278 TKANSPOETATION OP EOOK DEBEIS 



the onward movement of the ice, which in extreme cases may 

 have exerted a pressure of 200,000 pounds to the square foot. 

 On the final retreat of the glacier, this was left in the form of a 

 compact structureless mass of almost stony hardness, commonly 

 known as till or ground moraine. Materials falling upon the 

 surface from greater heights were likewise transported, so long 

 as the ice sheet continued to advance, and finally deposited in 

 the form of terminal or frontal^ medial and lateral moraines. 



Inasmuch as the ice sheet was almost continually melting 

 upon its surface, it is practically impossible to consider its 

 action wholly independent of that of water also. Thus, 

 streams resulting from such melting would gradually wear 

 channels in the ice, as on the land. In these channels would 

 accumulate sand and boulders of such size and weight as to 

 resist the current, and such accumulations, on the final melting 

 of the sheet, would be deposited on the surface of the ground 

 in the form of ridges known as eshers, or osars. Other forms 

 produced by water action on the materials of the ice sheet, are 

 hillocks of stratified sand and gravel deposited near the terminal 

 moraines, and known as Mmes. Since during the advancing of 

 the ice sheet existing rivers flowing eastward must have been 

 dammed, we can safely imagine the formation of large tempo- 

 rary lakes, on the bottom of which would be deposited the 

 glacial silt, like the so-called loess of the Mississippi valley. 

 Lake Agassiz, a glacial lake of this type, is supposed to have 

 occupied an area of more than 100,000 square miles in north- 

 western Minnesota, northeastern Dakota, and a considerable 

 portion of Manitoba. On the bottom of this lake there was 

 deposited during the comparatively brief time of its existence, 

 silt to a depth as yet undetermined, but known to be at least 

 100 feet.i 



"Waters issuing from the melting ice sheet tend to reassert the 

 material of the terminal moraine, redepositing it in approxi- 

 mately concentric zones beyond its margin. These deposits 

 are naturally thicker and coarser near the moraine and thinner 

 and finer at increasing distances. Their form and mode of 

 occurrence is such as to have suggested for them the name of 

 glacio-fluvial aprons, or frontal aprons. Their materials are 

 nearly always loose sands and gravels, the lithological nature 



^Ice Age in North America, by G. F. V^right, p. 355. 



