Download Chapter 10: Virginia`s Physiographic Provinces

Chapter 10: Virginia's Physiographic Provinces
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Difficulty Level: At Grade Created by: Melinda Mericle
GeoScience (Earth Science) Supplemental Textbook
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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.
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