CAUSE OF DEPARTURE OF ICE-SIIEET. 347 



on exceptional tnicts, displaying unusual evidences of pressure by an 

 advancing ice-slieet, need we appeal to such an invasion instead of the 

 more gradual, slow and gentle process by snow accumulation. 



Ice Departure attrihuled to Land Depression. — Beneath the weight of the 

 ice-sheet the formerly elevated land sank to its ]n'esent height or mostl}^ 

 somewhat lower, so that when the ice melted away the sea covered 

 coastal })ortions of the drift-bearing countries. A moderate subsequent 

 reelevation has since raised tlie fossil iferous marine beds which were de- 

 posited over the glacial drift upon these coastal tracts to altitudes having 

 a maximum of from 500 to GOO feet in both Canada and Scandinavia 

 above the present sealevel. The change of climate resulting along the 

 borders of the ice-sheet on account of land depression caused rapid 

 melting there, and this advanced inward until all the ice disappeared. 

 The depression from the former high altitude, as Dana remarks, would 

 transfer the southern part of the ice-sheet from a climate like that of 

 Greenland to the temperate climate of southern Canada and the north- 

 ern United States. Marginal melting then gradually i)ushed back the 

 boundary of the ice and thus gave to its front an increased steepness of 

 slope, whereby any slight halt or readvance due to a series of years 

 of unusual cold and snowfall became recorded in a marginal moraine. 

 Mainly, however, the temperate climate due to the subsidence of the 

 land prevailed over the accelerated currents with which the ice flowed 

 outward to its steeper border, so that, although the ice-action was then 

 most vigorous, it was almost continually being restricted within dimin- 

 ishing limits, and finally the drift-bearing regions became wholly un- 

 covered. 



Invasion by the advancing Border of an Ice-sheet. 



the a d va ncr no t con tin uo us. 



On some parts of the boundary of the glacial drift in the United States 

 terminal moraines were formed at or near this farthest limit of the ice- 

 sheet, contrasting remarkably with the recession of the ice mostly 50 to 

 'J( K) miles Inick from its early outer limits in the greater part of tlie Mis- 

 sissii)pi Ijasin before its moraines there were formed. The warm sun- 

 shine and rains of the temperate climate due to the land depression had 

 melted the ice away upon an area of more than 100,000 sijuare miles in 

 the MiHsissipi)i basin before the stage of the formation of the moraines, 

 which in that region, as already noted, seem readily explained by the 

 steeper slope then presented by the mostly waning but now and again 

 temporarily halting or slightly readvancing ice-border. Farther eastward, 

 however, as on the east side of the Wisconsin driftless area, and along 

 or near the limit of the ice-sheet and glacial drift for the whole distance 



