Physiography
By William E. Davies, U.S. Geological Survey
Appalachia, as an economic entity, is nearly co-extensive with the Appalachian Highlands, one of the eight principal geographic division of the United States (Fenneman, 1938, p. 121; 6 A, B, table 20). Because Appalachia's boundaries were established in recognition of certain social and economic factors, the area of Appalachia overlaps slightly into to other major geographic areas-the relatively flat country of the Coastal Plain province in Alabama and the Interior Plains division in Ohio, Kentucky, Tennessee, and Alabama. The Appalachian Highlands extend northward into New England and easternmost Canada, and eastward beyond Appalachia in New York, Pennsylvania, Maryland, and Virginia.
Appalachia comprises part or all of seven physiographic provinces, four in the Appalachian Highlands and three in nearby lands of lower elevation and more gentle topography. Characteristics of these provinces are listed in table 20. Distinctive landforms have developed in each province, in large part depending on the structure and type of underlying rocks have varied, the landforms in each province reflect the different periods of mountain building and crustal warping and the erosional forces of wind, water, ice, and gravity. The present landscape thus reflects the total geological environment, past and present.
Table 20. Rocks and landforms of the physiographic divisions of the Appalachian Region.
The eastern part of Appalachia in the Carolinas, Georgia, and east-central Alabama is in the Piedmont province. It is an upland of rolling hills with gentle slopes and relief generally less than 150 feet. Major steep-sided valleys, however, have been cut to depths of several hundred feet. Tributary valleys are generally broad with gently sloping sides. the expanse of rolling areas between deep streams is greatest in Alabama and Georgia where the upland extends for miles without interruption. Northwards in the Carolinas the relative amount of undissected upland diminishes, and steep-sided valleys are seldom more than a few miles apart.
In Georgia and Alabama the upland surface is at an elevation of about 1,400 feet, except at the western edge where it rises to 1,800 feet along the foot of the Blue Ridge. The upland is crowned by rounded ridges and knobs, called monadnocks, that rise above the general level of the hilltops. These monadnocks are erosional remnants reflecting contrasts in weathering processes and the strength of underlying rocks. Stone Mountain, Ga., outside Appalachia, is one of the most notable monadnocks in the southern Piedmont.
In the Carolinas the monadnocks are of greater size and include extensive outlier ridges similar to spurs of the Blue Ridge. Some of these ridges rise about 1,500 feet above the rolling upland and are separated from one another by broad valleys and rolling uplands.
The rocks of the Piedmont are primarily granite, gneiss, schist, slate, and relative metamorphic rocks. Except in Monadnocks the rocks are deeply weathered; the accumulation of soil and residual weathered material (saprolite) is as much as 120 feet thick.
The stream network and the other landforms in the Piedmont are located with little direct relationship to the types or structures of the rocks on which they are formed. The drainage system cuts across belts of granite, schist, and slate with no distinct change in pattern; similarly, the land surface is rarely in accord with beds of rock that lie under it.
Figure 6A. Physiographic provinces of the Appalachian Region (from Fenneman and Johnson, 1946, map).
Figure 6B. Landforms of the Appalachian Region (from Raisz, 1957).
The easternmost range of mountains in the Appalachian Highland is the Blue Ridge. These mountains rise abruptly from the Piedmont on the east and terminate with equal abruptness in the Great Valley to the west.
In Maryland the Blue Ridge consists of two parallel even-crested ridges rising to about 2,200 feet altitude. South of the Potomac the Blue Ridge continues as two parallel ridges; however, only the western one is within Appalachia. Angles of slope along the ridges are as much as 30°. Relief is 700-1,200 feet. The ridges are covered with forests, and on many parts of their west flanks large boulder fields extend from summits to valley floors. The valley separating the ridges is 1-2 miles wide and its surface is gently rolling.
