GLACIAL FEATURES 



297 



to one another as the squares of their similar dimensions we 

 have 1:4 or 1 : 16 as the ratio of their snow areas. If no com- 

 pacting took place, then to accommodate all the snow in the glacial 

 trough would require an increase in thickness in the ratio of 

 1:4. If the snow were compacted to half its original volume then 

 the ratio would be 1:2. Now, since the volume ratio of ice to 



Fig. 197 — Mode of cirque formation. Taking the facts of snow depth represented 

 in the curve, Fig. 195, and transposing them over a profile (the heavy line) which 

 ranges from 0° declivity to 50°, we find that the greatest excess of snow occurs 

 roughly in the center. Here ice will first form at the bottom of the snow in the 

 advancing hcmicycle of glaciation, and here it will linger longest in the hemicycle 

 of retreat. Here also there will be the greatest mass of nevS. All of these factors 

 are self-stimulating and will increase in time until the floor of the cirque is flattened 

 or depressed sufficiently to offset through uphill ice-flow the augmented forces of 

 erosion. The effects of self-stimulation are shown by "snow increase"; the ice shoe 

 at the bottom of the cirque is expressed by " ice factor." The form accompanying both 

 these terms is merely suggestive. The top of " excess snow " has a gradient char- 

 acteristic of the surface of snow fields. A preglacial gradient of 0° is not permissible, 

 but I have introduced it to complete the discussion in the text and to illustrate the 

 flat floor of a cirque. A bergschrund is not required for any stage of this process, 

 though the process is hastened wherever bergschrunds exist. 



snow is 1 : 9 and the thickness of the ice down valley is, say 400 

 feet, the equivalent of loose snow at the foot of the cirque must 

 be more than 1 : 4 over 1:9 or more than two and one-quarter 

 times thicker, or 400 feet thick; and would give a pressure of 

 (900 -f- 10) X 62.5 pounds, or 5,625 pounds, or a little less than 

 three tons per square foot. Since a pressure of 2,500 pounds per 

 square foot will convert snow into ice at freezing temperature, it 



