Amphitheater Valley Heads

Abstract

In an article recently published in the Journal of Geology, Freeman' described the topographic characters and mode of origin of certain valley heads in an anticlinal mountain range in central Montana; these valley heads, because of their similarity to glacial cirques, were called pseudo-cirques. The series of rocks present in the range consists of 2,300 feet of limestone, underlain, in order of position, by 700 feet of shale, 80-100 feet of quartzite conglomerate, and 200 feet of shale. Since no evidences of glaciation have been observed anywhere in the range, the origin of the amphitheater-like valley heads was ascribed to the undermining of the relatively easily eroded shales, and the continuous steepening of the resistant limestone faces by the collapse of unsupported masses of that rock. Freeman considered that certain structural conditions are specially favorable for the production of these great amphitheater valley heads: (i) an anticlinal fold with nearly flat, thick, resistant strata at the top or along the axis of the anticline, and steeply dipping strata along the limbs; (2) a sufficient area of flat-lying beds to permit the development of the amphitheaters; and (3) soft, easily eroded strata underlying the resistant beds. In regions of flat-lying or gently dipping strata, composed of thick, resistant members underlain by thinner beds of weak rocks, the development of amphitheater valley heads takes place if the streams have cut through a resistant layer into the weaker rocks below. The perfection of form and the size of the amphitheaters depend upon various conditions. Of prime importance are the thickness, the number, and the resistance of the harder strata. If more than one thick, resistant bed, separated by weaker layers, be