Controls on the form and development of rock slopes in fold terrane

Recent studies of the form and development of rock slopes have focused on the properties of materials as geomorphic controls. In the absence of structural controls, rock mass strength has been found to be an important determinant of slope form. In the fold mountains of the southern and southwestern Cape Province, South Africa, rock slopes on quartzite have developed to forms controlled by rock mass strength (strength equilibrium slopes), structure, and denudational processes. Contingency table analysis indicates that the rock properties of greatest significance as determinants of the form of strength equilibrium slopes are intact rock strength and the spacing of joints and other geological discontinuities. On undercut, unbuttressed rock masses and on buttressed (dip slope) forms, parting roughness is a key factor in slope development by virtue of the effect of parting roughness on shear strength. Roughness measurements, tilt tests, and field observation indicate that the effective friction angle of the quartzite is approximately 55°.