584 THE ICE AGE IN NORTH AMERICA. 



separated centers of accumulation to the northeast and north- 

 west. Positive evidence in support of this was found, as 

 already stated, by Prof. Williams in 1897 in the discovery of 

 a rolled piece of native copper from Lake Superior firmly 

 imbedded in till at East Warren, Pa., forty feet below the 

 surface, showing that the older ice-movement from the north- 

 west invaded the region now coverd with the later deposits 

 from the northeast to the extent of several hundred miles; 

 for, as already shown, northeastern drift extended some dis- 

 tance across the Mississippi at Burlington, Iowa. 



The most important evidence supposed to indicate the 

 Complete retirement of the continental ice-sheet between 

 successive deposits is found near Toronto, Canada. This was 

 first investigated by Dr. G. J. Hinde in 1878, but has since 

 been more thoroughly studied by Professor A. P. Coleman, of 

 Toronto University. Briefly stated the facts are that in the 

 valley of the Don River and at Scarboro Heights near Toronto 

 there is at the base a deposit of till which after having been 

 extensively eroded was covered by sedimentary deposits 150 

 feet in thickness which had been brought into standing water 

 by the stream to form a delta whose base extended twenty-five 

 or thirty miles along the shore. The lower strata of this delta 

 deposit are thirty-five feet below the present level of the lake, 

 and probably at about the same relative level as when laid 

 down. But the water from some unknown cause rose as the 

 accumulation progressed until it was 150 feet higher than 

 now, when the upper sediments of coarser gravel were depos- 

 ited the water began to fall, and a period of erosion succeeded. 



This proceeded until at Scarboro a V-shaped channel, 

 one mile wide at the top and 150 feet deep, was worn in the 

 sedimentary deposits, whereupon the ice advanced again 

 and covered the whole with sheets of bowlder clay and 

 assorted drift to a total depth of 200 feet. Here certainly 

 seems to be an interglacial deposit of unusual extent. 



Nor is the character of the fossil plants and animals 

 included in the interglacial deposits any less noteworthy. 



