84 W. UPHAM — DERIVATION OF KAMES, ESKERS, AND MORAINES. 



surface of the glacier and the great supply of its central current prevent 

 the drift so worn off and borne away from being carried into the axial 

 portion of the ice stream. Similarly the currents sweeping drift from 

 the land into the ice-sheet were probably limited, as before remarked, to 

 the lower fourth or third of its whole mass. 



The abundant englacial drift that was being transported very slowly 

 at heights not far above the ground appears, during the glacial recession, 

 to have been at last carried downward beneath the outer part of the ice- 

 sheet an J amassed in the usually flat or moderately undulating ground 

 moraine. Under certain peculiar conditions, due to climatic influences, 

 I suppose that great quantities of the ice-held drift, having once become 

 superglacial, were again covered by onflowing ice and heaped by its con- 

 vergent descending currents in the high oval hills of till called drumlins. 

 Rarely some of these hills, as Third and Fourth cliffs in Scituate, Massa- 

 chusetts, and the Capitol, University, and Observatory hills in Madison, 

 Wisconsin, contain a nucleus of subgiacial stratified sand and gravel 

 which is enveloped by the subgiacial till, with so much of the englacial 

 drift on the surface as still remained in the ice until its melting from 

 these areas. 



When the North American ice-sheet attained its greatest extent, erosion 

 probahly was in progress on all the ice-covered country, excepting near 

 its limits, being most rapid within some such distances as from 100 miles 

 to 200 or 300 miles back from the boundary. Deposition prevailed, we 

 may infer, on a somewhat broader peripheral belt than during the ensu- 

 ing time of disappearance of the ice-sheet, which was attended by the 

 recession of this belt, estimated to have been then 20 to 50 miles wide, 

 across all the previously eroded country. The englacial drift supplied 

 by the wide and long continued erosion was then laid down, according 

 to the view here presented, in great part and apparently more than half 

 of it all, as the ground moraine of subgiacial till, and the remainder 

 formed the retreatal moraines, kames, and eskers, and the valley drift in 

 its varieties of gravel, sand, clay, and loess. 



