WORK OF GLACIAL WATERS 155 



rock, which shows drift at a depth 600 feet lower than the deepest part 

 of the lake and 750 feet beneath sealevel. 



It seems probable that the valleys of the Finger lakes are blocked on 

 the north, along the drumlin belt, by deep drift filling, which can be de- 

 termined only by borings at close intervals. That these valleys were 

 gouged out by ice erosion, even by any number of continental ice-sheets, 

 seems to the writer extremely improbable. If they were so deepened, 

 then the basin of Lake Ontario was probably also scooped by ice erosion. 

 But if the Ontario Basin is a depressed river valley, then the valleys of 

 the Finger lakes must be fairly graded to the bottom of Ontario and be 

 of similar origin. If the Ontario and other basins were excavated by 

 river work and weathering, then it must be admitted that there have 

 been great changes in the height and attitude of the land in late geologic 

 time; but such changes are quite certain. It appears probable that the 

 valley-cutting occurred during a time of land elevation, and that the 

 Laurentian and the Finger Lake basins are the complex product of 

 land warping, land depression, and of glacial drift filling. Until the 

 later Tertiary and Pleistocene diastrophic movements of the area, in- 

 cluding New York, have been determined and the drift-buried valleys 

 mapped by borings, the deep lake basins may remain the subject of specu- 

 lation and dispute. 



Glacial Lake Succession 



The story of the succession of the glacial waters that laved the reced- 

 ing front of the Laurentian glacier is a dramatic episode in the geologic 

 history. Beginning in small pondings of water in the heads of the val- 

 leys along the north side of the morainic divide, the lakes were enlarged 

 as the ice-barrier receded, and were captured, drained, blended, or other- 

 wise affected by changes in outlets. The romantic story can not be satis- 

 factorily told in words alone, but requires cartographic representation, 

 and a series of maps have been constructed to show the better known and 

 more striking changes in the ice recession and the lake succession. 



The control of the glacial waters depended on the altitude of the low- 

 est passes affording immediate outflow, along with the relation of these 

 passes to some ultimate escape. The waters of the Laurentian Basin 

 outflow today by the St. Lawrence (240 feet). With that escape blocked 

 the lowest pass is at Home (460 feet for the water surface) to the Mo- 

 hawk-Hudson, and which for many thousands of years was the point of 

 escape of the waters while the ice-body lav over the Si. Lawrence Valley. 

 The next higher pass is at Chicago, which was occupied by the glacial 

 outflow for a very long time, hut to reach this ultimate escape the On- 