Throughout most of Virginia the Blue Ridge is east of the borders of Appalachia. In this section it consists of a main high broad ridge with low peaks flanked by a broad band of spurs and foothills. Altitudes of the ridge crest range from 3,000 to slightly more than 5,000 feet; the spurs and foothills are 1,000-2,000 feet lower. Slopes on the flanks of the ridge and on the spurs are 30°-45°, and rocky cliffs as high as 400 feet are common. Most of the ridge and spurs are forested, but at high elevations open meadows interrupt the forest cover. A small part of the west flank of this part of the Blue Ridge is in Appalachia in Botetourt County, Va.
The Blue Ridge province is within Appalachia in southern Virginia, Tennessee, the Carolinas, and Georgia. In the Great Smoky Mountains in eastern Tennessee the province increases to a maximum width of 70 miles, and has 46 peaks and 41 miles of divide (6,400 acres) above the 6,000-foot level and 288 peaks and 300 miles of divide (65,000 acres) at 5,000-6,000 feet (Secretary of Agriculture, 1902, p. 20; Ayres and Ashe, 1905, p. 14). The main ridge is along the east side of the province; it is flanked on the west by many subordinate parallel ridges and spurs. On the east, several mountain masses form outliers close to the main ridge.
Although of great height, the Great Smoky Mountains largely consist of subdued landforms. Crags, bare cliffs, and talus slopes are rather scarce. Summits are rounded and high domes are common, but sharp peaks are conspicuously absent. Forests cover most ridges and spurs but rounded grass-covered summits, called balds, are present on a few high domes (Wells, 1937).
Included within the Blue Ridge province in the Great Smokies are broad flat-floored oval valleys, known as coves, that cover 5-10 square miles. The valleys are on limestone, the same as exposed in the Great Valley to the west. The coves are 1,200-1,800 feet in altitude and are surrounded by mountains several thousand feet higher.
The rocks in the Blue Ridge province are dominantly metamorphic and consist of greenstone (metabasalt), schist, and slate. Granite and other igneous rocks also are found throughout the area. In Maryland and West Virginia and in the western part of the Great Smokies, conglomerate, quartzite, and other bedded rocks are present. Where the bedded sedimentary rocks occur, the ridges reflect differences in resistance of various rocks to weathering. Elsewhere the direction of the major ridges and minor divides show little relation to the structure of the bedrock.
West of the Blue Ridge a long narrow belt of mountains and parallel valleys, the Valley and Ridge province, forms a major part of the Appalachian Highlands. This mountainous belt is 1,200 miles long and is 80 miles wide in central Pennsylvania, 65 miles wide in Maryland, and 40 miles wide or less in Tennessee. The landforms are very closely related to the lithology and structure of the folded bedrock; the province is also known as the folded Appalachians (Fenneman, 1938, p. 122). A characteristic of the entire province is the trellislike pattern of the streams, which occupy valleys carved from less resistant belts of rocks.
Along the eastern part of the Valley and Ridge province is the Great Valley, a rolling lowland formed on highly folded limestone, dolomite, and shale. Locally the valley is known as the Hagerstown Valley (Maryland) and Shenandoah Valley (West Virginia and Virginia). Although this lowland extends throughout the length of the province, it is outside Appalachia in Pennsylvania and northern Virginia.
In Maryland and West Virginia the Great Valley is 20 miles wide and is 50-800 feet above sea level. Slopes are gentle and relief is about 50-100 feet except where the Potomac River cuts across the valley. The river is bordered by steep slopes and cliffs extending 100-150 feet below the main level of the valley. Discontinuous short low ridges parallel the axis of the valley. Sinkholes, some as much as 300 feet in diameter and 10-30 feet deep, are a common part of the landscape that has developed on limestone, and in many areas the land surface is billowy where numerous shallow sinkholes coalesce. Underground the drainage is extensive, and only trunk streams and major tributaries have water in them the year around.
From Roanoke, Va., southwest for 100 miles, the continuity of the Great Valley is disrupted by folds and faults. In this region the sandstone ridges that generally occur only in the western part of the Valley and Ridge Province are present in many places as far east as the Blue Ridge, and the valley is reduced to a series of narrow valleys connecting broader areas of lowland.
