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Chapter 10: Virginia's Physiographic Provinces Difficulty Level: At Grade Created by: Melinda Mericle GeoScience (Earth Science) Supplemental Textbook Read Resources Details A physiographic province is a landform region, an area delineated according to similar terrain that has been shaped by a common geologic history. Geographers and geomorphologists recognize more than 20 physiographic provinces in North America; Virginia intersects five of these. Each province is characterized overall by its elevation, relief, lithology, and geologic structure. As a consequence of the region's history of rock formation, deformation, and erosion, specific types of landforms or other geologic features may be associated with a given province. It is often possible to subdivide the provinces into subregions on the basis of the distribution pattern of these features. Virginia's five physiographic provinces [Figure1] [Figure2] [Figure3] Created by SLW, January 1997, last updated by slw January 2005. Chapter Outline Chapter Summary Atlantic Coastal Plain [Figure4] Virginia Beach coastal shore[Figure5] Fall Line [Figure6] The Coastal Plain is the youngest of Virginia's physiographic provinces, its rocks having been deposited after the Atlantic Ocean began to form early in the Mesozoic. Sediments eroded from the Appalachian Highlands were deposited offshore and later consolidated into sedimentary rock. Uplift of the North American continent tilted these beds so that today they dip gently seaward. Where the softer sedimentary rocks of the Coastal Plain abut the more resistant metamorphic rocks of the Piedmont, a low escarpment is visible on the landscape. (See geologic cross- section.) This is the Fall Line, so named because where rivers cross there are rapids or falls. On the Potomac a fairly broad "fall zone" extends from Great Falls downstream nearly to Alexandria. The falls were barriers to ocean-going vessels even in the Colonial Period and hence were to have a major influence on early European settlement patterns in Virginia. Physiographic Subregions of the Coastal Plain The tilted beds of the Coastal Plain extend offshore some 50 to 75 miles to the edge of the North American continent. Distinction may thus be made between that part of the Coastal Plain that is submerged, the continental shelf, and that that is exposed above sea level, merely known as the coastal plain. The continental shelf reaches from sea level (0') to 600 feet below sea level. At that depth the the margin of the continent is abrupt and the "continental slope" plunges down to the ocean floor. The continental shelf has not always been submerged. During the ice ages of the Pleistocene sea level was lowered, exposing the shelf. Streams flowed across the shelf and carved their valleys into it. One such large valley became Chesapeake Bay when sea levels rose again and flooded the shelf. The Coastal Plain proper extends from sea level inland to the Fall Line, where at about 200 feet above sea level it meets the Piedmont Plateau. Although the Coastal Plain is characterized by very low relief, it is not featureless. Several low, wave-cut terraces punctuate the landscape for the careful observer. Piedmont Plateau [Figure7] Rolling Hills of the Piedmont Plateau[Figure8] The surface of the Piedmont Plateau rises in elevation from approximately 200 feet above sea level at its eastern margin (the Fall Line) to a general base level of 1000 feet above sea level in the west. Individual peaks may rise to over 2000 feet in western parts of the province. Relief varies across the province, a fact which has led to the delineation of two subregions. The Outer Piedmont is characterized by low relief; the Inner Piedmont by high relief. The Piedmont is underlain by metamorphic rocks of various origins that werefolded during the Paleozoic as the North American and African plates converged. Later, in the Mesozoic, it was affected by rifting as Pangaea broke apart and the Atlantic Ocean formed. (See geologic crosssection.) Outer Piedmont The Outer Piedmont is a gently rolling upland; erosion and deep weathering have long since obliterated surface indications of the folded bedrock beneath. It is in this subregion that long, narrow rift valleys formed during the Triassic period of the Mesozoic era. Today the valleys-called Triassic lowlands--are filled with sandstones and basalts and can scarcely be distinguished by a casual observer. Inner Piedmont The Inner Piedmont is an area of rugged terrain where erosion has not yet leveled the most resistant of the metamorphic rocks. Softer materials have been worn away leaving a discontinuous belt of mountains, erosional remnants termedmonadnocks. Blue Ridge [Figure9] Blue haze on the mountains[Figure10] Anticline from upward thrusting of rock layer [Figure11] The Blue Ridge is composed of complexly folded and faulted igneous (granitic) andmetamorphic rocks. These rocks date to the PreCambrian and Paleozoic and represent parts of the basement rock of the North American continent. (Seegeologic cross-section.) When Africa and North America converged during the Paleozoic, these rocks were thrust upward and many miles westward over younger rock materials. Today the general surface of the Blue Ridge lies at about 3000 feet above sea level, with many peaks reaching another 1000 to 2000 feet higher. Elevations increase toward the southwest and culminate in Virginia's two highest peaks, Mt Rogers (5,729') and Whitetop (5,520'), located near the state's southern border. The Blue Ridge physiographic province is subdivided at Roanoke Gap into two distinct subregions. Blue Ridge Mountains North of Roanoke Gap, the Blue Ridge occurs as a narrow chain of mountain peaks. Shenandoah National Park lies atop the northern part of the Blue Ridge Mountains.When