GLACIATION IN THE BIGHORN MOUNTAINS 



219 



One of the most striking features of the youngest series of 

 glacial deposits is the extremely fresh appearance of the drift, 

 and the trivial extent of the inroads which weathering and erosion 

 have made on it and on the rock of the canyon walls and roches 

 Tnoiitonnees within the area where it occurs. The till often shows 

 scarcely a trace of surface oxidation. The numerous lakes in 

 the valleys occupied by the glaciers of the last ice epoch, many 

 of them confined by drift dams in valleys which have steep 

 gradients and copious drainage, are clear indications of the 



Fig. I. — Section from near Fort McKinney to the head of the South Fork of Clear 

 creek, showing the distribution of the Pleistocene formations: P, Bald mountain beds; 

 Pg, drift of the next to the last glacial epoch; Pg^-, drift of the last glacial epoch. 



recency of glaciation. The amount of postglacial stream erosion 

 has been slight in most cases. It is at a maximum where the 

 streams cut through the terminal moraines, where V-shaped 

 gorges, 50 to 100 feet deep, have been cut in the till ; but inas- 

 much as a single flood in Piney creek in the spring of 1902 cut a 

 channel 10 to 20 feet deep, these figures must be regarded as 

 small. In addition to the signs of freshness in the drift itself, 

 striae still remain on exposed surfaces of granite. 



Glacial drift antedating the last. — In most of the valleys exam- 

 ined there are bodies of glacial drift outside of the fresh moraines 

 referred to above, and of notably greater age. Their glacial 

 origin is established both by the disposition of the drift in the 

 form of well-defined lateral moraines, and by the finding of 

 striated stones in the fresh exposures. 



Their greater age is shown by the topography of the drift, 

 by the physical condition of the material, by its topographic 

 position, and by its patchy distribution {^Pg, Fig. i). The 