South of Roanoke in southern Virginia and through Tennessee, Georgia, and Alabama, the Great Valley is a lowland 800-2,500 feet above sea level, in which are parallel ridges and knobs of resistant limestone and sandstone rising to several hundred feet above the general level of the valley. Karst topography is more conspicuous here than farther north. Steep-sided sinkholes, some more than 500 feet in diameter, are abundant. In the region of Maryville, Tenn., southeast of Knoxville, the Great Valley is a broad undulating lowland as shown in figure 7, underlain principally by shale. It is bordered on the east by the Chilhowee Mountains, a rugged area of basal Cambrian clastic rocks.
Figure 7. Radar image of the Great Valley and Chilhowee
Mountain, Tenn.
Tennessee River southwest of Knoxville in upper
left;Chilhowee Mountain,
Great Smokies in lower right. Scale approximately 1:200,000
(distorted)
Along the east side of the Great Valley, abutting the lower flanks of the Blue Ridge, are extensive alluvial-fan deposits that have been dissected and reworked into terraces. Terraces a mile or two in width rise 300-700 feet above the level of the valley. Some deposits forming the terraces are more than 300 feet thick and consist of masses of residual clay overlain by sand and gravel. Similar deposits occur irregularly along the western side of the Great Valley.
The mountainous area west of the Great Valley consists of alternate long narrow even-crested ridges and narrow valleys that have been etched by erosion of alternating sequences of hard and soft rocks (fig. 8). As a general rule the major ridges are formed by prominent quartzitic sandstone that is prevalent throughout most of the Valley and ridge province. The Tuscarora Sandstone and its equivalent, the Clinch Sandstone, along with other closely associated sandstone formations such as the Keefer Sandstone are at or near the base of the Silurian and make the majority of high ridges. The Pocono Formation at the base of the Mississippian and the Pottsville Formation in the lower part of the Pennsylvanian consist of hard sandstone and conglomerate that form the backbone of other high ridges. Subordinate ridges consist of the Oriskany sandstone (Lower Devonian) and various sandstones of the Upper Devonian. In central West Virginia and parts of Pennsylvania and Virginia, Middle Devonian limestone also forms ridges equal in size to those of the Oriskany.
Figure 8. Radar image showing typical Valley and Ridge
topography
at Kingston, Tenn. Watts Bar Damon Tennessee river in lower
left;
city of Loudon in lower right. Scale about 1:280,000
(distorted)
The valleys and lower flanks of major ridges are underlain by shale and limestone. Valley floors contain rolling hills. Hills formed of shale are generally well rounded and smooth; those formed of limestone are somewhat irregular. The valleys are generally cultivated and have little woodland, whereas most major ridges and more than half of the subordinate ridges are forested.
The ridges in the northern part of Appalachia are narrow and have very level crestlines that extend for miles between major stream crossings (water gaps). They have only a few wind gaps cutting slightly below their crests. Adjacent ridges are more or less parallel, although the system has numerous sharp zigzags, and many ridges unite to form very intricate patterns (Ashley, 1933, p. 12). The zigzags have formed where resistant beds wrap around the axes of plunging anticlines and synclines. Wind gaps are more numerous and larger north of the Potomac river than they are to the south. In the southern part of Appalachia the crests of ridges are less uniform, having alternating broad swales and low domes. Throughout all of Appalachia the subordinate ridges are cut by many streams, but the intervening crests are relatively smooth.
In the northern part of Appalachia the major ridges are 1,300-4,500 feet in altitude, with relief as much as 2,500 feet. The subordinate ridges are 1,000-1,500 feet lower in altitude and correspondingly lower in relief. The flanks of major ridges directly reflect the attitudes of stratified rocks, and slopes range from 15° to 45°. Cliffs along the strike fronts of ridges of quartzitic sandstone are extensive and are 30-150 feet high. Steep slopes and cliffs as much as 1,500 feet high are present in most water gaps cutting major ridges. Where the hard sandstone beds are vertical, spines of rocks rise 50-700 feet along the axes of ridges.
