Abstract

Three groups of features including columns and column recesses, projecting plates, and arches result from mechanical disintegration of sandstone cliffs in Zion Canyon, Utah. The disintegration is in part due to frost action, runoff from rain showers, seep and stream undercutting, and gravity stresses in the rock masses. Rock slides from the canyon walls appear to occur randomly. Estimates of the quantity of material that could be released by failure of suspended columns and projecting plates were computed from elastic considerations and values of 20 kgm/cm2 and 700 kgm/cm2 taken to be representative of the tensile and crushing strengths of the sandstone.

Arches may originate from seep undercutting or intersection of joints paralleling the canyon wall; both of these factors contribute to the formation of vertical plates subjected to tensile stresses. Examination of simple mechanical models suggests that arches may develop upward as the result of rock failure along curved stress trajectories.

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