M2 - Rejuvenation

Late Cenozoic; 45 mya - present
Virginia's landscape is made distinctive by the existence of long parallel ridges of nearly the same elevation separated by narrow valleys, and the narrow, steep-sided water gaps that slash across those ridges. This indicates that rivers existed before the mountains, and the near equal elevations of the parallel ridges indicates that the underlying rocks were at one time eroded down to the same elevation and subsequently underwent uplift at the same time. Both of these lines of evidence lead to the conclusion that, sometime in the Cretaceous or Cenozoic, all of Virginia had been eroded down to another flat, broad peneplain and crossed by rivers draining the continent to the west. The gentle uplift that followed (rejuvenation) allowed the streams and rivers to cut their way back down into the surface. The rocks underlying most ridges are hard rocks that resist erosion while the valleys are underlain by softer rock that erodes easily. The ridges run parallel to each other because they were all folded and faulted into the same orientation during the Alleghenian orogeny.
The final stages of uplift of the Alleghenian Mountains have occurred over the last 45 million years, and today, the eastern coast of North America continues to settle into increased tectonic stability. Eventually all of the East Coast will again be eroded down to a peneplain.
Contributed by Lynn Fichter 
Friday, August 24, 2018
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