DEPOSITIOX OF THE EXGLACIAL DRIFT. 191 



Upon all the areas thus studied by me where the ice-sheet was bor- 

 dered hj great lakes or the sea, tracts of stratified sediments, as deltas of 

 gravel, sand, and silt, and somewhat more extensive deposits of finer silt 

 and clay, are found, and their distribution shows them to have been brought 

 into these bodies of water chiefly bv rivers flowing from the melting ice. 

 But a large portion of the englacial and superglacial drift, cori'esponding to 

 that which fell as wholly iTnstratified till on laud areas, was received from 

 the receding ice into these lakes or the sea with little change, being allowed 

 to fall to their bottom only very slightly modified by water action. "Within 

 the area of Lake Agassiz and the other associated glacial lakes, very exten- 

 sive tracts, probaljly half or a larger part of their whole extent, have a 

 surface of till, Avhich difters from its characters on the adjoining tracts that 

 were land during the ice retreat in having usually slight traces of stratifi- 

 cation within the 5 to 15 feet of the upper and englacial till, and in having 

 a more smooth and even contour. 



Bowlders, gravel, sand, and clay are mingled in this englacial till in 

 the same proportion as on the country outside these glacial lakes. There 

 was generally no noteworthy transportation of bowlders or other drift 

 by ice floes or bergs on these lakes ; nor was the fine clayey part of the 

 englacial drift washed away in any noteworthy amount from the sub- 

 merged and melting and receding ice margin by wave action, which would 

 have covered the till in front of the ice-sheet with beds of silt. Instead, 

 the englacial and finally superglacial drift that escaped the stream erosion 

 of the drainage from the glacial melting sank through the water to the 

 bottom as the ice gradually withdrew, and exhibits essentially the same 

 characters as on areas that were land, excepting its usually obscure traces 

 of stratification and its smoother surface. 