Extensive fields of talus and boulders devoid of vegetation lie along the flanks of major ridges, and in many areas the boulders, piled to a depth of 30 feet or more, form large rubble fields. These fields are extensive in Pennsylvania, but are less common southward and are rare south of Roanoke, Va.
In the southern part of the Valley and Ridge province within Appalachia, the major ridges are closely parallel and have fewer zigzags than do those to the north. The resistant rock layers have been brought to the surface repeatedly by extensive faulting. Altitudes are as much as 4,700 feet in the Roanoke area, but drop to 1,000 feet or less in the vicinity of Birmingham, Ala. Relief is 2,500 feet near Roanoke and drops to 500 feet in Alabama. Flanks of ridges are steep and are forest covered. Subordinate ridges are 400-1,000 feet lower than major ridges and are disrupted by many water gaps. Most of the subordinate ridges have woodlands on their summits and cultivated fields and pastures on their flanks.
Immediately west of and in sharp contrast with the Valley and Ridge province is a high upland dissected by many streams, the Appalachian Plateau province (fig. 9). The contrast in landforms is a result of the abrupt change in rock structure, from highly folded in the Valley and Ridge to nearly horizontal or very gently dipping strata in the Appalachian Plateaus. There is a major change in stream patterns, the linear valleys and trellislike pattern of streams in the Valley and Ridge province giving way to incised wandering stream courses with dendritic pattern.
Figure 9. Radar image of the dissected Cumberland Plateau
in Grundy County, Tenn.;
Collins River in left center of picture. Scale approximately
1:180,000 (distorted)
The eastern edge of the Appalachian Plateaus province is an escarpment that rises abruptly 1,000-3,000 feet above a valley that bounds it on the east. Maximum altitudes at this front are generally several hundred feet greater than those of the ridges in the Valley and Ridge province immediately to the east. This eastern ridge is the highest part of the Appalachian Plateaus. In Pennsylvania, altitudes range from 1,750 to about 3,000 feet, increasing to 4,800 feet in central West Virginia, and diminishing to 3,000 feet or less in Tennessee and to less than 1,000 feet in Alabama. Westward the altitude of the upland decreases to 900 feet in southwestern Ohio, 1,100-2,000 feet in east-central Kentucky and Tennessee, and 500 feet in northwestern Alabama.
Relief in the western part of the Appalachian Plateaus in central and southwest Pennsylvania, West Virginia, eastern Kentucky, and northern Tennessee is 500-1,500 feet. This area is carved into small steep-sided hills. Slopes of 45° are common, and steeper slopes, some vertical, prevail along the major rivers. The extreme dissection, high relief, and steep slopes have made it very difficult to build roads and railroads in this area.
The Allegheny Mountains (Ashley, 1933, p. 8), in the eastern part of the province in southern Pennsylvania, western Maryland, and northern West Virginia, are two broad low parallel ridges localized along gentle folds disrupting the normal westward dip of the stratified rocks. These ridges rise 500-1,200 feet above the upland. Their flanks slope 15° or less, and the crests are broad and level. A similar fold near and parallel to the front of the plateau has been eroded to form the broad lowlands of the Sequatchie Valley in central and southern Tennessee.
In northeastern and northwestern Pennsylvania and southern New York the upland surface has been glaciated. The glaciated section is at altitudes of 1,200-2,000 feet, and relief is generally 300-400 feet, although major valleys cutting it are as much as 800 feet deep. Glaciation rounded the ridges and filled major valleys with thick deposits of sand and gravel. As the result, old drainage systems were disrupted and extensive swamps and bogs developed.
Most of the Appalachian Plateaus province is underlain by rocks of Mississippian and Pennsylvanian age except in southwest Pennsylvania, northwest West Virginia, and adjacent parts of Ohio, where Permian rocks are present (plate 2). The rock formations typically are repetitious sequences of coal, shale, limestone, and sandstone.
Extensive karst areas occur where limestone is at the surface, as in parts of Kentucky, Tennessee, and southeastern West Virginia (fig. 16, p. 81).
