342 GEOLOGICAL EXCURSION TO THE ROCKY MOUNTAINS. 



the ice from the summit of the Gallatin range moved eastward across 

 Swan valley and passing over the top of Terrace mountain joined the 

 ice field coming from the east. The. united ice sheet plowed its way 

 northward down Gardiner valley to the Lower Yellowstone, where the 

 broad valley may be seen strewn with the material transported from 

 both the east and west rims of the Park. It has been named the Yel- 

 lowstone glacier. 



A second powerful glacier moved southward over the plateau and 

 down the broad valley of the Snake, receiving a number of tributaries 

 from both the Wind River and Teton ranges. 



Since the building up of the Park plateau, glacial erosion has greatly 

 modified all surface features of the Park. It has broadened and deep- 

 ened preexisting drainage channels, opened new waterways, and cut 



Fit.. 15. — Glacial boulder near the Yellowstone canyon, 



magnificent gorges in the rhyolite plateau. Such gorges as the Yel- 

 lowstone, Gibbon, and Madison canyons, in the strictest sense of the 

 word, have all been carved out in recent time. These canyons are 

 several miles in length and from 700 to 1,400 feet in depth. 



To the geologist one of the most impressive objects on the Park 

 plateau is a t ransported boulder of granite which rests directly upon the 

 rhyolite near the bank of the Grand Canyon, about three miles below 

 the Falls of the Yellowstone. In stands alone in the forest, miles from 

 the nearest glacial boulder. Glacial detritus carrying granitic material 



