area 1 repre- 4a and 4b, (7) the higher elevations of summits senting a later, partial peneplain. The pene- in the carbonate area than in areas 1 and 2, (8) plains were dissected in a subsequent erosion the westward slope of the carbonate area and cycle, and the present entrenchment of Swatara its lack of accordant summits. Creek in its flood plains indicates an even later Peneplain concept. According to the pene- erosion cycle. plain concept, the accordance of summits at The variation of summit elevations from 580 E 550 « 500 10 20 30 10 50 60 70 80 90 ' Percentage of total area at or above a given elevation Figure 4. Hypsometric curves of areas within Lebanon Valley, southeastern Pennsylvania 540 feet above msl in shale area 1 is the result to 720 feet above msl in shale area 3 presents a of peneplanation. The uniformity of mean basin problem. A descent from 720 to 580 feet above elevations in shale area 1 is the result of dissec- msl in only a few miles is too steep a slope for tion of the peneplain, which has proceeded to a peneplain that maintains an extensive 580- the same degree in each basin in area 1. The foot surface just a few miles farther down- accordance of summits at 580 feet above msl in stream. Differential erosion upon rocks of vary- shale area 2 is also the result of peneplanation. ing resistance to erosion is not a satisfactory ex- If the summits of areas 1 and 2 descended planation of the difference in summit eleva- gradually westward from 580 to 540 feet above tions, as the elevations cited are all on rocks of msl, one gently sloping peneplain would suffice virtually the same character. The 720-foot ele- for both areas. However, the abrupt 40-foot vation, therefore, must represent the remnants change of mean and summit elevations from of an older erosional surface peneplain. Thus, area 1 to area 2 indicates that two peneplains in order to explain the topography using the Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/9/1071/3442172/i0016-7606-73-9-1071.pdf by guest on 28 September 2021 1078 HAROLD MEISLER—EROSION SURFACES, LEBANON VALLEY, PA. peneplain concept it is necessary to postulate of interrupted erosion cycles, have been five erosion cycles, three of which resulted in seriously doubted by some geologists. Hack peneplains or partial peneplains. (1960, p. 80) states, "... it is unlikely that a That summit elevations are higher in shale landscape could evolve as indicated by the areas 4a and 4b than in areas 1 and 2 is difficult theory of the geographic cycle." to explain by the peneplain concept. The higher Hack uses as an alternative approach to land- summits may be remnants of the oldest pene- scape interpretation, the principle of dynamic plain in the area, also represented by the high- equilibrium, according to which "... every est elevations in shale area 3. Shale areas 4a and slope and every channel in an erosional system 4b, however, are as far down-valley as areas 1 is adjusted to every other. When the topog- and 2, and it seems unlikely that the highest raphy is in equilibrium and erosional energy re- and oldest peneplain would be preserved here mains the same all elements of the topography and yet be completely destroyed in areas 1 and are downwasting at the same rate." According 2. The explanation seems even more unlikely to him (p. 85) the number of former old-age when one considers that four subsequent surfaces that have been dissected to Davis' partial-erosion cycles must have occurred and stage of maturity is so great and the number of that the higher elevations in shale areas 4a and old-age surfaces is so small ("virtually non- 4b occur near the center of the valley—a loca- existent") that the end product of erosion ap- tion not favorable for the preservation of pene- pears to be a "maturely dissected" surface plain remnants. The westward descent of sum- rather than a peneplain. Hack (p. 89) gives the mits and their lack of accordance indicate the name "ridge and ravine" to this type of absence of peneplains in this area. topography. "In a typical ridge and ravine Summit elevations in the carbonate area are landscape the general character of the topog- higher, in general, than those in shale areas 1 raphy is probably maintained as the relief is and 2 but are not so high as those in areas 4a lowered" (Hack, p. 95). and 4b. To what peneplain should the car- With respect to accordant summits, which bonate summits be assigned? Are they rem- are often cited as evidence for peneplains, nants of either the 540- or 580-foot peneplains, Hack (p. 91) states: "The regularity of the which somehow have been preserved at higher landscape and the rather uniform height of the elevations on the carbonate rocks, or are they hills owe their origin to the regularity of the the differentially lowered traces of a higher and drainage pattern that has developed over long older peneplain? There is no such accordance periods by the erosion of rocks of uniform of summits in the carbonate area as there is in texture and structure." shale areas 1 and 2. The summits, mean-basin Ashley (1935, p. 1398), postulating