Piedmont Topography

Subdivision of Piedmont and
Blue Ridge

The Piedmont Province contains fourtopographic sub-regions, called zones,of which the Piedmont Lowlands Zone is the largest.The Piedmont Lowlands Zone containsfive topographic belts. The rocks in the Piedmont Lowlands Zoneare predominantly feldspathic gneiss and schist intruded by plutons, most of which are granitic. The area also includes metamorphosed sedimentary and volcanic rocks of lower metamorphic grade, which generally are more resistant to erosion. However, there are many exceptions where the latter rocks underlie low relief and the former underlie high relief. In the Carolina Volcanic Slate Belt, for example, feldspathic intrusive as well as volcanic rocks are quite resistant to erosion and generally underlie hilly topography. The Uwharrie Mountains belt, for example, exceed 300 m in altitude and has sharp relief (Hack, 1982).

Resistant rocks of the Kings Mountain Belt include quartzite, kyanite quartz rock, and conglomerate. They form chains of low hills. Crowders Mountain near Gastonia, NC, has a relief of 183 m and rises to an elevation of 460 m. The crest of this mountain, as well as that of Kings Mountain southwest of it, is held up by narrow outcrops of quartzite and conglomerate. The Pine Mountain Belt of Georgia and Alabama has rocks similar to those of the Kings Mountain Belt. Included in the Piedmont Lowlands are several Mesozoic basins formed by rifting when African and North America split apart in that era. Generally the relief of these basins is lower than that of the crystalline rock areas surrounding them because the continental sedimentary rocks associated with the basins tend to be less resistant than the crystalline rocks of the Piedmont.

The Northeastern Highlands Zone somewhat higher relief than that of the Piedmont Lowlands Zone. It extends from northern Virginia to the Delaware River and is bounded on the southeast by the Fall Zone and on the west and north by the Blue Ridge Mountains. The area southeast of the Mesozoic basins in Maryland and southeastern Pennsylvania is relatively high, and several investigators have suggested that it has been subject to late Cenozoic uplift. Other evidence for such uplift include convex-up stream profiles and deep, incised valleys, as well as the nearby presence of the Stafford fault zone which shows Cenozoic fault movement (Mixon and Newell 1977).

The Southwestern Highlands Zoneresembles the Northeastern Highlands in some ways. Both have moderate relief which, in places, is much higher than that of the Piedmont Lowlands. Many local features of the topography in this region seem to be related to rock hardness, but the topography and relief on all rock types become lower closer to the Coastal Plain on the southwest. The general altitude of the landscape also declines to the southeast. The profiles of the Coosa and Tallapoosa Rivers suggest substantial uplift of the area relative to the Coastal Plain in late geologic time.

The Foothill Zone extends from the Potomac River southwestward through the inner Piedmont to near the South Carolina-North Carolina boundary. Its boundary on the northwest is defined by the southeast margin of the Blue Ridge Province. The southeastern boundary is defined approximately by the outer limit of hilly areas and isolated hills within the inner Piedmont and by a slight increase in slope. This subprovince can be divided into three parts.

1. The part north of the Roanoke River consists mostly of small ranges of hills and mountains underlain by resistant rocks and separated by extensive lowlands. The Catoctin Formation, consisting of metamorphosed mafic lava flows, forms many ranges of high hills and seems to be one of the most resistant rocks (Hack 1982).

2. The middle part of this subprovince is between the Roanoke River and the Yadkin River. Here the Foothill Zone is more clearly defined and takes the form of an upland that has a relief uniformly greater than that of the Piedmont to the east. Locally, quartzite forms isolated mountains higher than their surroundings.

3. The third part of this zone lies between the Yadkin River and the South Mountains, where the Foothill Zone ends. Rock type controls the topography to some extent, although to a lesser extent than it does in the Virginia part of the Foothill Zone. For example, the Brushy Mountains and South Mountains have many peaks with altitudes of 750 to 900 m, and are underlain by the more resistant rocks of the region, including a biotite gneiss, an augen gneiss, muscovite and sillimanite schist and minor quartzite and sandstone, and a biotite gneiss unit that contains metasandstone. However, all these "resistant" units also occur in low relief areas. Thus, tectonic processes as well as rock resistance may have played a major role in shaping the topography. Tilting (up to the northwest) is suggested by the greater height of hills near the Blue Ridge than farther out on the Piedmont (Hack 1982).