METEOROLOGIC EXPLANATION OF ICE INVASION. 351 



Doubtless tlie prevailing course of storius during the Glacial and Cham- 

 plain epochs, as at the })resent time, was from west to east and north- 

 east. AMth the restoration of a temperate climate by the subsidence of 

 the land to its present heiglit, or mostly somewhat lower, the sunshine 

 and rains began to melt tlie ice away. Its border in general retreated and 

 became steeper, but with interruptions, ranging in length from decades 

 to centuries, when the snow accumulation and ice outflow caused im- 

 portant extensions of the glaciation. The warm air currents, bringing 

 rainstorms and therefore rapidly melting the front of the ice where they 

 first swept over it at the west, would, however, be chilled as they passed 

 onward, giving principally snowfall on more eastern parts of the ice 

 margin. The western ice-melting also contributed much to the supply 

 of moisture for this snowfall from the eastwardly moving storms. 



Instead of receding along all its extent the front of the North American 

 ice-sheet appears thus to have remained nearly stationary or to have ad- 

 vanced in its great lobe from Salamanca, in southwestern New York, to 

 Long island and Nantucket, and probably onward in the eastern prov- 

 inces of Canada, during the time of the mainly rapid melting and de- 

 parture of the ice upon its extensive windward area from the Dakotas 

 and Iowa east across lakes Superior, Michigan, Huron and Erie, which 

 then were united in the glacial lake Warren. The retreat of tlie ice and 

 the accompanying uplift of the land from the Champlain subsidence 

 here passed from southwest to northeast, so that northern New England 

 was the latest part of the United States, at least eastward from the Rocky 

 mountains, to be uncovered from the ice-sheet and elevated to its pres- 

 ent height. 



Criteria of Ice Accumulation and Invasion. 



Near the margin of drift-bearing areas, glaciation chiefly due to snow 

 and ice accumulation, with less supply by inflow from central and thicker 

 tracts of the ice-sheet, is indicated, as the present writer thinks, l)y a 

 gradual attenuation of the drift, absence of morainic knolls and hills, 

 and scanty glacial erosion of the bed-rocks. 



Conversely, the evidences of an invasion by the ice-sheet upon its 

 marginal tracts consist in thick drift deposits, hilly morainic belts, and 

 much planing and striation of the rock surface. Displacement and fold- 

 ing of soft strata beneath morainic deposits seem to be especially con- 

 clusive ])roof of vigorous incursion by a steej) ice-front. 



Readvanccs of the ice within its drift area may be recognized by very 

 clearly defined belts of morainic hills, pushed out on smooth tracts of 

 till, or by the very rare occurrence of disrupted or deeply crumpled 

 underlying beds. In most cases where moraines belonging to the time 



XL— Bui.f.. Gkot,. Soc. Am.. Vol. 0, 1894. 