the ex- elevations, and low-basin elevations descend istence of only one peneplain (Schooley pene- westward at about the same rate. No evidence plain) in the Appalachian region, states "... exists of any peneplains in the carbonate area. that the present surface . . . has been lowered Thus, the surface features of shale areas 4a by not less than 100 feet for the hardest rocks and 4b and the carbonate area apparently can- and by several hundred feet for the softer not be explained adequately by the peneplain rocks, for each million years since the begin- concept. The surface features of shale areas 1, ning of uplift." From Ashley's statement the 2, and 3 can be explained by the peneplain con- present topography in the Appalachian region cept only by the use of five erosion cycles that is apparently the result of differential erosion include three peneplains or partial peneplains. reflecting the character and structure of the Rich (1938, p. 1714) states: "Theoretical con- rocks, and the geographic cycle and the pene- siderations are strongly opposed to the idea planation had no part in the development of that remnants of older surfaces of more than the present landscape. one or possibly, in rare cases, two cycles ad- Ashley (p. 1398) accounts for the "... level vanced beyond maturity, can exist in a region surfaces and imaginary surfaces touching the of uniform rocks." The foregoing analysis in- tops of accordant hills and mountains ... by volves three cycles which have advanced be- (a) local base-leveling or district base-leveling; yond maturity. or (b) the stripping of flat-lying or nearly flat- Erosional-equilibrium concepts. Davis' (1889) lying hard rocks; or (c) as the result of parallel concepts of geomorphic development, which lowering because of uniformity of rock and include the geographic cycle, the peneplain, structure." The third explanation for accordant and the formation of mountains by a succession summits is very similar to that of Hack. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/9/1071/3442172/i0016-7606-73-9-1071.pdf by guest on 28 September 2021 EROSIONAL FEATURES 1079 The process of lowering of the land surface Quittapahilla Creek) in this valley follows postulated by Ashley could have proceeded three lines of reasoning: (1) one continuous just as readily from a ridge and ravine topog- valley still exists in which the sections not raphy as from a peneplain; hence, if Ashley's presently occupied by streams are at such low theory concerning the lowering of the land elevations, compared to the rest of the car- surface is correct, no peneplain need ever have bonate area, that they indicate erosion by one existed in the Appalachian region. A discussion stream; (2) a high shale ridge continues almost of the existence or nonexistence of peneplains unbroken between the carbonate rock valley in the Appalachian region, however, is outside and Swatara Creek westward from where the scope of this paper. Quittapahilla Creek crosses the shale ridge to The theory of geomorphic development of join Swatara Creek; (3) the pattern of many the Lebanon Valley proposed by the writer streams near the divide between the carbonate uses some of the ideas inherent in the concept valley and Swatara Creek indicates that these of dynamic equilibrium and some of the ideas streams once flowed southward into the car- of Ashley. The theory is based upon the follow- bonate valley and have been captured by ing principles: (1) a ridge and ravine topog- northward-flowing tributaries of Swatara Cieek. raphy, in which streams and interfluvial areas The accordance of summits and the uni- are in dynamic equilibrium, occurs early in the formity of mean basin elevations in shale area 1 erosional development of a humid area; (2) in a are the result of uniform lowering of rocks of state of equilibrium all elements of the topog- uniform character and structure in a series of raphy are lowered at the same rate; (3) areas basins whose discharge points are at approxi- of rocks of uniform character and structure mately the same elevation. The stretch of have uniform topography and local relief; (4) Swatara Creek adjacent to shale area 1 must changes in stream regimen due to stream have had a very gentle gradient, so that tribu- capture or to uplift will cause streams to de- tary streams from the individual basins drained velop new equilibrium conditions; (5) equilib- into it at approximately the same elevation. rium of stream channels is reached much more Even now, there are stretches along Swatara rapidly than equilibrium of interfluvial areas Creek as long as 14 miles that descend only 20 with the streams, and the length of time re- feet. The accordance of summits in shale area 2 quired for the latter is a function of the is similarly explained. lithology and structure of the rocks. The abrupt 40-foot drop in elevation from The Lebanon Valley is interpreted as a ridge shale area 2 to shale area 1 is caused by andesite and ravine topography which is the result of lava at the junction of Swatara and Little erosion within two stream systems (Swatara