ABSTRACT
The spectacular cliffs of the Plateau Country of southeastern Utah generally result from the collapse of massive sandstone caprock due to loss of support following weathering and erosion of subjacent thin-bedded sandstones and shales. However, prior to the exposure of a weaker substrate, major slope breaks develop within the massive sandstone itself, producing near-vertical faces rising above straight or convexo-concave “slick rock” slopes in the same material. Slope segmentation is usually associated with discontinuous horizontal partings in uniform sandstone, with the rock below the parting acting as a surrogate for a weaker substrate. By extrapolation it is suggested that in an arid climate the highest plane of weakness, even if only centimeters in depth (such as that at the base of any caprock), may play a larger role in the differentiation of cliffs and footslopes than does the more obvious contrast between the rock types involved. Vertical faces and slick rock slopes in massive sandstone are discontinuous, succeeding one another both vertically and laterally. Seepage of moisture at high-level partings appears to be a significant but not essential factor in slope segmentation, mainly causing collapse below the seepage plane rather than above it, to create long slick rock slopes below inconspicuous ledges. Slick rock slopes of this type have not been formed recently and may reflect greater moisture infiltration at some time in the past. In massive sandstone with no exposed substrate, scarp forms alternate through space and time as bedding discontinuities appear and die out at80 different levels, so that forms are constantly readjusting to changing physical conditions. The scarp retreat process is spasmodic, and a time-independent steady-state development exists only in the very broadest sense. The advantage of the principle of allometric change as opposed to the equilibrium concept is vividly demonstrated in this landscape.
