No. 4.] CLAYPOLE — PRE-GLACIAL GEOGRAPHY. 225 



district by the contineutal glacier. Omitting the two measure- 

 ments in Summit and Cuyahoga counties, which as shown above 

 in the note are altogether exceptional and accidental, and taking 

 the mean depth wherever two extreme limits are given, we find 

 that the average thickness of the drift over all these northern 

 counties of Ohio scarcely exceeds 50 feet. The thinning out to 

 the southward of the clays and sands of which it is composed is 

 moreover easily noted even in the list above quoted, the upper 

 names being those of counties near the lake and the lower of 

 those more distant. 



From the facts and figures now given we may draw the infer- 

 ence that as the mass of deposited glacial matter in northern 

 Ohio does not exceed fifty feet in average thickness, and as this 

 matter was mainly derived from the region lying immediately 

 north of Ohio, therefore the average thickness of surface-ero^sion 

 accomplished by the great ice-sheet during its whole duration 

 did not overpass this limit. 



It may be uriijed that some of this material has since been re- 

 moved by streams. This amount, however, except in the stream- 

 valleys is not large. On the uplands and plateaux this factor in 

 the problem may safely be disregarded, and these are not the 

 places in general where the greatest depth is found. Moreover 

 an allowance in the opposite direction must be made for that 

 portion of the drift which was brought from land yet farther 

 north in Canada. Though not very large in comparison with 

 the whole it is large enough to form an important ofi'set to the 

 portion removed by the action of fresh water. 



Returning now to the line of argument followed above, we 

 may conclude if the great continental glacier prevailing over the 

 lake region and Ohio for so many ages, possessing a thickness of 

 many hundred feet (which can scarcely be doubted) and probably 

 moving at a rate (for a glacier) exceedingly rapid, could only 

 remove from the surface a layer of earth and rock (surface soil 

 included), not exceeding fifty feet in thickness, that the local 

 glaciers in the lake-beds, thinner and smaller by far, shorter-lived 

 and probably less rapid in movement, must have been utterly 

 powerless to scoop out those beds to their present depth. Is it 

 rational to believe that one of these puny ice-tongues lying on 

 the site of Lake Ontario, could excavate in a short time to the 

 depth of 450 feet, those rocks from which its gigantic ancestor 

 of longer duration could only scrape off at most some fifty feet ? 

 Vol. IX. p No. 4. 



