EXTINCT AND EXISTING GLACIERS OF COLORADO 63 



our fine series of photographs. The neve of the dead south lobe 

 and one narrow intermediate lobe of ice and snow between the two 

 neves have shrunken markedly since 1906. This general shrinkage 

 must not be confused with annual shrinkage and recuperation, but 

 is the sum of the annual differences between shrinkage and recupera- 

 tion. As long as shrinkage and recuperation are exactly balanced, the 

 front at the same time of each year will appear about the same, but 

 when one overbalances the other, a change in the position and shape 

 of the front must be expected. Shrinkage at the head of the neve is 

 even more important than shrinkage at the foot of the glacier, for if 

 the neve cannot hold its own it will soon lose the power of sending 

 down ice each year to check the shrinkage at the foot of the ice. To 

 make them available for comparison, our photographs are taken on 

 September i of each year. The best time to see the details of our 

 mountain glaciers is late August and early September, after the 

 preceding winter's snow is about gone from the ice and before the next 

 winter's snows have begun to cover it. 



Isabel Glacier. — This glacier is located in a cirque with extremely 

 precipitous walls at the head of the South St. Vrain, west of Ward, 

 Boulder County, Colorado, a short distance above Isabel Lake. It 

 was discovered by Mr. Fred A. Fair in about 1908, and on September 

 17, 1 910, I went with him to examine it. Unlike most glacier rem- 

 nants in Colorado, this is in a cirque facing southeastward. It is 

 about 2,500 feet wide and 1,500 feet long, a very distinct tongue 

 extending out in front of the center and passing beneath the moraine. 

 The moraine forms a ridge rising from 10 to 30 feet above the end of 

 the ice-tongue and bearing little evidence of the deposition of fresh 

 glacial mud, which is to be expected from the fact that the ice passes 

 beneath it. A well-defined bergschrund extends across the upper end 

 of the ice, gaping from 5 to 20 feet wide. Below this in the center 

 the surface of the ice flattens out, then passes abruptly into the front 

 with a slope at first of 20°, gradually flattening to 15°, then to 10°, 

 and finally at the end to less than 5°. The shape of the cirque is such 

 that the ice is not crevassed below the bergschrund. The bergschrund 

 reveals everywhere at least 40 feet of well-consoUdated but somewhat 



