﻿Vol. 64.] CHEONOLOGT OF GLACIA.L EPOCH IN NOETH AMERICA. 149 



8. Chronology of the Glacial Epoch in North America. By 

 Prof. George Frederick Wright, F.G.S.A. (Communicated 

 by Prof. E, J. Garwood, M.A., Sec.G.S. Eead January 8th, 

 1908.) 



[Abstract.] 



In the case of Plum Creek, Lorain County (Ohio), the study of the 

 activity of the stream and of the amount of work which it has done 

 since a certain stage of the Glacial Epoch, has yielded important 

 results. This stream began the erosion of its trough when the 

 temporary lake, held up in front of the ice, was maintained for a 

 considerable period at the level of its Fort- Wayne outlet ; it has 

 never had anything more resistant than Till to act upon. From 

 a given section, 5000 feet long, it has excavated 34,000,000 cubic 

 feet of Boulder-Clay, removing it from exposed banks 1600 feet long. 

 Twelve years' erosion of a 500-foot length of a part of the trough 

 of the stream under observation, and from banks 1000 feet long, 

 gives a rate of 8450 cubic feet per annum. Therefore, the removal 

 of 34,000,000 cubic feet from the 5000-foot section would give a 

 period of 2505 years. Considerations tending to lengthen the 

 estimate are the former forestation of the area and the increased 

 gradient in the artificial cut-off. Those tending to shorten the 

 estimate are the present wider flood-plain, the time taken for 

 forests to grow, and the probably greater former water-flow. 



The erosion of the Niagara Gorge began considerably later than 

 that of Plum Creek, and probably dates from midway between the 

 disappearance of the ice from Northern Ohio and from Quebec. If 

 conditions have been uniform, the age of the Gorge would be 7000 

 years. As the Niagara Limestone is thinner at the mouth of the 

 Gorge, and the Clinton Limestone has dipped out of sight at the 

 Whirlpool, there is nothing in the stratigraphy to indicate a slower 

 recession in the past than in the present. Moreover, nearly one- 

 third of the erosion has been accomplished by two pre-Glacial 

 streams, one from the south and a smaller one from the north. 

 Therefore, the Author concludes with considerable confidence that 

 the Gorge is less than 10,000 years old, and that the ice of the 

 Glacial Epoch continued down to that time, to such an extent over 

 the lower St. Lawrence Valley and Central New York that it 

 obstructed the entire eastern drainage of the Great Lakes. 



There is nothing which would lead to a longer estimate of the 

 time which has elapsed since the Kansan stage of the Glacial Epoch 

 than that approved by Prof. Calvin of Iowa, and agreed to by Prof. 

 "Winchell. These assume 8000 years as the limit for post-Glacial 

 time, and that a multiple of this by 20, amounting to 160,000, 

 would carry us back to Kansan time. This, however, would still 

 leave as long a period still earlier, for the advance of the ice. Tlie 

 Author's impression is that the whole epoch may well have been 

 compassed within 200,000 years. 



