Ridge-and-valley Appalachians

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Shaded relief map of Cumberland Plateau and Ridge and Valley Appalachians on the Virginia/West Virginia border

The Ridge-and-valley Appalachians are a belt within the Appalachian Mountains extending from northern New Jersey westward into Pennsylvania and southward into Maryland, West Virginia, Virginia, and Tennessee. They form a broad arc between the Piedmont and the Allegheny and Cumberland Plateaus.

These mountains are notable because they form long, even ridges, with long, continuous valleys in between. From a great enough altitude, they look almost like corduroy, except that the widths of the valleys are somewhat variable and ridges sometimes meet in a vee.

These curious formations are the remnants of an ancient fold-and-thrust belt, west of the mountain core that formed in the Alleghenian orogeny.(Stanley, 421-2) Here, strata have been folded westward, and forced over massive thrust faults; there is little metamorphism, and no igneous intrusion.(Stanley, 421-2) The ridges represent the edges of the erosion-resistant strata, and the valleys portray the absence of the more erodable strata. Smaller streams have developed their valleys following the lines of the more easily eroded strata. But a few major rivers, such as the Delaware River, the Susquehanna River, and the Potomac River are evidently older than the present mountains, having cut water gaps that are perpendicular to hard strata ridges. The evidence point to a wearing down of the entire region (the original mountains) to a low level with little relief, so that major rivers were flowing in unconsolodated sediments that were unaffected by the underlying rock structure. Then the region was uplifted slowly enough that the rivers were able to maintain their course, cutting through the ridges as they developed.

Valleys may be synclinal valleys or anticlinal valleys.

These mountains are at their highest development in central Pennsylvania, a phenomenon termed the Pennsylvania climax.

Reference:

Stanley, Steven M. Earth System History. New York: W.H. Freeman and Company, 1999. ISBN 0716728826

Ridge-and-valley Appalachians

Reference:

Significant ridges

See also