the piedmont landscape of maryland: a new look at an old problem

EARTH SURFACE PROCESSES AND LANDFORMS, VOL. 9, 59-74 (1984)

THE PIEDMONT LANDSCAPE OF MARYLAND: A NEW LOOK AT AN OLD PROBLEM

JOHN E. COSTA*

Department of Geography, University of Denver, Denver, Colorado 80208, U.S.A. AND

EMERY T. CLEAVES MaryInnd Geological Survey, 711 West 40th Street, Baltimore. Maryland 21218, U.S.A.

Received 5 March 1982 Revised 8 July 1983

ABSTRACT

The Piedmont upland of Maryland has been variously interpreted as a peneplain, a series of peneplains, a surface of marine planation, and a landscape in dynamic equilibrium. These different perspectives of landform evolution are related to different scales of time and space. Both equilibrium and episodic erosion features can be recognized in the modern landscape. An equilibrium condition is suggested by adjustment of first and second order streams to rock structure and lithology, entrenchment of some streams against gneiss domes, altitudinal zonation of rock types around gneiss domes, correlation of lithology with overburden thickness on uplands, decreasing overburden thickness on uplands related to decreasing degree of metamorphism of crystalline rocks, and correlation of secondary mineral assemblages with subsurface drainage and slope. The long-term episodic character of erosion is suggested by clastic wedges on the adjacent Coastal Plain, an upland of low relief that truncates non-carbonate rocks of different lithologies, isovolumetric chemical weathering of alumino-silicate rocks, clastic deposition in marble valleys, and weathering profile truncation by modern drainage.

The Maryland Piedmont may have been an area of positive relief subject to subaerial erosion since Triassic and possibly Permian time. The upland surface preserved in the eastern Piedmont developed by the Late Cretaceous. In the interval from the Late Cretaceous to the Late Miocene, low input of terrigenous sediments to the Coastal Plain, dominance of marine sedimentation, and spotty evidence of saprolite formation on crystalline rocks, suggest that the Maryland Piedmont was an area of low relief undergoing intense weathering. Incised valleys were formed during a cycle of erosion probably initiated in the Late Miocene and extensive colluvial sediments were deposited on hillslopes by periglacial processes during the Pleistocene.

KEY WORDS Piedmont Landform evolution Weathering Saprolite

INTRODUCTION

The morphogenesis of the Piedmont Province of Maryland (Figure 1) has been a subject of controversy for many years. Some authors have interpreted the Piedmont upland as a peneplain or series of peneplains (Campbell, 1933; Knopf, 1924) representing the terminal phase of a Davisian cycle of erosion. Barrel1 (1913, 1920) proposed that the relief of the Appalachians from New England to the Potomac River consisted of a series of marine terraces carved by marine planation, a theory refuted by Sharp (1929) and Johnson (1931) but recently restated by Meyerhoff (1975). Others have questioned the genetic implications of peneplains and erosion surfaces and have interpreted the landscape in the context of'equilibrium of action' (Hack, 1960,1975).

Present address: U.S. Geological Survey, Mail Stop 413, Denver Federal Center, Lakewood, Colorado 80225, U.S.A.

0 197-9337/84/010059-16$0 1.60 0 1984 by John Wiley & Sons, Ltd.

60 JOHN E. COSTA AND EMERY T. CLEAVES

3 9 O -

370-

E l 0 7 7 O

Figure 1. Map of the middle eastern United States showing physiographic Provinces. Stippled area shows location of Figure 6. Abbreviations: CP, Coastal Plain; P, Piedmont Province; BR, Blue Ridge Province; VR, Valley and Ridge Province; md, Maryland;

va, Virginia; cb, Chesapeake Bay

According to the theory of equilibrium of action, topographic forms can be explained by differences in the rocks, and the processes presently acting upon them. The close relationship between present form and present processes required by equilibrium theory rejects the possibility that some landscapes might be relic, formed by processes no longer operating there (Higgins, 1975). In applying the concept of dynamic equilibrium in the Piedmont Province, Hack (1960, p. 91) states, ‘The regularity of the landscape and the rather uniform height of the hills owe their origin to the regularity of the drainage pattern that has developed over long periods by the erosion of rocks of uniform texture and structure.’ This viewpoint deems the concept of cycles unnecessary to explain the dissected landscape of the Piedmont.

