124 MAN AXD THE GLACIAL PERIOD. 



Juan range, in southwestern Colorado. Here a surface of 

 about twenty-five square miles, extending from an eleva- 

 tion of 12,000 feet down to 8,000 feet, shows every sign of 

 the former presence of moving ice. The greater part of 

 the glaciation in Colorado is confined to elevations above 

 10,000 feet. 



The whole range of the Sierra Nevada through Ore- 

 gon, and as far south as the Yosemite Valley in Califor- 

 nia, formerly sustained glaciers of far greater size than 

 any which are now found in those mountains. In general 

 these glaciers were much longer on the western side of the 

 Sierra Nevada than on the eastern. On the eastern side 

 glaciers barely came down to Lake Tahoe and Lake Mono 

 in California. The State of Nevada seems to have been 

 entirely free from glaciers, although it contains numer- 

 ous mountain-peaks more than ten thousand feet high. 

 In the Yosemite Canon glaciers extended down the Mer- 

 ced River to the mouth of the canon ; while in the Tuo- 

 lumne River, a few miles to the north, the glaciers which 

 still linger about the peaks of Mount Dana filled the val- 

 ley for a distance of forty miles. 



It is a question among geologists whether or not the 

 glaciation west of the Rocky Mountains was contempo- 

 raneous with that of the eastern part of the continent. 

 The more prevalent opinion among those who have made 

 special study of the phenomena is that the development of 

 the Cordilleran glaciers was independent of that of the 

 Lauren tide system. At any rate, the intense glaciation of 

 the Pacific coast seems to have been considerably later than 

 that of the Atlantic region. Of this we will speak more 

 particularly in discussing the questions of the date and 

 the cause of the Glacial period. It is sufficient for us 

 here simply to say that, from his extensive field observa- 

 tions throughout the Cordilleran region, Dr. George M. 

 Dawson infers that there have been several successive al- 

 ternations of level on the Pacific coast corresponding to 