Swatara creeks and along Little Swatara Creek Creek and the ancestral Quittapahilla Creek, for 1000 feet above the junction. This andesite see Figure 5b), in which the streams and inter- lava caused the very high, steep hills immedi- fluvial areas were at one time in a state of ately south of Little Swatara Creek. Because erosional equilibrium. While all elements of the of the resistant rock at the mouth of Little land surface were lowered at the same rate, Swatara Creek, a higher base level of erosion each drainage basin underlain by rocks of uni- was established for shale area 2, which resulted form character retained its uniform topography in higher mean and summit elevations in the and relief above its local base level. Conse- area. quently the area underlain by shale that was in The range of summit elevations from 580 to equilibrium with the higher ancestral Quitta- 720 feet above msl in shale area 3 reflects the pahilla Creek lies at higher elevations than that equilibrium between streams and interfluvial which was in equilibrium with the lower areas. The higher summit elevations correspond Swatara Creek. A relatively recent entrench- to higher stream elevations and occur on the ment of streams has taken place, and sufficient divides and in the more distal parts of the time has not elapsed for the interfluvial areas basin. The lower summit elevations are in to come into equilibrium with the present equilibrium with the lower stream elevations stream regimen. nearer the mouth of the Little Swatara Creek. The ancestral Quittapahilla Creek (Fig. 5a) The long, gently sloping stretches of Swatara occupied the carbonate valley that is now Creek, which resulted in accordant summits in partly occupied by parts of Quittapahilla and shale areas 1 and 2, did not occur in shale area 3. Spring creeks. Evidence supporting the ex- Shale areas 4a and 4b constitute the area istence of one continuous stream (ancestral lying between Swatara and the ancestral Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/9/1071/3442172/i0016-7606-73-9-1071.pdf by guest on 28 September 2021 1080 HAROLD MEISLER—EROSION SURFACES, LEBANON VALLEY, PA. Quittapahilla creeks. Because Swatara Creek must have taken place in the Lebanon Valley was the main stream in the valley, its great since the deposition and lithification of the load-carrying capacity and low elevation sediments and their subsequent folding and caused those parts of shale areas 4a and 4b in faulting cannot be traced in detail. Undoubt- equilibrium with it to stand at a lower eleva- edly thousands of feet of limestone, dolomite, tion than those parts that were in equilibrium and shale of Cambrian and Ordovician age, and with the much higher ancestral Quittapahilla perhaps younger rocks, have been eroded from Creek. Because of the lower equilibrium posi- the area. tion of the land surface in the Swatara Creek What is of concern primarily is the sequence drainage area, the divide between the two of events that has taken place in relatively re- creeks migrated southward, so that most of cent times and has resulted in the surface areas 4a and 4b are now within the drainage features now visible in the Lebanon Valley. area of Swatara Creek, and the divide between Stages in the development of the drainage pat- the two streams is adjacent to the ancestral tern in the Lebanon Valley are shown in Figure Quittapahilla Creek valley. The summit eleva- 5. tions in areas 4a and 4b that are higher than A major stream, the ancestral Quittapahilla those in areas 1 and 2 occur predominantly Creek was developed on the carbonate rocks along the divide between Swatara and Quitta- near their contact with the Martinsburg shale pahilla creeks. The high summit elevations (Fig. 5a). The ancestral Quittapahilla Creek along the divide and their westward descent was probably the major stream in the valley, thus reflect past equilibrium with the west- and the present southwest-trending segments ward descending ancestral Quittapahilla Creek. of Swatara Creek are probably parts of streams The divide area has not yet come into equilib- that were tributary to the ancestral Quittapa- rium with the present stream systems. Almost hilla. By a series of stream captures, the all the higher summits that do not occur along northern tributaries of the ancestral Quittapa- the divide are underlain by andesite lava, hilla were joined together to form Swatara basalt, and other highly resistant rocks found Creek (Fig. 5b). The ancestral Quittapahilla in shale area 4b. and Swatara creeks probably joined in the Summit elevations are higher in the car- vicinity of Hummelstown and flowed as one bonate area than in shale areas 1 and 2 (Fig. 3), stream into the Susquehanna River, either by because the land surface in the carbonate area the present southward route of Swatara Creek was in equilibrium with the ancestral Quitta- to Middletown or (more likely) by a westward pahilla Creek, whereas areas 1 and 2 were in route. equilibrium with the lower Swatara Creek. After the capture of the northern tributaries However, in the area where the ancestral