The introduction of the concept of ‘dynamic equilibrium’ (Hack, 1960) has polarized the thinking among many geomorphologists about landform development. Some geomorphologists adhere to the concept of time dependence. This is essentially historical in approach, such that ‘every erosional process leaves a largely indelible imprint upon surface forms and deposits’ (Howard, 1965, p. 302). Other geomorphologists favour the concept of ‘dynamic equilibrium’ (Hack, 1960) where landforms are viewed as in equilibrium with the processes acting on them, such that all elements of the landscape are evolving at the same rate. Some studies favour the historical approach (Holmes, 1964) while others favour the equilibrium concept (Flint, 1963).

A middle course involving both episodic erosion and equilibrium conditions has been suggested by Schumm and Lichty (1965), Howard (1965), and Slaymaker (1972). These studies consider landform evolution according to the span of time involved, and the scale of the geomorphological system. Within the framework of time and scale, episodic erosion and dynamic equilibrium both find a home. During short periods of geological time ( 10°-105 years) for mesoscale (slopes and stream reaches) and microscale features (sites), landforms may be in equilibrium with the processes acting upon them. However, for long periods of geological time (greater than lo6 years) and megascale features (i.e., whole river basins or provinces), the average relief and mass volume of material in the system will decrease as erosion continues. It has been argued that it is highly unlikely that a geomorphological steady-state even exists (Bull, 1975). For landforms requiring geological time to develop (greater than lo6 years), changes in climate, base level, and erodibility of surficial materials are sufficiently rapid to preclude the attainment of dynamic equilibrium. Howard (1965, p. 310) also concludes that ‘ . . . the nice adjustment (quasi-equilibrium) of stream parameters to small-scale variations in climate does not imply that the coarser features are at equilibrium.’

We believe that the interpretation of the relief of the Piedmont of Maryland is a problem of the scale and time frame in which the system is viewed. For example, the stratigraphy of the adjacent Coastal Plain records conditions affecting the Piedmont with periodicities of n x lo6 years; but surficial deposits in the Piedmont on

T H E PIEDMONT LANDSCAPE OF MARYLAND 61

slopes and floodplains record events with a time frame of n x 10' to n x lo5 years. Elements in the landscape reflecting dynamic equilibrium include the adjustment of smaller drainage elements to structure and lithology, and local correlation of topography with lithology, the correlation of saprolite thickness with landforms and metamorphic rank, and saprolite mineral assemblage as related to topography. On the other hand, Coastal Plain clastic wedges, truncated weathering stratigraphy, truncated marble lowland gravel horizons, and distribution of stream gradients and floodplain formation indicate that erosion has been episodic.

EQUILIBRIUM EVIDENCE

The Maryland Piedmont contains a variety of micro and mesoscale geomorphological evidence supporting the concept of dynamic equilibrium.

1. First and second order stream patterns show exceptional dependence upon jointing and foliation. Control of stream alignment in the Piedmont by joints has been inferred from joint measurements (Nutter and Otton, 1969) and confirmed by detailed mapping of stream tributaries. In some rock units, joints are the primary direction along which the rock splits, and stream segment alignments follow joint alignments (Figure 2A, C). In other areas, foliation becomes a primary splitting direction and strongly controls stream alignments (Figure 2B).

U U

- 0 2 4 0 K m 2

Map scale for A ond B Number of lomh

w - 0 K m 0 . 5 0 2 4

Number of p n t s Mop scale for C

Figure 2. Joint and foliation control ofthree typical Piedmont streams: A. Slade Run, joint control; B. Blackrock Run, joint and foliation control; C. Dulaney Valley Branch, joint control

2. Rock lithology and regional structure appear to control the location of some third and fourth order tributary stream alignments. In the Maryland Piedmont west and north of Baltimore, five Precambrian gneiss domes outcrop along the crest of a broad regional upwarp (Figure 3). Streams are nestled against the north flank of these gneiss domes in valleys underlain by marble bedrock. The marble is more susceptible to chemical and physical erosion than the adjacent Setters Formation (which includes quartzite), and the tributary has eroded its valley in the marble against the domes. Streams flowing from north to south down the regional slope continually force the marble valley streams against the gneiss domes. Because of constant saturation by ground and surface water, the marble valleys are the location of accelerated solution and


N Gunpowder Falls i

. . . . . . . . . . .

