THE DATE OF THE GLACIAL PERIOD. 343 



River, one of the small streams which empty into Lake 

 Erie from the south. Its post-glacial gorge, worn in 

 sandstone which overlies soft shale, is only about two 

 thousand feet in length, and it has as yet made no ap- 

 proach toward a V-shaped outlet. 



The same impression of recent age is made by examin- 

 ing the outlets of almost any of the lakes which dot the 

 glaciated area. The very reason of the continued exist- 

 ence of these lakes is that they have not had time enough 

 to lower their outlets sufficiently to drain the water off, as 

 has been done in all the unglaciated region. In many 

 cases it is easy to see that the time during which this 

 process of lowering the outlets has been going on cannot 

 have been many thousand years. 



The same impression is made upon studying the evi- 

 dences of post-glacial valley erosion. Ordinary streams 

 constantly enlarge their troughs by impinging against the 

 banks now upon one side and now upon the other, and 

 transporting the material towards the sea. It is estimated 

 by Wallace that nine-tenths of the sedimentary material 

 borne along by rivers is gathered from the immediate 

 vicinity of its current, and goes to enlarge the trough of 

 the stream. LTpon measuring the cubical contents of 

 many eroded troughs of streams in the glaciated region, 

 and applying the tables giving the average amount of 

 annual transportation of sediment by streams, we arrive 

 at nearly the same results as by the study of the recession 

 of post-glacial waterfalls. 



Professor L. E. Hicks, of Granville, Ohio, has published 

 the results of careful calculations made by him, concern- 

 ing the valley of Raccoon Creek in Licking County, Ohio.* 

 These show that fifteen thousand years would be more 

 than abundant time for the erosion of the immediate val- 

 ley adjoining that small stream. I have made and pub- 



- See Baptist Quarterly for July. 1884. 



