222 ROCHESTER ACADEMY OF SCIENCE. [Oct. 23, 



The forests probably surpassed those of to-day in the size and 

 luxuriance of individual plants, for the climate was sub-tropical. 



At last there came a remarkable change. It was felt at first in a 

 slow lowering of temperature. Then with a decided change of 

 climate, due to the height of land and the southward creeping of the 

 polar ice-cap, came a change in the fauna and flora. In course of 

 time the arctic ice-sheet reached and buried this region and expanded 

 to the southward. For many thousands of years accumulation of 

 snow in the highlands north caused a slow movement of the conti- 

 nental glacier over this area. The depth of ice at its maximum may 

 well have been as much as two thousand feet. (') By its weight and 

 motion the superficial decomposed rocks were pulverized and 

 removed, and in some places the erosion ate deep into the solid 

 strata. The old drainage channels were either obliterated or filled 

 with debris'; a sheet of heterogenous drift was spread all over the 

 uplands ; huge masses of unassorted rocks and clay were accumu- 

 lated at numerous places, while ridges of partially assorted matter were 

 piled up by the action of the sub-glacial rivers. The Pinnacle Hills are 

 a deposit of the latter kind. (*) Some American glacialists believe 

 that there were two or more glacial epochs, with one or more inter- 

 glacial epochs, when the ice sheet retreated far to the north, and that 

 during the interval drainage channels were established, and vegetation 

 took possession of the surface, or peat accumulated in the swamps, to 

 be destroyed or buried under the re-advance of the ice. During the 

 latter part of the Ice age the prevailing motion here was from the 

 northeast, and the country is strewn with a great variety of hard 

 boulders brought from the St. Lawrence and Canadian regions and 

 intermediate points. 



The evidence of human occupation of this northern region 

 during glacial times has probably been over-estimated, and is not 

 established to the satisfaction of the geologists. 



The lake basin of Ontario was produced during the Ice age, 

 probably by enlarging a pre-existent Tertiary valley and river 

 channel, with some movements of depression and elevation of 

 adjacent areas. After the recession of the ice-sheet from this area 

 the Ontario depression was flooded, due to subsidence of the land, 

 and the barrier of the ice in its northward retreat, so that a larger 



(7) The distance from Rochester southward to the glacial boundary is about 75 miles. An 

 average gradient of the glacier surface of 30 feet per mile would give something over 2,000 feet. 

 See Professor T. C. Chamberlin's maps of the glaciated area in Third Ann. Rep., U. S. Geolog. 

 Survey. 



(8) " Eskers near Rochester, N. V." By Warren Upliam, in this volume, pp. 181-200. 