Potopsco River

I BALTIMORE I

5 MILES

I I

R KllOMFTFRS

Figure 3. Relationships of stream courses to gneiss domes north of Baltimore, Maryland. Abbreviations: W, Woodstock Dome; C, Chattolanee Dome; P, Pheonix Dome; Tx, Texas Dome; Tw, Towson Dome. Third and fourth order streams with drainage areas less than 155 km2 show structural and lithologic controls. Larger master rivers (Patapsco and Gunpowder Falls) cut across regional lithologic

and structural trends

erosion. A series of marble strath benches has formed on the”north side of valley walls. Occasional thin deposits of subrounded quartzite gravels resting on some of the straths indicate the former presence of streams (Figure 4).

3. In the vicinity of the gneiss domes, relief seems to be well adjusted to lithology. For example, measurements of altitudes of different rock types around one gneiss dome (Table I) indicate a zonation of relief and lithology. The more easily dissolved and eroded Cockeysville Marble underlies the areas of lowest elevation, and the much more resistant Setters Formation (with quartzite lenses) underlies the highest elevations. Similar relationships between lithology and altitude have been established in Piedmont terrain in Connecticut (Flint, 1963, pp. 688490).

4. The thickness of the saprolite which mantles alumino-silicate rocks in the eastern Piedmont of Maryland is related to landform and lithology (Cleaves, 1974a, 1975). Where rocks of contrasting mineralogy or texture underlie similar landforms, the thickness of saprolite developed on the rock consistently differs (Figure 5). In Figure 5, uplands underlain by the augen gneiss member of the Baltimore Gneiss have a shallow saprolite cover (1.5 to 6m), in contrast to uplands underlain by the Loch Raven Schist which have overburden thicknesses exceeding 6 m. The quartzite member of the Setters Formation is resistant to chemical weathering. Wherever this unit occurs, regardless of landform, overburden thicknesses are generally less than 1.5m.

THE PIEDMONT LANDSCAPE OF MARYLAND 63

Figure 4. Photograph of marble strath bench formed against north side of Phoenix Dome. Arrows point to strath

Table I . Altitudes of different rock types around the Phoenix Dome near Baltimore, Maryland. (Percentage of area underlain by rock unit within the given range of elevation)

Average 90-120m 120-150m 150-180m 180-210m elevation m

Cockeysville Marble 60 % 40 % 0 % 0% 120 (marble)

(gneiss)

(schist)

(quartzite)

Baltimore Gneiss 10 % 55 % 30 % 5 % 150

Wissahickon Group 1 % 9 % 30 % 60 % 180

Setters Formation 0 % 10% 15% 75 % 200

Textural control of overburden thickness is illustrated by the response of the Baltimore Gneiss to weathering. Coarse-grained layered gneiss phases are mantled by saprolite which generally exceeds 6 m in thickness, in contrast to finer-grained, more massive phases where the overburden mantle is between 1.5 and 6 m thick.

5. Thickness of saprolite is also related to degree of metamorphism. The more strongly metamorphosed Loch Raven Schist, adjacent to gneiss domes, has a thicker saprolite mantle than the Prettyboy Schist which outcrops north and west of the domes. The Loch Raven Schist is a uniform, medium to coarse-grained biotite-oligoclase-muscovite-quartz schist, with garnet, staurolite, kyanite, and, in a few places, sillimanite. The Prettyboy Schist is a uniform, fine-grained albite-chlorite-muscoviteguartz schist, with garnet. The depth of overburden on the uplands based upon water well records and outcrop mapping indicates that the saprolite is thicker on the Loch Raven Schist than on the Prettyboy Schist (Table 11). The thinning of saprolite away from centres of metamorphism is also reflected in the increasing abundance of Mt. Airy-Glenelg-Linganore residual soils on the uplands. These shallow soils are commonly developed on fine-grained schists and phyllites (Reybold and Matthews, 1976). Overall, the chemical weathering response of the schistose rocks is consistent with LeChatelier’s rule, which Krauskopf (1967, p. 19) equates to a system in a dynamic equilibrium.


