June 30, 2003
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.