 Stage
M-2 (No cross section) (Next Cross Section) (Previous Cross Section) (Home) Rejuvenation Late Cenozoic; 45 my - present Under Construction
One of the things that makes Virginia's landscape so beautiful and distinctive are the long parallel ridges of nearly the same elevation separated by narrow valleys, and the narrow, steep sided water gaps that slash across those ridges. A little thought shows that the rivers flowing through the water gaps could not have cut those gaps after the mountains formed. To do so the water would have to flow up hill - and then erode down. The conclusion is the rivers existed before the mountains. Similarly, the near equal elevations of all the ridges seems to indicate that rocks underlying all the ridges were at one time eroded down to the same elevation, and that they have all been lifted up 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 peneplain, flat all the way down to sea level, crossed by rivers draining the continent to the west. Furthermore, all this land then underwent gentle uplift (rejuvenation) allowing the streams and rivers to begin to erode and cut their way back down. The ridges are the hard rocks resisting erosion (such as those holding up the Blue Ridge, and Massanutten, Little North, Great North, and Shenandoah mountains), and the valleys softer rock eroding away. The ridges run parallel to each other because they were all folded and faulted the same way during the Alleghenian orogeny.Thus, what we are seeing are the final stages of uplift of the Alleghenian mountains, as the east coast of North America settles into greater and greater tectonic stability. The short term future is that eventually all the east coast will again be peneplained flat.
|
