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Spider Rock is a pinnacle in Canyon de Chelley. The oldest (and lowest) sedimentary rock formation in the canyon is the Supai Formation, deposited in Pennsylvanian time (about 280 million years ago) in the era before dinosaurs when the Colorado Plateau region was a hot and humid coastal plain. Today, these dominantly redish-brown mudstone layers are exposed along the base of the canyon. The massive, high cliffs that form the walls of the canyon are De Chelley Sandstone. The De Chelley Sandstone consists of sand deposited in dunes in a subtropical to arid environment in Early Triassic time (about 250 to 230 million years ago). The caprock along the canyon consists of a comparitively thin unit of Shinarump Conglomerate. The Shinarump Conglomerate of the lower Chinle Formation is of latest Triassic to Jurrasic age (around 200 million years old) and consists of stream deposited sand and gravel. Younger strata that once covered the region has long since eroded away. The canyon is carved into the Defiance Plateau, a broad, upland surface covered with juniper and pinyon pine in northwestern Arizona. |
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