Figure 5 . Sketch illustrating relationships of landforms, overburden, and stream valleys in the Maryland Piedmont. Abbreviations: U, uplands; S, slopelands; and L, lowlands. In cross-section, stipple pattern represents saprolite and modern soil, diagonal lines represent jointing; dashed lines represent foliation. Note truncation of saprolite on hillslopes. Bedrock abbreviations: Ir, schist; cm, marble; sq,

quartzite; sg, gneiss and schist; b. gneiss. A-A and B-B refer to cross-sections shown in Figure 8

Table 11. Thickness of overburden on Loch Raven Schist compared to Prettyboy Schist on Upland areas

Number of water wells in which thickness of overburden (m) is:

0 -2 2 -6 > 6 Loch Raven Schist 0 36 414 Prettyboy Schist 1 143 70

6. During chemical weathering of crystalline rocks to saprolite, secondary mineral assemblages which replace rock-forming minerals appear to correlate closely with subsurface drainage and slope (Cleaves, 1974b). The parent rock in the example is hornblende-plagioclase-quartz gneiss. Near the edge of the uplands, the weathering profile is well drained; and saprolite consists mainly of ferric oxide and hydroxide compounds, kaolinite, and quartz with lesser amounts of vermiculite. On adjacent side slopes greater than 20 degrees, the profile is well drained, and the saprolite consists of ferric oxide and hydroxide compounds, gibbsite, and kaolinite. At the base of slopes passing beneath floodplains, drainage is poor and the saprolite consists of quartz, kaolinite, and montmorillonite.

The relationships among parent alumino-silicate minerals, secondary mineral assemblages derived from chemical weathering, topography, and internal drainage indicate a rapid interaction between groundwater and rock minerals (probably n x 10’ years). A similar conclusion was drawn by Bricker et al. (1968, pp. 126138)


from a study of the geochemical balance in a small (042 km’) basin on felsic schist in Baltimore County, Maryland. The dominant clay mineralogy may reflect a broad regional geochemical environment (e.g., a leaching environment characterized by kaolinite in the humid Eastern United States), but the localized saprolite environment is a complex of many different microenvironments wherein a variety of clay minerals may be in equilibrium at a given site.

EVIDENCE OF EPISODIC EROSION

Even though there is evidence on both the micro and mesoscales supporting equilibrium conditions on the Maryland Piedmont, there is also strong evidence to suggest an episodic character for the long-term fluvial erosion processes shaping the Piedmont landscape. The Coastal Plain sedimentary units record long-term (n x lo6 years) events in the weathering and erosional history of the adjacent Piedmont source area. The Potomac Group sediments of Lower Cretaceous age in the Maryland Coastal Plain record a major period of Piedmont fluvial erosion. Marine deposits of Late Cretaceous to Late Miocene age apparently indicate a prolonged period characterized by low rates of input of terrigenous sediment. Renewed vigorous erosion of the Piedmont is recorded by fluvial sediments of Late Miocene, Pliocene, Pleistocene, and Holocene age in the Coastal Plain. Overall, the Coastal Plain sediments record two periods of significant fluvial erosion which may reflect tectonism, climatic change, or eustatic fluctuations in the source area and/or the basin of accumulation (Reed, 1981).

Like the Coastal Plain sediments, the Maryland Piedmont source area contains evidence of long-term regional erosion and weathering. The evidence is both geomorphological and stratigraphical: (1) the presence ofa surface of low relief with a thick weathered mantle, which truncates rocks of varying lithologies; (2) clastic deposits in marble valleys; (3) truncation of saprolite profiles by modern drainage; and (4) isovolumetric chemical weathering of the Piedmont crystalline rocks.

1. Landform mapping in the Maryland Piedmont near Baltimore has documented an extensive surface of low relief which has been dissected (Figure 6). The surface has been recognized and described in general terms by Abbe (1899, pp. 117-1 19) and by Knopf (1924,1929, p. 63). Along the Maryland-Pennsylvania border in the north, the surface averages 244-262m in elevation. The upland surface declines gradually to 152-171 m in the southern part of the area. The upland surface extends eastward and northeastward to and across the Susquehanna River, and westward and southwestward to the Potomac River. This upland surface truncates non-carbonate rocks of varying lithologies. Comparison of the underlying geology with the upland surface shows that the surface cuts across numerous non-carbonate geological formations of varying lithology (Crowley et al., 1976).

