Archive - Geological Unconformity

June 28, 2020


Each Sunday we present a notable item from our archives. This EPOD was originally published June 30, 2003.

Provided and copyright by: Tom McGuire
Summary authors & editors: Martin Ruzek; Tom McGuire

An unconformity is a buried erosion surface. This one is located south of Cannonville, Utah. (Thanks to Mr. Bill Lesniak of Sodus HS, NY) The folded siltstones on the bottom were eroded to a horizontal surface before the pebble rich conglomerate on top was deposited, perhaps by streams, meandering over a flat land surface. Uplift of the Colorado Plateau, possibly caused by a descending Pacific plate that turned horizontal, resulted in a massive regional uplift in the past several million years. Among the legacies of this uplift is the Grand Canyon of Arizona.

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