Queenston clastic wedge. Graptolite bearing flysch unconformable overlying Chopawamsic arc volcanics and batholiths. Mafic volcanics cut by granite plutons at mya, plus flysch deposits. Probably created by a west dipping subduction. Deposition of eastward thickening wedge of tidal flat and shelf carbonates as rifted margin cools and subsides. Rifts are filled. Michelle Stanislaus and her classmates from their Historical Geology class and Government class ran the petition for this law starting […].
See the Featured Images Map. Virginia is not as flat as a pancake today, despite million years of rain and wind and gravity. Some ridges have resisted erosion, and "differential erosion" has created a low Shenandoah Valley adjacent to a high Blue Ridge. All those years of erosion should have worn Virginia down to a flat peneplain, but the key reason we have ridges today is recent uplift.
Tectonic forces squeeze the North American Plate as it moves, creating stresses that might be pushing some rocks up.
More significantly, the deeply-buried "roots" of the ancient Appalachians continue to float upward as erosion lowers the weight of the surface deposits above those roots. In addition, parts of Virginia are moving up or down in response to the very recent disappearance of the Laurentide ice sheet. The last ice age ended only 18, years ago.
The earth's crust in Pennsylvania is still rebounding upwards after the removal of the heavy weight of ice, and that rebound may be flexing the crust around Norfolk so it moves down. The downward motion is substantial enough to increase the threat of flooding as sea level rises. To understand why there are individual ridges in some places and valleys in others, it helps to know the history of three major mountain-building events when chunks of silica-rich crust - even an entire continent - collided with Virginia.
Since the locations of towns, highways, even soccer fields is determined in part by the geology of Virginia, it helps to understand the bedrock and the topography if you want to understand the people and the culture of the state. Could we have mountain music or "mountain dew" moonshine whiskey without mountains?
Also in this history we try to reconstruct what Virginia may have been like, what the environments were like, what the climate was, during each of the stages. Theoretically the simplest model for Virginia's geologic history is the Wilson cycle, the opening and closing of an ocean basin. This cycle is implicit in the pages to follow, but the cycle is not dealt with directly.
To get a quick review go to a single page summary of the Wilson cycle. For a more thorough discussion go to this link.
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