CONDITIONS OF FORMATION OF MARGINAL MORAINES. 



25 



mile, li feet of drift; the fourth mile's ice stratum, 120 feet thick, 1 foot 

 of drift; and the stratum of 100 feet in the fifth mile, seven-tenths of a 

 foot. The amount of englacial drift above the altitude of 970 feet, reached 

 at the end of five miles, would be about 5 feet in a thickness of about 

 800 feet of ice, the upper limit, as before noted, being assumed to be 1,760 

 feet above the land surface. 



The rate of ablation of the ice in the warm summers of the Champlain 

 epoch, with alternating sunshine and still more efiicient rains, probably 

 averaged from two to four inches daily during 200 days of the warm por- 

 tion of each year. In the remaining five and a half months we may 

 suppose that the snowfall and ablation counterbalanced each other, while 

 the ice advance, though diminished on account of the lower temperature, 

 would produce some thickening of the border. Rarely the border thicken- 

 ing during many years of prevailingly low temperature would accumu- 

 late drumlins in the manner before described. Frequently when a series 

 of years had a small mean rate of ablation, the ice front remained nearly 

 stationary, giving the conditions necessary for the formation of a mar- 

 ginal moraine ; but when the ablation was more rapid, no belt was occu- 

 pied by the front so long as to be marked by morainic hills and ridges. 

 An average ablation of two inches per day during 200 days of each year 

 may be assumed as permitting the front to remain on the same line, or 

 with advances and recessions not exceeding a half mile or one mile from 

 that line. The resulting moraine would be heaped irregularly on a belt 

 one to two miles wide. 



Conditions of morainic Drift Accumulation. 





Ascent of ice surface. 



Glacial advance. 



Morainic drift, in feet. 



Ice stratum terminating in suc- 

 cessive miles. 



6 

 S 

 ft 



o 



_6 



cS 





o 



CO 



"o 



a 



Becoming su- 

 perglacial in 

 30 years. 



.2 iJt 



Oh 



1 



400 

 200 

 150 

 120 

 100 



400 

 600 

 750 

 870 

 970 



1:13 

 1:26 

 1:35 

 1:44 

 1:53 



2.1 

 4.3 

 5.8 

 7.3 

 8.8 



2.5 

 5.3 

 6.7 

 8.7 

 10.4 



5.0 

 2.0 

 1.5 

 1.0 

 0.7 



12.3 



10.6 



10.0 



8.7 



7.3 



10.0 

 80 

 6.5 

 5.5 

 4.8 



2 



3 



4 



5 





Total average thickness of moraine from these five miles, 83.7 feet, if amassed 



on a belt one mile wide. 



48.9 



34.8 



To supply the ice by onflow equivalent to the ablation of two inches 

 daily in summer upon the first mile from the frontal line would require 

 an average forward current of 26 inches daily for the lowest 400 feet of 



IV— Bull. Geol. See. Am., Vol. 7, 1895. 



