334 GEOLOGY. 



The ice of the Puget Sound region ' came from three sources: the smallest 

 part came from the Olympics on the west; another and larger part from the 



Cascades to the east, while the third and largest pari came from the north. This 

 northern glacier sent a branch westward into the strait of Juan de Fuca, as well 

 as one south into Puget Sound. The southern edge of this Puget ice sheel lay 

 south of Tacoma and Olympia. 



East of the Cascades also, glaciation was extensive. As already noted great 

 tongues of ice, altogether beyond the size of valley glaciers, descended from 

 the north into the basins of the Okanagon, the Columbia, and the Colville Rivers. 2 

 Glaciation was also widespread in northern Idaho and northwestern Montana. 

 From the Rocky mountains of the latter state, mountain glaciers descended and 

 spread for miles on the plain to the east. Just south of the national boundary, 

 the drift from the Keewatin ice-sheet overlaps that from the mountains. 3 Far- 

 ther south, the extension of the ice east of the mountains was less. Although 

 they have not all been well studied, it is safe to say that all the principal moun- 

 tains of Montana, Wyoming, Idaho, Oregon, and Washington harbored glaciers, 

 some of which were very large. In the Yellowstone Park, in the eastern part 

 of the mountains, glaciation was so extensive as to belong to the ice sheet, 

 rather than the valley glacier type . The aggregate number of glaciers which existed 

 in these northwestern states has never been determined, but it must have risen 

 into the thousands. 



The glaciers of the Bighorn mountains of Wyoming 4 (Fig. 471) were per- 

 haps typical for those of the lesser ranges in this section of the United States. 

 The glaciers of this range were numerous, the longest being about 17 miles in 

 length. None of them, however, reached the surrounding plains. 



Farther south, in Colorado, the Front range 5 was more or less generally 

 glaciated for a width of 16 miles in latitude 40°, while the Park range was gla- 

 ciated somewhat generally over an area 60 miles long by 10 miles wide. Gla- 

 ciation in the Medicine Bow range was less extensive. On the east side of the 

 Sa watch range, an elevation of about 11,000 feet was necessary to produce gla- 

 ciers. 6 Glaciers of great size (one 65 or 70 miles long) existed in the mountains 

 of southwestern Colorado, where their sources were at altitudes of 11,000 feet 

 or more. 7 In no part of Colorado thus far studied does there appear to have 

 been a body of ice which extended beyond the limits of a single drainage system. 



South of the Front range of Colorado, the eastern ranges of the PcOckies were 

 the site of numerous glaciers as far south as northern New Mexico 8 (lat. 35° 45'), 



1 Willis, Tacoma folio, U. S. Geol. Survey. 



2 B'ackwelder and Garrey, Jour, of Geol., Vol. IX, pp. 721-724. 



3 Calhoun, Jour, of Geol., Vol. IX, p. 718. 



4 Blackwelder, Jour, of Geol., Vol. XI, p. 216. 



5 King, Geol. Surv. of the 40th Parallel, Vol. I. 



6 Leffingwell and Capps, Jour, of Geo!., Vol. XII, p. 698. 



7 Stone, Mono. XXXVII, U. S. Geol. Surv.; also Hole- and Everley, unpublished 



data. 



8 Salisbury, Jour, of Geol., Vol. IX, 1901 