2. In the Baltimore area, clastic deposits have been identified and mapped in the valleys underlain by the Cockeysville Marble (Crowley and Cleaves, 1974; Crowley et al., 1975). The deposits are a mixture of colluvial and alluvial materials that are mineralogically and texturally distinct from the underlying carbonate residuum developed on the marble. Some deposits are clearly alluvial fans deposited by streams where they flow from uplands out into marble lowlands. Other clastic deposits occur well out in the marble valleys and, where exposed, are characterized by a gravel layer overlain by loam (Figure 7). The gravel consists exclusively of quartz clasts.

Analysis of heavy mineral fractions (Table 111) demonstrates the contrast between marble residuum and overlying surficial deposits. Diopside, sphene, and tremolite are found only in the residuum and are primary minerals from the marble bedrock. Kyanite, hornblende, garnet, and staurolite characterize the gravel layer. Some mixing of dolomite has occurred. The nearest source of kyanite, hornblende, garnet, and staurolite is the schist bedrock outcropping in the slopes above the valleys. These gravel deposits do not appear to be related to any presently active process. The age or ages of these deposits are unknown, but they have some antiquity, since the zones are truncated by the present drainage.

3. The episodic character of erosion in the Maryland Piedmont is further indicated by the truncation of saprolite by modern drainage. Evidence indicating truncation is both direct (saprolite profiles and overburden thickness maps) and indirect (stream low-flow characteristic, gradients, and floodplain


OKM. 5 10 I 5 r . . . . .

Liitle Gunpowder Falls

\ BALTIMORE CITY

Figure 6. Upland surface in the Piedmont area of Baltimore County, Maryland

distribution of streams). Two profiles illustrate our contention that the Piedmont upland is appreciably older than the valley sides and valley bottoms, thus precluding the possibility that the reliefas a whole can be explained by processes presently acting on the landscape. The location of cross-sections in Figure 8 are shown in Figure 5. On uplands, saprolite is thickest and many first order streams flow on saprolite (Profile A-A). On the valley side slopes, saprolite is thin or absent (Profile B-B). Truncation of the weathered mantle by deeply incised streams indicates that the valley slopes are younger than the upland surface, and that different components of the Piedmont landscape have different ages. The saprolite developed on side slopes of Profile B-B has been removed partly or completely by mass wasting. During incision of the stream saprolite has been eroded as rapidly as it formed. From B' downslope to the stream saprolite formed during the period of incision, but it has not been eroded as fast as it is formed. Saprolite on the valley side is younger than the saprolite on the upland.

Contour maps of the thickness of residuum show that the distribution of the weathering products closely follows the drainage patterns. Saprolite thickness appears to be related to stream order. Ephemeral and first order streams generally flow on saprolite in wide valleys (A-A, Figure 8). Where present, the alluvial flats bordering channels are poorly defined. Colluvium is abundant. Second and higher order streams generally have cut down through the saprolite into weathered rock and bedrock (B-B', Figure 8). The higher the

THE PIEDMONT LANDSCAPE OF MARYLAND

~

Loam

Grovel zone

67

Residuum

Figure 7. Gravel zone exposed in marble lowlands. Gravel horizon separates marble residuum (below) from transported loamy sediments above. Shovel is approximately 1 m long

Table 111. Heavy mineral analyses, surficial deposits of Figure 7. (< 2mm sand fraction)

Residuum Gravel zone Loam (percentage) (percentage) (percentage)

Staurolite Sillimanite Kyanite Diopside Sphene Tremoli te Dolomite Rutile Tourmaline Hornblende Garnet Zircon

Number of grains counted

12 10

Tr 8 4 5

46 8 3

Tr Tr 4

445

35 10 10

- 10 5 2 8

Tr 20

350

57 8 20 -

- 1 2 3 1

Tr 8

400

stream order, the more deeply the stream is incised into bedrock. One such map for Western Run watershed north of Baltimore (Figure 9) illustrates the coincidence of thin or absent saprolite with increasing stream order, and thick saprolite on interfluves. Maps showing the thickness of overburden in other areas of the Maryland Piedmont display an identical situation (Cleaves, 1974a, 1975).