first set aside in the 1930s, the parkland was deforested and deeply eroded; much of its natural vegetation and wildlife has since been restored to make Shenandoah one of Virginia's main tourist attractions. Blue Ridge Plateau South of Roanoke Gap, the Blue Ridge widens into a high plateau. Scattered mondnocks rise above the general surface level. Mt. Rogers and Whitetop Mt. are on the Blue Ridge Plateau, but, geologically, are not part of it. They are composed of volcanic rocks, the origins of which have yet to be fully understood. Valley and Ridge [Figure12] Shenandoah Valley [Figure13] Limestone Outcrops [Figure14] The Valley and Ridge province has developed on thick, folded beds of sedimentary rock deposited during the Paleozoic. The long axes of the folded control the shapes and orientations of a series of long, narrow parallel ridges and intervening valleys. The differing degrees of resistance to erosion of the sandstones, shales, and carbonate rocks comprising the lithology determine local relief. In general, the more resistant sandstones cap the ridgetops, protecting softer bedrock below from erosion; limestones and other carbonate rocks form the lowlands and valleys. (seegeologic cross-section.) The Valley and Ridge is subdivided into two subregions, the Valley of Virginia and the Allegheny Mountains. The Valley of Virginia is the regional name for what is actually part of one of the major landform features of eastern North America, the Great Valley, which stretches from New York state southwest to Alabama. Underlain primarily by carbonate rocks, the Valley of Virginia is a region of karst. Solution of the carbonate bedrock has created sinkholes on the surface and many caves and large caverns beneath the surface. The Valley has been an important north-south route of travel since prehistory. The Valley of Virginia can itself be subdivided into a string of river basins. From northeast to southwest, one encounters the Shenandoah Valley, the James River Valley, The Roanoke Valley, the New River Valley, and the Holston Valley. Elevations vary from basin to basin. The highest divide occurs near Rural Retreat in Wythe County; waters flowing north to the New River or south to the Holston system separate at about 2400 feet above sea level. The valley floor slopes in both directions away from the New/Holston divide. The Holston leaves Virginia at an elevation of about 1400'; the Shenandoah crosses Virginia's northern boundary at about 400 feet above sea level. Throughout the Valley of Virginia, hills and ridges rise above the general elevation of the valley floor. Massanutten Mt., a 50-mile long folded mountain in the middle of the Shenandoah Valley section, attains an elevation slightly higher than 2000 feet above sea level. Allegheny Mountains The Alleghany Mountains are a series of long, parallel folded mountains with narrow valleys in between. Elevations of ridgelines range from about 3000 above sea level in the northern part of the subregion to about 4000' above sea level in the south. The highest peak in the Alleghenies is Beartown Mountain (Tazewell County) at 4700'. Beartown forms part of the rim of a large, bowl-shaped valley, Burkes Garden. This landform is conspicuous in satellite imagery and is the result of erosion of an upward bending fold. Appalachian Plateau [Figure15] Appalachian Plateau [Figure16] Coal in mountain. [Figure17] Only a very small portion of Virginia extends onto the Appalachian Plateaus, a surface underlain by the same Paleozoic sedimentary rocks as the Valley and Rdige. The difference is that in the Plateaus physiographic province, these rocks have not been deformed and still occur today in horizontal beds. See geologic cross-section. The average elevation of the plateau surface in Virginia is between 2000 and 2500 feet above sea level. Elevations decrease westward across West Virginia, Kentucky and Ohio. The Appalachian Plateaus are only structurally a plateau. The ancient surface has been eroded by stream action over millions of years into what is today a region of high of relief. Small, narrow valleys (or hollows) twist through the resulting mountains. The older surface is evident in the pattern of hilltops all tending to reach the same elevation. Such an eroded plateau is known as a dissected plateau. The upturned edge of the Appalachian Plateaus, where the horizontal beds of the plateau give way to the folded beds of the Valley and Ridge, is observable in certain places. This feature is called the Allegheny Front. Physiographic Subregions of the Appalachian Plateaus The Appalachian Plateaus are composed of two major plateaus, the Allegheny Plateau and the Cumberland Plateau. This separation has no significance in Virginia's physiography, since all of Virginia that lies in the province is in theCumberland Plateau subregion. A major distinction between the two plateaus is that the northern Allegheny surface was glaciated during the Pleistocene; the southern Cumberland surface was not. The border between the two lies near the New River in West Virginia. Image Attributions 1. 2. 3. 4. [1]^ License: CC BY-NC 3.0 [2]^ License: CC BY-NC 3.0 [3]^ License: CC BY-NC 3.0 [4]^ License: CC BY-NC 3.0 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. [5]^ Credit: CK-12 Foundation; License: CC BY-NC 3.0 [6]^ Credit: CK-12 Foundation; License: CC BY-NC 3.0 [7]^ License: CC BY-NC 3.0 [8]^ Credit: CK-12 Foundation; License: CC BY-NC 3.0 [9]^ License: CC BY-NC 3.0 [10]^ Credit: CK-12 Foundation; License: CC BY-NC 3.0 [11]^ Credit: CK-12 Foundation; License: CC BY-NC 3.0 [12]^ License: CC BY-NC 3.0 [13]^ Credit: CK-12 Foundation; License: CC BY-NC 3.0 [14]^ Credit: CK-12 Foundation; License: CC BY-NC 3.0 [15]^ License: CC BY-NC 3.0 [16]^ Credit: CK-12 Foundation; License: CC BY-NC 3.0 [17]^ Credit: CK-12 Foundation; License: CC BY-NC 3.0