Upham.] 436 [Dec. 21, 



of the Niagara gorge, which is one of the best measures of post- 

 glacial time which has yet been studied, is greatly reduced by what 

 we now know of the rate at which erosion is proceeding at the falls. 

 Ten thousand years is now regarded as a liberal allowance for the 

 age of that gorge. But, finally, the term "close of the glacial pe- 

 riod" is itself a very indefinite expression. The glacial period was 

 a long time in closing. The erosion of the Niagara gorge began 

 at a time long subsequent to the deposit of the gravel at Trenton 

 and at Madisonville. Between those two eA T ents time enough must 

 have elapsed for the ice-front to have receded a hundred miles or 

 more, or all the distance from New York to Albany ; since only at 

 that stage of retreat would the valley of the Mohawk have been 

 freed from ice so as to allow the Niagara River to begin its work. 

 The deposits at Trenton and Madisonville took place while the ice- 

 sheet still lingered in the southern watershed of New York, Penn- 

 sylvania and Ohio. 



THE RECESSION OF THE ICE-SHEET IN MINNESOTA 



IN ITS RELATION TO THE GRAVEL DEPOSITS 



OVERLYING THE QUARTZ IMPLEMENTS 



FOUND BY MISS BABBITT AT 



LITTLE FALLS, MINN. 



BY WARREN UPHAM. 



How far back in geologic time can the existence of man be 

 traced ? Answers to this inquiry come from various parts of Europe 

 and North America, bearing testimony of man's great antiquity as 

 measured by years, though recent in contrast with the long record 

 of geology. The observations on this subject gathered in Great 

 Britain, France, Germany and other countries of Europe, extend 

 the human period with certainty back to the time when confluent 

 ice-sheets covered Ireland, Scotland, and the northern borders of 

 England, the whole of Scandinavia, northern Germany and north- 

 western Russia. Within this Ice Age, the latest in the series of 

 geologic ages preceding the present, are found evidences of man's 

 appearance in Europe. His rudely chipped stone implements 

 occur there in caverns and in beds of fluvial gravel and sand high 