Additional indirect evidence for incision of the Piedmont drainage can be found in the gradients and distribution of floodplains of the Piedmont tributaries to the Susquehanna and Potomac Rivers. Where tributaries draining the Piedmont join these large rivers, they have little or no floodplain and steep (3.8 4 . 7 m/km) gradients. Farther upstream, the same tributaries have broad floodplains and more gentle


A

Figure 8. Schematic cross-sections of Maryland Piedmont valleys. Stipple pattern represents structured saprolite; short vertical lines beneath land surface represents massive saprolite and modern soil; black represents alluvium and colluvium. Section A-A: first order perennial streams flow on saprolite in thin alluvium; Section B-B: major perennial streams flow on bedrock; saprolite truncated on valley

sides

Figure 9. Isopach map showing saprolite thickness, Western Run basin (see Figure 3 for location). Isopach contours in metres. White areas: saprolite; open stipple pattern with large dots: weathered rock; tight stipple with small dots: unweathered bedrock; white area along

master stream: alluvial fill


(0.95-1.9 m/km) gradients. This relationship of Piedmont tributaries to main streams was noted by Mackin (1 936, p. 13), who remarked, ‘Each of these tributaries flows in an open valley in the upland to a point where, as it approaches the main stream, abrupt falls or rapids mark the head of a narrow inner ravine extended from the main gorge.’ This relationship between the major rivers and their tributaries draining the Piedmont has been reiterated by Reed (1981). An excellent example of such a Piedmont tributary is Rock Creek in the Washington, D.C., West Quadrangle. From its junction with the Potomac River upstream to the gauging station north of Miller Cabin, the stream has little or no floodplain and a gradient of 4.1 m/km. Above the gauging station, the floodplain widens to more than 305m; and the gradient decreases to 1.46 m/km. This change in gradient and floodplain widening seems to occur about 5-8 km upstream from the confluences of tributaries with the Potomac and Susquehanna Rivers in the vicinity of Baltimore and Washington. An alternative explanation is offered by Hack (1973), who attributes a sharp increase in gradient of the Piedmont tributaries to large, resistant lag streambed materials. However, these oversteepened reaches are not directly related to local lithological differences (Reed, 198 l) , as required for dynamic equilibrium.

4. Where the upland surface is underlain by alumino-silicate crystalline rocks, chemical weathering has resulted in the formation of saprolite, in which 20 to 50 per cent of the original rock mass has been removed in solution. Detailed study of saprolite-rock profiles of weathering demonstrates that the chemical alteration of rock to saprolite is an isovolumetric process (Cleaves, 1974b). Mass subtracted from the rock by chemical weathering is removed by groundwater, and rock-forming minerals are replaced by secondary minerals. The process results in a marked decrease in density and a great increase in porosity. Even though there are major mineralogical and physical changes accompanying the formation of saprolite, there is no decrease in volume. The saprolite occupies the same volume of space that the original rock occupied, a matter of critical geomorphological importance. Specifically, chemical weathering of rocks in which alumino-silicate minerals are major components apparently does not result in significant lowering of land surfaces. However, the saprolite is much more susceptible to erosion by running water and mass movements.

AGE AND ORIGIN O F THE PIEDMONT LANDSCAPE

The upland surface on the eastern Piedmont of Maryland can be indirectly dated from Coastal Plain sedimentary sequences. The pre-Cretaceous sedimentary record in the Appalachians shows that the Piedmont has apparently been an area of positive relief for a considerable time prior to the Cretaceous, but the record is too scanty to reconstruct earlier episodes of erosion. There is no evidence to support Barrell’s (1920) marine planation theory for the upland surface in Maryland. There are no marine deposits nor identifiable marine features in the Piedmont Province in Maryland.

The courses of the major rivers in the Maryland Piedmont are inherited from prolonged periods of subaerial weathering and erosion that extend into the Jurassic and possibly well into the Triassic. The drainage may be as old as Permian (Meyerhoff, 1972). The present eastward drainage of the major rivers may have been established in the Jurassic and may be related to early rifting and subsidence of the continental margin consequent upon the opening of the Atlantic Ocean (Le Pichon, 1968, p. 3689). Rifting of the continental margin of North America may have been initiated in the Triassic (Dewey and Bird, 1970, p. 2630; Dietz and Holden, 1970, p. 4946). Our evidence for the antiquity of Piedmont drainage is mainly sedimentological. The rivers were not superimposed from a cover of Cretaceous age sediments as suggested by Johnson (1931) and reiterated recently by Dott and Batten (1976, p. 406). Deposits of marine Cretaceous sediments are absent west of the Fall Line, and the mineralogy of non-marine lower Cretaceous sediments indicates that metamorphosed Piedmont bedrock was exposed and being eroded (Groot, 1955; Glaser, 1969). Pre-Cretaceous and early Cretaceous erosion had exposed the core of at least one gneiss dome in the Baltimore area, since early Cretaceous sediments rest unconformably upon Baltimore Gneiss in the Towson Dome (Crowley and Cleaves, 1974). This suggests that present rivers such as Gunpowder Falls and the Patapsco probably had established their courses across the Piedmont landscape by at least early Cretaceous time.


