Spider Rock on a day after a winter snowstorm on the southern Colorado Plateau.
Spider Rock is a pinnacle in Canyon de Chelly.
The canyon is carved into the Defiance Plateau, a broad, upland surface covered with juniper and pinyon pine in northwestern Arizona (within the greater Colorado Plateau). The Defiance Plateau has been a structural high throughout much of Paleozoic time, so many of the older rock formations exposed in the Grand Canyon or surrounding regions were either not deposited or preserved in the Canyon de Chelly region (Rascoe and Baars, 1972). The Defiance Plateau was also uplifted during regional tectonism of the Laramide Orogeny at the end of the Cretaceous Period and probably lasting into Paleocene to Eocene time (Cather, 2003).
The caprock along the canyon consists of a comparatively thin unit of Shinarump Conglomerate of Late Triassic age. The Shinarump Conglomerate consists of stream deposited sand and gravel. Younger strata that once covered the region has eroded away.
The massive, high cliffs that form the walls of the canyon are De Chelly Sandstone. The De Chelly Sandstone also crops out throughout the Monument Valley area nearby on the Navajo Reservation. The De Chelly Sandstone consists of sand deposited in dunes in a subtropical to arid environment in Early Permian time. Large-scale cross beds of sandstone are exposed throughout the canyon. These beds have been interpreted as representing a large sand sea (erg) deposit that existed in the Defiance Plateau region when the De Chelly Sandstone was deposited (Stanesco, 1991; Stanesco and others, 2000).
The oldest (and lowest) sedimentary rock formation in the canyon is the Organ Rock Formation, deposited in Early Permian time when the Defiance Plateau region was a low, coastal floodplain setting. Today, these dominantly reddish-brown mudstone and fine sandstone layers are locally exposed along the base of the canyon. Rocks at the base of the Organ Rock Formation have been correlated with the Hermit Shale in the Grand Canyon (Blakey and Baars, 1987), whereas rocks of the De Chelly Sandstone have been interpreted to intertongue with layers equivalent to the Supai Formation in the Grand Canyon (Stanesco, 1991).
Click here to see a stratigraphic chart for Canyon de Chelly National Monument.
