Gravel as a Resistant Rock
In a recent2 number of the Journal of Geology, Mr. John L. Rich presents the thesis that "gravel, in its relation to the agencies of denudation, is under certain geological conditions a highly resistant rock. To these agencies it will, in general, offer greater resistance than ordinary igneous or sedimentary rocks, with a few possible exceptions."3 The writer is in accord with this general thesis; but the specific evidence presented in arriving at this conclusion is not wholly accurate, and certain deductions affecting the physiographic history of the region should in the writer's opinion be interpreted differently. Mr. Rich divides his paper into three parts: (i) to point out the theoretical reasons for the resistant nature of gravel deposits; (2) to show, from an actual occurrence in nature, that the gravels do behave as the theoretical considerations would lead us to expect; and (3) to sketch by way of suggestion the normal course of development of topography in a region where alluvial fans of coarse material are accumulating at the base of mountains. It is especially with No. 2 that the following has to deal. It is shown that a plain-like lowland lies between the northern edge of the gravel deposits and the high mountains to the north. This lowland is in places ioo feet below the base of the gravel south of it, and is crossed by the mountain streams flowing southward out upon the desert deposits. To explain this feature Mr. Rich presents three alternative hypotheses: (i) The gravels may have been removed by erosion