614 THE ICE AGE IN NORTH AMERICA. 



of the one hundred and sixty thousand years or more which 

 have been claimed. 



The adoption by some of a term of eighty thousand 

 years for the post-Glacial period has been very much the 

 result of the belief that no shorter time would account for 

 the excavation of the valleys supposed to have been formed 

 during this period, on the assumption of a "uniformitarian" 

 rate of denudation. This rate, based on observations made 

 at the present time, always seemed to me open to grave 

 objections, and in this belief subsequent experience has 

 confirmed me. . . . and I would for the same reasons limit 

 the time of the so-called post-Glacial period, or of the 

 melting away of the ice-sheet, to from eight thousand to 

 ten thousand years or less.* 



Summary. — The terrestrial facts brought to light as 

 clearly bearing on the question of the date of the glacial era 

 are much more numerous than they have heretofore been 

 supposed to be. Scarcely more than a beginning has been 

 made in their collection and interpretation ; but, as far as we 

 have gone, the investigation has been most interesting and 

 suggestive. For the most part these facts imply a later date 

 for the Glacial period than the current astronomical theory 

 would admit, and so far they go to disprove that theory. 



The glaciated area seems a vastly newer country than 

 the unglaciated. In the glaciated region the waterfalls have 

 hardly more than begun to recede ; the valleys and gorges 

 are both narrower and shallower than in the unglaciated por- 

 tion of the country ; the lakes and kettle-holes are yet unfilled 

 with sediment, and their outlets have not yet to any great 

 extent lowered the drainage lines ; the striated rocks have 

 resisted disintegration to a remarkable degree during post-gla- 

 cial times, and the moraines and kames have retained their 

 original forms with little signs of erosion. Niagara Falls 

 and the Falls of St. Anthony can neither of them be over 

 ten thousand years old. The waves of Lake Michigan can 



See Prestwich's " Geology," vol. ii, pp. 533, 534. 



