528 THE PLEISTOCENE EPOCH 



because the formation of similar deposits has not been observed 

 in connection with modern glaciers ; and several geologists do not 

 accept the commonly received view of the origin of till, but regard 

 it as the product of water and floating ice. 



Stratified D?'ift is made by water, either alone or assisted by 

 the action of ice. Much of the great mantle of Pleistocene drift 

 is more or less completely stratified, because the border of the 

 ice-sheets, whether they were advancing or retreating, was melting, 

 and the drift left by the melting ice in its retreat was worked 

 over more or less by water. Subglacial streams are active agents of 

 deposition, both while still under the ice and after they have 

 emerged. In long, winding tunnels beneath the ice they may 

 leave the gravel ridges called eskers. Subglacial streams often are 

 under great pressure and rise like fountains from beneath the ice- 

 edge ; the relief of pressure on escaping causes deposition at the 

 edge of the ice-sheet. Drift thus piled in irregularities of the 

 ice-front formed kames, which are hillocks or short ridges of strati- 

 fied drift, shaped by the recesses of the ice in which they gathered, 

 and are often connected with the terminal moraines. Beyond the 

 free border of the ice the escaping waters spread out stratified drift 

 for considerable distances. When these waters descended valleys 

 that were not too steep," they deposited sand and gravel in their 

 descent, forming valley trains, which are of coarser materials and 

 steeper grade near their heads than below. When the waters 

 escaped from the ice away from valleys they spread out their 

 deposits as 7norainic or overwash plains. The ice-front in several 

 places entered the sea, and in others formed lakes by damming 

 valleys and depressions ; the ice-derived materials were, in such 

 cases, rapidly deposited in quiet waters, as deltas and subaqueous 

 overwash plains, and the finer silt and clay were carried further 

 out into deeper water. 



When the ice-cap was retreating, the processes described were 

 continued along the shrinking border, until every part of the area 

 once covered by the ice had been subjected to them, and thus 

 the moraines left by the ice were covered with a more or less 

 extensive and thick mantle of the stratified drift. 



