de Chelly Sandstone

de Chelly sandstone

The de Chelly sandstone is not the usual horizontally deposited sandstone. It is of a type called crossbedded sandstone. This means that there are many depositional surfaces, highly inclined to the horizontal. This deep layer is composed of many, many wedges, with steeply dipping (angles greater than 30 degrees).

It is believed that this structure indicates a windblown origin for the rock layers. The cross stratification seen is typical of windblown dune deposits. Such wind deposited strata attaining a depth of 50 feet are forming today (Great Sand Dunes National Monument, the dune field in Death Valley National Monument, the Kelso dunes in the Mojave desert). The sand is blown up the relatively gently inclined windward slope of the dunes and deposited on the leeward side of the dune. When the slope there exceeds the angle of repose, avalanches take the excess sand downward and restore the equilibrium situation, which is an angle of approximately 34 degrees.

If this interpretation of the bedding structure in the de Chelly sandstone is correct, then the direction of the prevailing winds can be determined fthe spatial orientation of the steep cross bedding seen in the exposed rock layers. Studies of this structure indicate that this sand came from the north and northeast directions, from the Ancestral Rockies (the Uncompahgre uplift in Utah and Colorado). These windblown sands (as opposed to the water deposited sands of the Cutler red beds) filled in the basin south of Canyonlands and east of The Monument Upwarp.

If, like me, you are confused by the terms used to designate geologic time, a chart, based upon one given in Roadside Geology of Arizona by H. Chronic is available.

In Association with


Basin and Range, John McPhee, Noonday Press.
The Colorado Plateau : A Geologic History, Donald L. Baars, Univ of New Mexico Press.
Navajo Country : A Geology and Natural History of the Four Corners Region, Donald Baars, Univ. New Mexico Press.
Roadside Geology of Arizona, Halka Chronic, Mountain Press.

In Association with

© 1994 Karen M. Strom

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