Subaerial erosion of the Piedmont terrain was apparently initiated in mid-Ordovician time, and the area contributed sediments into the Appalachian geosyncline up to mid-Pennsylvanian time (Hopson, 1964, p. 204). Triassic alluvial sediments derived from the Piedmont are recorded along the western boundary of the Piedmont (Stose and Stose, 1946, pp. 83-87,128-131), and Triassic sediments are present on the metamorphic terrain buried beneath the Coastal Plain in eastern Maryland (Jacobeen, 1972, p. 7). Jurassic rocks are also present beneath the Coastal Plain (Maher, 1971), and the Piedmont apparently was an emergent area subject to subaerial weathering.

Extensive subaerial erosion of Maryland’s Piedmont is indicated by the Jurassic-early Cretaceous sediments filling the Salisbury Embayment (Maher, 1971, pp. 25, 37). The Salisbury Embayment is a prominent pre- Cretaceous feature in crystalline basement rocks on the Coastal Plain which was nearly filled with fluvial, brackish, and marginal marine sediments prior to the Tertiary. Much of the inorganic particulate matter presumably came from the adjacent Piedmont. The edge of this fill in Baltimore County was deposited by southeasterly flowing streams (Glaser, 1969, p.75), similar in direction to the present Piedmont drainage. Early Cretaceous sediments were mainly derived from intensively weathered Piedmont crystalline rocks (Glaser, 1969).

The Potomac Group sediments of early Cretaceous age represent a major period of alluvial sedimentation. The sediments generally consist of a lower gravel-sand lithofacies (apparently deposited in a braided river environment), and an upper sand-silt-clay lithofacies (deposited on a deltaic plain by low gradient, meandering rivers) (Glaser, 1969, pp. 71-72). These deposits represent a significant episode of fluvial erosion in the Piedmont of Maryland. Beneath the Maryland Coastal Plain, saprolite developed in crystalline rocks prior to deposition of the early Cretaceous sediments (Cleaves, 1968, p. 15), and early Cretaceous sediments unconformably overlie saprolite along the Coastal Plain/Piedmont boundary near Baltimore. Locally, the Cretaceous sediments contain reworked blocks of saprolite, indicating that at least some of the saprolite is pre- Cretaceous in age.

By Late Cretaceous time, sediments of the Magothy Formation and Monmouth Group indicate a major decline in fluvial sedimentation. In the Maryland Coastal Plain, the outcropping formations from Late Cretaceous to Late Miocene generally represent marine and estuarine sediments rather than fluvial-deltaic sediments common in the early Cretaceous Potomac Group. Although the marine sediments are composed mainly of clastic materials (sands, silts, and clays), their volume is much less than that of the Potomac Group, and in the Maryland area there are no major fluvial equivalents. The marine sediments represent a series of marine transgressive and regressive sequences. These data suggest that the time span from the Late Cretaceous to the Late Miocene was generally a time of intense chemical weathering and saprolite formation in the adjacent Piedmont. The small input of clastic fluvial sediments to the Coastal Plain during this time suggests that the Piedmont in Maryland was an area of low relief, and its surface was near sea level.

A Late Miocene (?) to Holocene period of renewed fluvial erosion of the Piedmont is recorded in alluvial sediments deposited in the Coastal Plain, and thick colluvial deposits on the hillslopes in the Piedmont. Plio- Pleistocene and Holocene fluvial sediments in various areas of the Maryland Coastal Plain have been mapped by Schlee (1957), Glaser (1971), Hansen (1966), Owens (1969), and Hack (1955), among others. Sedimentation may have started as early as Late Miocene (Glaser, 1971, p. 33; Hack, 1955, p. 37) although fossil evidence is lacking.

