220 ROLLIN D. SALISBURY AND ELIOT BLACKWELDER 



details of the topography of this older drift, and in many cases 

 even its larger features, are no longer glacial, but erosional. 

 Any lakes which once existed in its surface seem to have disap- 

 peared long before the advent of the glaciers of the last epoch. 

 This is true at any rate of that part of the drift which lies outside 

 the newer drift, the only part now exposed. The surfaces of 

 the roches moutotinees within the area of the older drift have been 

 weathered into roughness, many of them being angular crags, 

 while deep valleys have been cut into the drift, and lOO to 500 

 feet into the underlying crystalline rock. In most cases no 

 traces of terminal moraines were found at the limit of this series 

 of older drift deposits. From their position, such moraines were 

 the first part of the drift to be removed. Portions of old lateral 

 moraines are, however, preserved in some of the valleys, such as 

 that of North Fork of Clear creek, East Fork of Big Goose 

 creek, and at Penrose park. They have suffered considerably 

 from stream erosion, but the even crest-line and ridge-like char- 

 acter are sometimes still distinct. 



In general, this body of drift has been rendered notably dis- 

 continuous by erosion. The remnants are often so meager that 

 accurate mapping of the outlines of the glaciers which made it is 

 not practicable, though their approximate limits can usually be 

 ascertained. In many places the remnants are restricted to 

 hilltops. 



A formation of still older {^glacial f) drift. — From the foregoing, 

 it is clear that the Bighorn mountains were subjected to two 

 distinct glacial regimes in the Pleistocene period. That there 

 was a still earlier epoch of glaciation is suggested, though not 

 proved, by the data now in hand concerning a series of deposits 

 which antedate the older of the drift formations referred to 

 above. Gravel, some of it containing bowlders, is widespread 

 about the mountains. It covers the dissected plains and caps 

 the hills which rise above them. Most of this gravel is regarded 

 as the work of waters flowing out from the mountains and 

 spreading on the plains. These gravels are of various ages, as 

 their varying topographic positions show, and the oldest ante- 

 date, by some considerable period, the glacier deposits of the last 