In southwestern Ohio, central Kentucky, Tennessee, and northwestern Alabama, the Appalachian Plateaus grade into the Interior Low Plateaus province. The actual boundary between these provinces is a low west-facing scarp formed by the Pennsylvanian rocks. West of this low scarp are nearly flat-lying shale and limestone of Mississippian age. The large segment of the Interior Low Plateaus in Appalachia is the Highland Rim. Altitudes are 1,100-1300 feet in Kentucky and 700-1,100 feet in southern Tennessee and northwestern Alabama. Relief is low, exceeding 200 feet only along the valleys of major rivers. The upland is rolling and along the eastern edge has knobs that are capped by Pennsylvanian rocks, outliers of the Appalachian Plateaus. Karst landscape with deep sinkholes, long blind valleys, and large caves are extensive in the areas underlain by limestone (fig. 19).
In north-central Kentucky north of Lincoln County, Appalachia laps over a small part of the Lexington Plain, and in Carthage County, Tenn., it includes a very small part of the Nashville Basin. These sections are underlain by nearly horizontal beds of lower and middle Paleozoic rocks, dominantly limestone and shale. The region has many conical knobs that stand as isolated hills separated by wide, flat valleys and lowlands. The knobs reach altitudes of 1,400 feet and have relief of 500-200 feet. The soil in the Appalachian part of the Lexington Plain and Nashville Basin is thin and poor.
Central Lowland and Coastal Plain
In southwestern Ohio, a small area of Appalachia lies within the Central Lowland province. The strata underlying the area are continuous with those of the adjacent Interior Low Plateaus but are covered with a thin mantle of glacial deposits. Soils tend to be thicker here than in the adjacent provinces and relief more subdued.
Along Lake Erie in New York and Pennsylvania a distinct low scarp separates the Appalachian Plateaus from the narrow, terraced plain of the Central Lowland province which lies along the lake at an altitude of 575 feet with terrace levels as high as 780 feet.
The western and southern extremes of Appalachia in Alabama are within the inner part of the Coastal Plain province, known as the Fall Line Hills (Fenneman, 1938, p. 67). The hills are in a belt as much as 50 miles wide and are formed of Cretaceous sand. In northwestern Alabama, altitudes are more than 700 feet but southward they decrease to about 300 feet. The hills are bounded by many valleys 100-200 feet deep; in the central part of the belt, in western Alabama, valleys are 250 feet deep along major streams. Slopes as steep as 45° are common along valley sides and 5°-15° on the upland.
Because of the relief, many valleys cut through the Cretaceous deposits and expose deeply weathered Mississippian and Pennsylvanian sandstone, shale, and limestone in western Alabama and older Paleozoic rocks in the south-central part of the state.
Except for the geologic formations present the Fall Line Hills bear closer resemblance to the adjacent mountain and plateau areas than to typical parts of Coastal Plain adjacent to other parts of Appalachia, and most inhabitants of the region do not designate their region as Coastal Plain. The hills are mainly in woodland because of poor soil, rugged relief, and steep slopes.
The pattern of landforms in the several physiographic provinces of Appalachia reflects a long geologic history. At the close of the Paleozoic Era the Appalachians were probably a high, massive mountain range. Erosion slowly cut away the weak rocks and formed the valleys, while the strong rocks remained as ridges. Because the rock strata are deformed, the location of ridge lines changed as lower parts of the inclined beds of rock were exposed. Since completion of major deformation and throughout most of its erosional history the Appalachian Highlands province has resembled the present landscape.
The origin of the even, concordant ridge tops and the wind and water gaps has been a challenge for geomorphologists studying Appalachia. One school of thought reflects classical theories developed by W. M. Davis (1889, 1890), which envisioned the Appalachian Highlands as being eroded to a near sea-level plain (peneplain) by the end of the Cretaceous. Rivers on this plain established southeastward courses unrelated to the bedrock structure. Subsequently the region was uplifted and the main rivers became entrenched. Erosion by tributary streams resulted in the etching of resistant beds into high-standing ridges. Several subsequent periods of widespread leveling in the early Tertiary were believed to have produced the concordant levels of subordinate ridges.