There is some evidence to indicate possible causes of the latest erosion period in the Maryland Piedmont. In Pliocene and Quaternary time, sea level in the vicinity of Washington, D.C., has fluctuated at least 113 m (Hack, 1973, p. 426; Hack, 1955, pp. 28-33; Darton, 1951, p. 752). This implies achange in base level sufficient to instigate a period of erosion.

Tectonic upwarping has also contributed, in part, to renewed Quaternary fluvial erosion (Reed, 1981). The northern Virginia-Maryland part of the Piedmont has been identified as an area of upwarping (Hack, 1980, pp. BlCL11; Hack, 1982, pp. 15-19). Deformation and faults displacing Pliocene (?) deposits along the Fall Line in this area have been reported by Mixon and Newel1 (1977), Spoljaric (1973), and Darton (1951). Large clastic wedges deposited on the Atlantic Coastal Plain during Cretaceous and Late Miocene-Pliocene-Pleistocene times have been interpreted as evidence for episodic tectonic uplifts of the central Appalachian source area (Owens, 1970; Gibson, 1970). Bollinger (1973) reported that recent seismic


activity in the Southeastern United States results from strain concentrated in old Appalachian structures and is caused by recurring crustal uplifts. This recent seismicity may represent isostatic adjustment of the Piedmont and Appalachians following the erosion of large volumes of material.

During the Pleistocene, the Piedmont in Maryland was subjected to a rigorous periglacial climate (Godfrey, 1975; Maxwell and Davis, 1972; Sirkin et al., 1977). Frost action and increased soil moisture initiated creep and

Collurium

Creeping saproli te

Saprolite

Figure 10. Saprolite with gneissic layering, overlain by creeping saprolite. Saprolite overlain by colluvium containing spruce and fir pollen. Age of colluvium is probably Pleistocene (?). Shovel handle is 1 m long


other mass movements in saprolite, forming colluvial deposits on sideslopes (Figure lo), and lowering the upland surface.

Colluvial deposits of Quaternary Age in the inner Piedmont from South Carolina, North Carolina, and Virginia have been described by Overstreet et al. (1968, pp. 31-36). Although some of the deposits are being formed by present accelerated erosion due to human activities, other deposits clearly date back into the Pleistocene (Overstreet et al., 1968, pp. 32,35). Detailed studies of similar colluvial deposits in North and South Carolina by Whitehead and Barghoorn (1962, p. 367) led them to conclude that the colluvial sediment overlying stonelines and Pleistocene organic horizons probably accumulated during glacial advances when increased precipitation and frost action combined to promote downslope movement of surficial sediments. In upland Piedmont valleys in Maryland, we found abundant spruce and fir pollen in colluvial deposits similar to those in Figure 10, suggesting that they too are Pleistocene in age.

CONCLUSIONS

Coastal plain clastic wedges, truncated weathering stratigraphy, truncated marble lowland gravel horizons, and distribution of stream gradients and floodplains indicate that erosion in the Maryland Piedmont has not been uniform over long periods of geological time (n x lo6 years). The upland surface of low relief in the present Piedmont landscape appears to be a feature that dates back to the Late Miocene. This upland surface is a relic feature that is currently being dissected by fluvial erosion. The adjustment of low order streams to structure and lithology, local correlation of topography with lithology, correlation of saprolite thickness with topography and metamorphic rank, and the relationship of saprolite mineral assemblages to topography, record events of n x 10’ to n x lo5 years and suggest an equilibrium condition between process and landforms.

Erosion on the Piedmont since the Late Miocene apparently has been primarily by stream incision and mass wasting during periglacial conditions in the Pleistocene. A t present, the interfluves seem relatively immune to natural erosion processes but have been greatly affected by man in the past 200 years (Costa, 1975). The modern landscape, therefore, exhibits elements that can be attributed to long-term episodic erosion, and to short-term equilibrium of processes and form. Whether one believes that relief adjusts as quickly as the equilibrium proponents favour, or slowly as the historical proponents believe, depends upon the scale and time frame in which the changes are viewed.

ACKNOWLEDGEMENTS

Our concepts and ideas materially benefited from the advice and criticism of many colleagues. This manuscript was significantly improved by the constructive observations and comments of John Glaser, Maryland Geological Survey; Wayne Newel1 and Milan Pavich, U.S. Geological Survey; Andrew Godfrey, U.S. Forest Service; and M. Gordon Wolman, the Johns Hopkins University.

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the piedmont landscape of maryland: a new look at an old problem

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