by Swatara Creek, the main sources of water Quittapahilla and Swatara Creeks joined and in the valley then supplied Swatara Creek the two streams were at the same elevation, rather than the ancestral Quittapahilla, and summits in the carbonate area are lower than Swatara Creek became the major stream. The summits in shale area 1. Furthermore, summit part of the area drained at that time by elevations in shale areas 4a and 4b that were in Swatara Creek was eroded and lowered more equilibrium with the ancestral Quittapahilla rapidly than the part drained by the ancestral Creek are higher than the summit elevations in Quittapahilla. As a result of the unequal ero- the carbonate area. This points out that within sion, the divide between the two streams mi- the same stream system, or within parts of two grated southward toward the ancestral Quitta- stream systems having a common drainage level, pahilla Creek. Eventually the divide between the shale lies at a higher elevation than the the two streams was breached by a northward- carbonate rocks. flowing tributary to Swatara Creek, and Quit- Summit and mean elevations of individual tapahilla Creek was captured and made basins in the carbonate area descend westward, tributary to Swatara Creek (Fig. 5c). Quitta- because they are in equilibrium with the main pahilla Creek then was entrenched. streams of the carbonate area (previously the Swatara Creek also has been entrenched, ancestral Quittapahilla, now Quittapahilla and causing the creek to be out of equilibrium with Spring creeks), which descend westward. the present interfluvial topography. The en- GEOMORPHIC HISTORY trenching of Swatara Creek was probably caused by shortening and steepening the The long history of geomorphic events that stream's path to the Susquehanna River as a Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/9/1071/3442172/i0016-7606-73-9-1071.pdf by guest on 28 September 2021 SHAL^E CARBONATE ROCK \ \ HUMMELSTOWN O w O os a Figure 5. Stages in development of drainage pattern in Lebanon Valley, southeastern Pennsylvania: (a) development of ancestral Quit- tapahilla Creek; (b) integration of northern tributaries of ancestral Quittapahilla Creek to form Swatara Creek; (c) capture of part of ancestral Quittapahilla Creek by northward-flowing tributary to Swatara Creek; (d) capture of Swatara Creek by shorter and steeper stream flowing from near Hummelstown to the Susquehanna River near Middletown Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/9/1071/3442172/i0016-7606-73-9-1071.pdf by guest on 28 September 2021 1082 HAROLD MEISLER—EROSION SURFACES, LEBANON VALLEY, PA. result of its capture by a stream flowing from higher elevation than that of Swatara Creek; near Hummelstown to the Susquehanna River consequently, interfluvial areas within the near Middletown (Fig. 5d). former system were also at a higher elevation. The carbonate rocks, which occur in the an- CONCLUS10NS cestra] Quittapahilla Creek system he at a The peneplain concept does not adequately lower elevation than shale in the same system account for the landforms of the Lebanon but, generally, at a higher elevation than shale Valley. in adjacent parts of the Swatara Creek system. The carbonate valley was at one time occu- Accordance of summits is the result of uni- pied by one major stream—the ancestral form lowering of rocks of uniform character Quittapahilla Creek—which was later be- and structure in a series of basins whose dis- headed by a tributary to Swatara Creek. charge points are at approximately the same The land surface of the Lebanon Valley is the elevation. Lack of accordant summits in the result of erosion within two streams systems, ancestral Quittapahilla Creek system and in Swatara and the ancestral Quittapahilla creeks, part of the Swatara Creek system is the result in which interfluvial areas were in erosional of the lowering of rocks in a scries of basins equilibrium with the stream systems. The whose discharge points are at different eleva- stream system of Quittapahilla Creek was at a tions. REFERENCES CITED Ashley, G. H., 1935, Studies in Appalachian Mountain sculpture: Geol. Soc. America Bull., v. 46, p. 1395-1436 Bethune, P. F. de, 1948, Geomorphic studies in the Appalachians of Pennsylvania: Am. Jour. Sci., v. 246, p. 1-22 Campbell, M. B., 1903, Geographic development of northern Pennsylvania and southern New York: Gcol. Soc. America Bull., v. 14, p. 277-296 Davis, W. M., 1889, The rivers and valleys of Pennsylvania: Natl. Geog. Mag., v. 1, p. 183-253 Fenneman, N. 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America Bull., v. 49, p. 1695-1722 Ver Steeg, Karl, 1942, A study in Appalachian physiography: Jour. Geology, v. 50, p. 504-511 MANUSCRIPT RECEIVED EY THE SECRETARY OF THE SOCIETY, NOVEMBER 28, 1961 PUBLICATION AUTHORIZED BY THE DIRECTOR, U. S. GEOLOGICAL SURVEY PREPARED IN CO-OPERATION WITH THE PENNSYLVANIA GEOLOGICAL SURVEY, DEPARTMENT OF INTERNAL AFFAIRS, COMMONWEALTH OF PENNSYLVANIA Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/9/1071/3442172/i0016-7606-73-9-1071.pdf by guest on 28 September 2021