In a preliminary paper in the American Naturalist for September, 1890,* there was a brief sketch of the phenomena herein described in greater detail and illustrated by photographs reproduced, mechanically (plate 18). These phenomena, consisting of grooves, striæ and potholes, were discovered by the writers in the course of a survey of the Pleistocene formations of the valley of the Beaver river. The description of the particular phenomena in which we are interested is here prefaced by a short account of some of the features and the geological history of this portion of western Pennsylvania.
The Beaver river is formed at Lawrence Junction, on the Pittsburgh, Youngstown and Azhtabula railroad, by the confluence of Mahoning and Shenango rivers, and, after a course of about twenty-one miles in a southerly direction, terminates by emptying into the Ohio river at Beaver. In its course it . . .