GLA CIA L J VORK IN WES TERN MO UN TA INS 7 2 7 



appears to be younger. The ice of the earlier epoch, therefore, 

 seems to have been more extensive. 



3. Two distinct sheets of drift were found at several points 

 in the valleys of both the North and South Forks of the Ameri- 

 can Fork. These sheets of drift are separated by a soil twelve 

 to eiofhteen inches thick. In the same localities, the soil on the 

 surface of the upper drift sheet, formed since the last retreat of 

 the ice, is from four to six inches thick. So far as the thickness 

 of soil is a basis for estimating time, it would indicate that the 

 interval between the ice epochs was longer than the time since 

 the last. 



The topography of the region examined was somewhat modi- 

 fied by glaciation. The valleys which were occupied by ice are 

 usually U-shaped, and their slopes are commonly smoothed off 

 as far up the sides as the ice reached. Such forms are in sharp 

 contrast with the V-shaped canyons and rugged slopes of the 

 valleys not occupied by ice. In some of the glaciated valleys 

 massive moraines were built up. These are sometimes in the 

 canyons, and sometimes at their debouchures, according to the 

 position which the ends of the glaciers reached at the time of 

 their maximum extension. When tributary glaciers joined the 

 main, medial moraines were formed, and as the ice melted, these 

 moraines were left as ridges, parallel with the course of the val- 

 ley. Recessional moraines are frequently found crossing the 

 valleys as crescentic ridges, convex down stream. These ridges 

 often served as dams, above which lakes accumulated, rose, and 

 overflowed. The outlet streams of such lakes cut gorges in the 

 moraines. Near the heads of many valleys the ice gouged out 

 rock basins, 50 to 200 feet in diameter and from 5 to 20 feet in 

 depth. At least thirteen of the thirty-six glacial lakes mapped, 

 are in rock basins apparently made by the ice. 



The relation of the moraines at the west base of the Wasatch, 

 to the fluviatile (or shore) deposits in the Bonneville basin, is 

 such as to indicate, as Gilbert has pointed out, that the last 

 advance of the ice from the mountains occurred during a late 

 period in the history of Lake Bonneville. The close correlation 



