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Mountain Topography and Geomorphology
The formation of the highly weathered mountain range began with mountain building in the Proterozoic Era that continued in the Paleozoic Era through Permian Period (between about 1 billion and 265 million years ago). Uplift renewed during the Cenozoic Era about 65 million years ago, and the modern landscape began to form. The range now consists of gneiss and schist bedrock formed from the re-crystallization of sedimentary, volcanic, and igneous material. Over time, the geology of the region has become a dissected landscape of rounded peaks and wide concave valleys defined on the east by a steep escarpment rising 1,800 feet over the neighboring section. The ranges consist of low (<2,300 feet), moderate (2,300 – 4,000 feet), and high (4,000 – 6,560 feet) peaks. The range has 82 peaks greater than 5,000 feet and 43 peaks greater than 6,000 feet. Mid-elevation soils are deep, well-drained, and acidic, and are classified as “infertile sandy” or “gravelly loam,” while high-elevation soils typically have high organic content.
The Mountain Resources Commission (MRC) counties lie in two distinct physiographic provinces, the Blue Ridge and Piedmont. Separating these two is the Blue Ridge Escarpment, a steep, highly dissected mountain front that marks the change from the mountainous Blue Ridge province to the lower, rolling topography of the foothills zone of the Piedmont province. Ongoing geologic processes through the millennia shape the terrain and impart the unique characteristics that define our landscape. With the advent of high-resolution digital elevation maps derived from LiDAR (Light Detecting And Ranging), many more landscape features can be seen and accurately mapped than before.
In the Blue Ridge Province, erosion of uplifted mountains of resistant metamorphic and igneous rocks has produced a rugged landscape. The overall southwest to northeast trend of the landscape results from a similar pattern in the underlying rocks and structures imparted during Paleozoic mountain building events. Streams and rivers cut down through the mountains along less resistant rock types, and along faults and fracture zones. A prime example is the long, linear topography along the Brevard fault zone. Many streams that flow northwest or southeast follow trends of post-Paleozoic fracture zones in the bedrock that favor preferential down cutting and erosion by water.
Regolith (soil and highly weathered rock) derived from the underlying bedrock is generally thinner in the Blue Ridge than in the Piedmont. Both provinces contain residual soil derived from the chemical and physical weathering of bedrock, a process that takes as much as 250,000 years to produce three feet of regolith. Furthermore, soil on mountainsides is moved down slope by water and gravity more readily than on the flatter Piedmont slopes. This transported soil is known as colluvium and forms large deposits along many mountain foot slopes. These deep deposits can yield productive forest soils, sources of groundwater, and can also indicate where future landslides may deposit material. Some debris deposits date back to 780,000 years ago, during the Pleistocene Ice Age, and demonstrate the antiquity of some features still present in the modern landscape.
The transition zone between the Blue Ridge and Piedmont provinces is the Blue Ridge Escarpment, an abrupt change in elevation with a vertical relief that ranges from about 1,300 to 2,500 feet. Its upper boundary generally coincides with the Eastern Continental Divide, which separates river systems that flow westward into the Gulf of Mexico from those that flow eastward into the Atlantic Ocean. The origin of the Blue Ridge Escarpment is uncertain, but it appears to be a product of Cenozoic tectonic uplift and the erosive power of streams. Streams flowing eastward to the Atlantic Ocean generally are shorter, straighter, and more energetic than those flowing westward across the Blue Ridge into the Mississippi River system and into the Gulf of Mexico. Consequently, the Escarpment and Eastern Continental Divide are slowly migrating westward by headward erosion. Although the base of the Escarpment coincides with the Brevard fault zone in a few places, the last known activity along the Brevard fault zone likely occurred over 200 million years ago. Other major faults have not been identified that coincide with the escarpment.
The Escarpment influences weather patterns, tourism, and transportation, among others. The region’s highest annual rainfall is in Transylvania County, where weather systems collide with the Escarpment, thereby enhancing rainfall. Higher amounts of rainfall fell along the Escarpment during the storms of July 15-16, 1916, and August 10-17, 1940, that impacted much of the MRC region. The record rainfall for North Carolina of 22 inches in 24 hours fell during the July 15-16, 1916, storm at Alta Pass, near the crest of the Escarpment in Mitchell County. Many waterfalls are present along the Escarpment, including the much-visited Whitewater Falls in Transylvania County, which drops 411 feet in North Carolina and 400 feet in South Carolina, making it the highest waterfall east of the Rocky Mountains. The Old Fort grade on I-40 and the Saluda grade on I-26 are where these Interstate highways cross the Escarpment.
Southeast of the Escarpment, in the foothills zone of the Piedmont, the terrain is generally less rugged, and the depth of weathering of bedrock is generally greater than in the Blue Ridge. The influence of bedrock on the landscape is less prominent than in the Blue Ridge, but is readily apparent in the linear orientations of streams and ridges. In general, transportation routes are easier to construct, and the thicker regolith in the Piedmont means greater storage capacity for groundwater. Conversely, it often means that the depth to hard rock useable for construction is greater. Exceptions to these generalizations are the South Mountains, Brushy Mountains, and the Kings Mountain areas within the Piedmont, where the physiography and soil types are similar to those in the Blue Ridge.
Elevation Map: LIDAR, NC One Map. Accessed from: www.nconemap.com.
Hack, J.T. 1982. Physiographic divisions and differential uplift in the Piedmont and Blue Ridge. U.S. Geological Survey Professional Paper 1265, 49 p.
North Carolina Geological Survey. Accessed from: http://www.geology.enr.state.nc.us/.
Physiographic Provinces and Geomorphic Features Map: County LiDAR DEMs (digital elevation models). Accessed from: http://floodmaps.nc.gov/fmis/Download.aspx.