The term cliff-former is a geologic term indicating that a unit of bedrock is more resistant to erosion than overlying or underlying units and consequently outcrops have high slope angles. It is more or less equivalent to ridge-former, and may be contrasted with slope-former. In humid environments, sandstones are typically cliff-formers. In arid environments, limestones are often cliff-formers also. Recent extrusive igneous units may be cliff-formers as well.
In the Ridge-and-Valley Appalachians of the eastern United States the major ridge-formers are the Tuscarora, Pocono, and Pottsville Formations. [1]