Topography and Physiography

The Appalachian Highland is one of eight major physiographic divisions of the United States. The part of this division considered here includes, from southeast to northwest, the Piedmont, Blue Ridge, Ridge and Valley, and the Appalachian Plateau Provinces. The Blue Ridge is separated into the southern and northern sections. The Ridge and Valley is separated into the southern and middle sections. The Appalachian Plateau is separated into the Cumberland and Kanawha sections. Of these, the Blue Ridge contains the highest topography, and the Appalachian Plateau the second highest. The Ridge and Valley is somewhat lower, and the Piedmont is by far the lowest.

Physiographic provinces

The Piedmont Province is a dissected plateau whose inner boundary is the Blue Ridge Province. The outer boundary is the Coastal Plain. The general slope is from the mountains toward the Coastal Plain. The landscape is characterized by hilly relief. Elevations range from a few tens of meters near the Coastal Plain to as much as 600 m near the Blue Ridge.

The Blue Ridge Province is divided into two subprovinces, the Northern Blue Ridge and the Southern Blue Ridge. The Northern Blue Ridge lies north of the Roanoke River and is a narrow range of high mountains that is 395 km long but nowhere exceeds 22 km in width. In its narrowest part it is a single ridge that stands about 365 m above the Great Valley of the Ridge and Valley Province and about 610 m above the Piedmont Lowlands on the east. It is underlain by a sequence of Precambrian and Cambrian rocks that form the northwest limb of the Blue Ridge anticlinorium (Espenshade, 1970). This sequence consists largely of resistant rocks. Only three rock units support the crest of the range: (1) a complex assemblage of medium to coarse granitoid rocks, of which hypersthene granodiorite (Pedlar Formation) is the most resistant; (2) a metavolcanic unit, the Catoctin Formation; and (3) a thick series of quartzite, arkose, and phyllite called the Chilhowee Group ofearly Cambrian in age. The topography of the Northern Blue Ridge is largely due to rock resistance and width of outcrop, although the average altitude of the mountains north of the Potomac River is little more than half that of its southeastern part, despite similar lithology. This decline may be due simply to less uplift in the northern region (Hack, 1982). The Southern Blue Ridge extends from the Roanoke River southwestward into north Georgia.

The Ridge and Valley Province extends from north-central Alabama to beyond the late-Wisconsinan glacial border in Pennsylvania. It is underlain by Paleozoic sedimentary rocks that have been folded and thrust from the southeast. Tight folds characterize the northern part of the province, whereas stacking of thrust sheets characterizes the southern part. The southeastern border of the province is a broad, linear lowland called the Great Valley, but most of the province consists of alternating ridges and valleys. Elevations generally range from 300 to 900 m,but extremes are 120 to 1,300 m (Fenneman, 1938).

Geologic Cross Section

The contrast in bedrock resistance to erosion is greater in the Ridge and Valley than in any other province, with some sandstone and conglomerate units, for example, being many times more resistant than the abundant shale and carbonate rocks. Differential erosion, in which resistant rocks come to form ridges and mountain peaks while easily eroded rocks form valleys,has beenvery prominent in shaping the landscape. Thus, lithology and structure are more dominant in determining the topography than in other province. Certain resistant units repeat themselves and are widespread ridge makers. To the north, these include Pottsville Sandstone of Pennsylvanian age, Pocono Sandstone of Mississippian age, and Tuscarora Sandstone of Silurian age. To the south, such units include Pottsville, Mississippian Fort Payne chert, and Silurian Clinch (equivalent to Tuscarora).

The Appalachian Plateau Province extends from Alabama to beyond the glacial border in Ohio and Pennsylvania, bordering the Ridge and Valley. It differs from the other provinces in several ways:

1. Rocks are dominantly clastic, to include conglomerates, sandstones, and shales, with some interbedded coal. Limestones are uncommon.
2. Strata are mainly Mississippian and Pennsylvanian in age,but some northern areas are underlain by the Dunkard Series of Permian age. Thus, they are generally younger than those of other Appalachian provinces, except for the Mesozoic rocks of the Piedmont rift basins.
3. Rocks have undergone little deformation relative to the other Appalachian provinces; exceptionsare a few gentle folds and thrust faults adjacent to the Ridge and Valley.
4. The Appalachian Plateau is bounded on all sides by outfacing escarpments, reflecting the regional synclinal structure of the Plateau.
5. Altitudes in the Plateau nearly everywhere are higher than those in adjacent provinces.
6. Most of the province, with the exception of that part in Tennessee and Alabama, is highly dissected, and has a higher average slope than any other province. The topography north of Tennessee is a "plateau" only in the sense that the hilltops are concordant and suggest a former flat surface. Near the eastern margin, the plateau is so dissected that the topography commonly is referred to as "mountains" (Thornbury, 1965).