With the accumulation of geologic evidence over the last half century the concept of peneplains has been questioned, and some geomorphologists believe that the topography of Appalachia reflects systematic and continuous erosion by streams developing headwards from the Atlantic side of the highlands (Hack, 1965, p. 64-65).
In more recent times, Pleistocene glaciers covered much of the plateaus in New York, northern Pennsylvania, and Ohio. Landforms were smoothed by the ice, and valleys were widened and filled with morainal and outwash material. Repeated freezing and thawing accompanying the cold climate during glacial times caused rapid mechanical weathering of hard rocks, the broken material forming large fields of rubble. During the same period, large fans formed along the foot of major mountain ridges, and many river terraces were cut as streams eroded their older flood plains.
Appalachia is noted as an area containing large and beautiful caves in the widely distributed limestone of the region. Almost 2,900 caves have been recorded. The most extensive cavern area in the eastern United States lies along the Great Valley (fig. 10); altogether about 1,500 caves are known in the Great Valley of Maryland, West Virginia, southern Virginia, Tennessee, and Alabama. To the west, in the mountainous part of the Valley and Ridge province, several hundred caves are found in many belts of limestone, and about 700 are known in the Appalachian Plateaus and the Interior Low Plateaus provinces. Contrary to popular opinion, most Appalachian caves are small; fewer than 300 are recorded as exceeding 500 feet in length. Very few of the caves contain stalactites, flowstone, and similar ornamentations. Large, well-ornamented caves number less than 50 in Appalachia, and barren or sparsely ornamented cavern passages are the rule.
Figure 10. Caves in the Appalachian Region.
Caves form as soluble material is removed from rocks by moving fluids. Movement of the fluids may be by slow percolation until a solution channel forms, usually along fractures, joints, or especially favorable horizons; once formed, the solution channel may continue to enlarge to form passageways. Slow percolation of small amounts of moisture toward the openings continues to dissolve mineral matter, and under some conditions this material may precipitate in a variety of forms collectively called speleothems.
Most large caves of the region are south of central West Virginia and Virginia. Deep pits and caves of great vertical extent are found primarily in southern Appalachia. The deepest is Bull Cave, Blount County, Tenn., that has interconnected opening over a vertical distance of 680 feet.
Most caves in the Valley and Ridge province consist of a few simple, rather straight passages. In a few caves the pattern is similar to city streets, consisting of a grid of numerous intersecting passages. About 20 percent of the caves in the Valley and Ridge province have two or more distinct interconnected levels. In the flat-lying rocks of the Appalachian Plateaus and Interior Low Plateaus, caves are generally on one level and consist of branching passages or a maze of intersecting passages. Passages in most caves are large enough to permit walking but vary in height and width, from low crawlways to galleries 100 feet or more high and wide. Abrupt changes in size of passageways are common.
Most caves are damp and have abundant water seeps and standing pools. About a fifth have streams, many of which fluctuate greatly in amount of flow; flooding of passages occurs frequently.
Within Appalachia, some 33 caves have been developed as tourist attractions, and most of these are now operated as commercial enterprises. Distribution of these caves is shown in figure 10l Between 250,00 and 500,000 visitors are estimated to spend about $1 million annually for admission fees. Successful operation of caves as tourist attractions seems to depend on three principal factors - good location, adequate provisions for safe passage for visitors to the cave, and an attractive cave.
Several additional caves in Appalachia appear to warrant consideration for commercial development (fig. 10). The potential for successful development depends, however, on a great increase in tourist travel throughout the region. Many of the present commercial caves are marginal operations and at least a three-fold increase in tourism is apparently needed if most or all the additional caves are to be developed.
In the last 20 years the sport of spelunking, the exploration of caves for recreation, has increased greatly until now an estimated 10,000 visits per year are made to undeveloped caves in Appalachia. This number will continue to grow and will probably double in the next 10 years.
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