1881.] _ The Ancient Glacters of the Rocky Mountains. 5 
across mounds of glacial débris among which huge boulders of 
granite and granitoid gneiss are conspicuous. The transported 
character of these materials is all the more evident from the fact 
that the platform of solid rock on which they rest often consists 
of various lavas and other volcanic masses. Some parts of the 
route present long smooth slopes dotted with boulders precisely 
like some Scottish boulder-clay moors. The granitic blocks are 
conspicuous objects even from a distance, owing to their size, a 
length of six to eight feet being common among them. These 
signs of glaciation can be traced up to and across the water-shed 
leading over to the Yellowstone valley. They prove beyond 
question that not only was that valley filled up with ice, but that 
the glacier plowed over the ridge one thousand feet above the 
valley bottom and passed into the country lying to the westward. 
No old glacier valley in Europe presents a more characteristic 
scene of ice-drift erratics than does that of the Yellowstone for 
three miles below Lower falls. The large blocks of granite, 
gneiss and other crystalline rocks are scattered about so profusely 
that one might cross the ground for some little distance by leap- . 
ing from boulder to boulder. The blocks are heaped upon 
mounds of moraine-stuff, perched on ice-worn hammocks of | 
gneiss, and stream over the horizontal volcanic sheets through 
which the ravines have been cut. 
As, the determination of the distribution of the erratics gives 
an approximate indication of the thickness of the ice, I noted 
with the aneroid the positions of blocks of granite, gneiss and 
other non-volcanic rocks on the way up to Mount Washburn, 
which projects so conspicuously into the valley of the Yellow- 
stone. These blocks get fewer in number and smaller in size as’ 
they are followed upwards ; but I observed one of three feet long 
at a height of 8650 feet on the west side of the ridge which rises 
southward into Mount Washburn, and another about one foot in 
diameter at a height of 8900 feet. From the position in which 
these erratics lie, the traveler looks clear over the Yellowstone 
valley for many miles to the east and west. The general level of 
the valley-bottom above the edge of the cafion, on the north of 
Mount Washburn, may be taken at between 6000 and 7000 feet. 
South from that eminence it rises to more than 8000 feet. The 
general slope is about 2000 feet in fifteen miles, or roughly, about 
one hundred and thirty-three feet per mile. The ice, after pass- 
