Southern Blue Ridge Topography
The Southern Blue Ridge Province extends from the Roanoke River southwestward into northern Georgia. It is of similar length to the Northern Blue Ridge, but is much broader (as wide as 112 km) and higher. The northwest border of the Southern Blue Ridge follows the outcrop of the uppermost quartzite beds in the Chilhowee Group of Early Cambrian age. The southeast border is an erosional escarpment related only locally to the underlying rocks. From the Georgia-South Carlina border northward, the crest of the escarpment divides the Atlantic and Gulf drainages. Viewed as a whole, the escarpment cuts diagonally across the entire Blue Ridge anticlinorium.
Hack (1982) divided the Southern Blue Ridge into several topographic areas. Some of these areas correspond to distinctive geologic areas, but some do not. In topography the Chilhowee-Walden Creek Belt on the northwestern margin of the province corresponds closely to rock resistance.In itszone elongate mountain ridges and valleys 5 to 50 km wide, underlain by sedimentary and metasedimentary rocks of low metamorphic grade. Long, steep-sided ridges separated by parallel valleys are characteristic of the belt. Northeast of the French Broad River, the quartzite of the Chilhowee Group forms most of the ranges, whereas the valleys are underlain by infolded or faulted carbonate and shale of Cambrian age. Southwest of the French Broad River, metasedimentary late Precambrian rocks of the Walden Creek Group form a belt of intermediate relief between the Chilhowee ridges and the mountain highlands. Some ridges underlain by quartzite of the Chilhowee Group exceed 1,200 m in altitude, and local relief exceeds 750 m Hack (1982).
The largest part of the Southern Blue Ridge Province consists of ranges of high mountains, many of which exceed 1,500 m in elevation. Mount Mitchell, the highest mountain in the Eastern United States, is 2,040 m in elevation. Local relief is generally high, exceeding 900 m. Along the northwest and southeast borders of the Mountain Highlands the ranges tend to be roughly parallel to the northeast regional strike of the rocks. In the center, however, the ranges have various trends that seem to be determined by the complex pattern of deep basins and valleys that cut through the mountains in various directions. Some rock control of topography is evident. The Great Smokies, for example, have many peaks exceeding 1,800 m. The highest part of the range is underlain by massive conglomerate and sandstone of the Thunderhead Formation. The trends of many valleys in the Mountain Highlands parallel the strike of the rocks, and some deep basinswere once covered withreadily weathered rock types. On the other hand, the general impression given by the region does not suggest rock control like that seen in the Ridge and Valley Province or in the Chilhowee-Walden Belt. The intermontane basins are irregular, have angular boundaries, and are often connected with other basins by deep valleys that look like trenches. Valleys tend to be straight and to cut across drainage divides at high angles. They are referred to here as trench valleys. The intersecting pattern of linear valleys and interconnected basins suggests tectonic influences as well as variations in weathering resistance of rock.
Several trenches and basins have features that shed light on their origin. The most impressive of these is the long deep trench that corresponds toan outcrop area of Murphy Marble and Andrews Schist of Cambrian age. These formations are the youngest in a great syncline folded into the rocks of the southwestern part of the Mountain Highlands (MM, in Figure above). The trench is approximately 150 km long and extends through Georgia into North Carolina. In North Carolina the marbleis a continuous outcrop and forms a spectacular trench far below the mountains on either side. The trench culminates in the Nantahala Gorge, whose floor is 450 to 900 m below the mountaintops on either side, and is in places less than 500 m wide. The gorge ends where the marble outcrops end. The trench cuts directly across the main westward-flowing drainage system with little change in altitude, demonstrating that where contrasting rock types occur, rock control of topography is important in the Blue Ridge. The local relief along the trench is as great as the relief in the Appalachian Valley where carbonate rocks abut wide outcrop belts of resistant rocks such as quartzite and massive sandstone (Hack, 1982).
The trenches in the Mountain Highlands result from outcrops of nonresistant rocks or from shear zones that have the same effect as narrow belts of nonresistant rock. Aside from the Murphy Belt, the Brevard zone is the most impressive major feature of this kind. It forms a narrow zone of low relief in an elongated zoneof faults and fault slices that extends from Virginia to Alabama. It has a long geologic history involving several periods of activity, possibly extending as far back as the Precambrian.
Intermontane basinsare thought to have formed when the rivers that drained them were flowing at a lower grade. These basins aregenerallyon the Harrisburg erosional surface (see Thornbury 1965, p. 107). Wright (1931), thought that rock resistance had a strong influence on these basins. Hack (1982) opined that a sill of erosion resistant rock along the streamscould permita low-relief areato migrate up the tributaries and that some such mechanism could have played a role in basin origin. However, he also noted that the basins have patterns and outlines related to rock structure and to trench valleys--conditions that suggesta more complex origin. The Asheville Basin is the largest. Hack (1982) thought that there was no evidence that the landscape of the basin has features caused by dissection of an older surface. He noted that the relief in the basin is significantly lower than that of the mountains around it, and that the broader outlines of the area, at least, are not controlled entirely by rock types. He suggested as a possible explanation that the area is affected by intensive faulting and shearing along many zones.
