QUATERNARY 



659 



TABLE ROCK HOUSES^} 



known. When allowances are made for fluctuation in the volume of 

 the rivers at different times after the ice had retreated so as to permit 

 the streams to flow over their new courses, it is seen that 10,000 to 

 50,000 years must have elapsed since the cutting of the Niagara gorge 

 began. For the recession of the St. Anthony Falls between 12,000 

 and 16,000 years seem necessary. To these estimates must be added 

 the time required for the 

 retreat of the ice from its 

 terminal moraine to the 

 Niagara and Minnesota 

 rivers. For this 10,000 

 to 30,000 years more 

 should be added. This, 

 then, would give 20,000 

 to 80,000 years since the 

 beginning of the retreat 

 of the last ice sheet. It 

 is evident upon compar- 

 ing the weathering of the 

 Wisconsin drift with that 

 of older drifts that the 

 time which has elapsed 

 since the last ice sheet is 

 a small fraction of some 

 of the interglacial stages. 

 This has led to the sug- 

 gestion that perhaps we 

 are now in an interglacial 

 stage. It is interesting 

 in this connection to note 

 that the extent of the 

 area at present covered by glaciers is one tenth that of the maxi- 

 mum glaciation. 



Marine postglacial clays in Sweden have furnished an interesting 

 basis for determining the length of postglacial time. These clays 

 have been deposited in regular layers, with different colors and 

 composition, the same succession being repeated time after time. 

 The layers laid down in summer are brown, due to oxidation, and 

 thicker; those laid down in the autumn are darker as a result of the 

 greater amount of organic matter, and thinner. Counting these 



Fig. 573. — Outline map of a portion of the crest 

 line of Niagara Falls, showing the recession of the 

 brink during various intervals since 1842. (After 

 Taylor.) 