In the small Mount Rogers area,relief correspondsclosely withrock resistance. Topographic highs, including Mount Rogers (1,750 m) and White Top Mountain (1,630 m), are underlain by porphyritic welded rhyolite tuff, whereas the surrounding lowlands are underlain by weaker metasedimentary rocks. North of this local area are the Iron Mountains (1,410 m), which are underlain by the resistant Chilhowee Group.
The northeastern end of the Southern Blue Ridge is referred to here as the New River Plateau. It has relatively low relief, and generally is graded to the New River in the Great Valley at an elevation of about 610 m. Most of the plateau is less than 900 m in elevation. Excluding areas with low mountains, the average relief is about 200 m, much less than other parts of the Blue Ridge. The streams cross the New River Plateau in sinuous courses, reflecting the low slope. The plateau appears to owe its topography to rock control, as most of the area is underlain by relatively nonresistant thinly layered schist and gneiss of the Ashe Formation, in contrast to the resistant quartzite in the Chilhowee-Walden Creek Belt, the massive gneiss of the Elk Park plutonic group, and the large bodies of massive amphibolite in the Ashe Formation in the Mountain Highlands to the west of the plateau.
The Blue Ridge Escarpment, where the Blue Ridge Province drops off to the Piedmont to the southeast, is one of the most striking topographic features in the Appalachians. It extends southwestward from the Roanoke River in Virginia, to Rossman, NC, on the headwaters of the Keowee River, a major tributary of the Savannah River. The most spectacular section lies northeast of the Catawba Riverheadwaters. Here, the relief is highest and the slope the steepest. The height of the escarpment above the Piedmont ranges from 600 m near Mount Mitchell to about 300 m near Roanoke, VA. The width of the escarpment averages 5.5-7.5 km. As mentioned above, the crest of the escarpment corresponds generally to the Atlantic-Gulf drainage divide.
Many hypotheses have been suggested for the origin of the Blue Ridge Escarpment. One group of hypotheses attributes the escarpment primarily to tectonics. Hayes and Campbell (1894), for example, suggested that it is essentially a monoclinal flexure. White (1950)hypothesized that the scarp was produced by faulting on a series of en echelon faults at the foot of the escarpment. Stose and Stose (1951) criticized this concept. They pointed out that the youngest large fault zone in the region is the Brevard Zone, which is a thrust fault in which the Piedmont, not the Blue Ridge, is on the upthrown side. In addition, the Brevard Zone coincides with the escarpment for only 50 to 60 km. It departs from the escarpment both to the northeast, where it is farther east in the Piedmont, and to the southwest, where it is within the Blue Ridge Mountains. More recent geological mapping also makes it unlikely that faulting is the direct cause of the escarpment.
A second group of hypotheses attributes the escarpment to erosion. The earliest of these was by Davis (1903), who pointed out that the streams on the southeast side of the Blue Ridge flow directly to the Atlantic, whereas those on the northwest side, most of which reach the Gulf of Mexico via the Mississippi River, have a much greater distance to travel. As a result, the streams on the southeast side have a much steeper average gradient, which allows them to erode headward at a faster rate than can those on the northwest side. The effect is to push the divide to the northwest. Hack (1982)noted, however, that there is a problem with the idea that the distance to the sea determines the Escarpment, for the altitude of the base of the Blue Ridge escarpment on the southeast in some places is higher than that of the base of the mountain front on the opposite side of the Blue Ridge Province.
A third group of hypotheses attributes the escarpment to a combination of faulting and erosion. One suggestion is that although faults were involved, the faults were some distance from the present scarp. The discontinuity was maintained as the divide slowly shifted to the northwest (Thornbury 1965, p. 105). Hack (1982) thought that this idea or the monoclinal flexure idea of Hayes and Campbell provided the best explanation.He also suggested that the divide asymmetry represented by the escarpment is caused in part by massive and resistant rocks far downstream in the northwesterly drainage system. These belts of hard rock are responsible for the long and steep reaches in the Blue Ridge and Appalachian Valley, as well as the Appalachian Plateau. They help to maintain the highland northwest of the divide. He also noted that differences in rocks along the escarpment itself cannot be the cause of the steep slopes on the southeast side, because in one segment, the escarpment foot and cresthave nearly homogeneous rocks for more than 150 km.
Hack (1982) argued that although the escarpment does retreat to the northeast, such retreat is very slow and the drainage divide has been approximately where it is for a long time. Reasons supporting this inference include; (1) the divide in the Southern Appalachian Highlands corresponds closely to a topographic high and a negative gravity anomaly, and (2) the major valleys on the escarpment are structurally controlled, suggesting a long history of landscape evolution in which the drainage became established along major structural trends such as shear zones and joint sets. Hack (1982) suggested that the divide is probably not very far from its position in the Mesozoic. On the other hand, the relief of the escarpment is probably the product of a broad late-Cenozoic uplift that provided more energy for erosion. Hack (1982) appealed to work by Ahnert (1970b), mentioned above, to demonstrate the recency of such uplift.
Encyclopedia ID: p1563
