GLACIAL DAMS, LAKES, AND WATERFALLS. 395 



On coming to the region of the Great Lakes, the influence 

 of ice-barriers is very conspicuous. South of Lake Erie there 

 is an ascending series of what are called lake ridges. These 

 are composed of sand and gravel, and consist largely of 

 local material, and seem to maintain throughout their entire 

 length a definite level with reference to the lake, though 

 accurate measurements have not been made over the whole 

 field. The approximation, however, is sufficiently per- 

 fect to permit us to speak of them as maintaining a uniform 

 level. These ridges can be traced for scores of miles in a 

 continuous line, and in the early settlement of the country 

 were largely utilized for roads. In Lorain county, Ohio, 

 an ascending series of four ridges can be distinguished at 

 different levels above the lake.* The highest is from 200 

 to 220 feet above it; the next is approximately 150 to 

 160 feet; the next lower is from 100 to 118 feet; and the 

 next lower less than 100 feet, while some appear on the 

 islands near Sandusky which are not over 70 feet above the 

 water-level. Eastward from Buffalo portions of this series 

 have been traced, according to Gilbert, until they disappear 

 against the highlands near Alden, on the Erie Railroad. 



Lake Ontario is likewise bordered by similar ridges upon 

 its southern and eastern sides, but the investigations in that 

 region are not yet complete enough to be altogether satisfac- 

 tory. From what has already been done it is evident that 

 they do not maintain so nearly a uniform level as on the 

 shores of Lake Erie. Mr. G. K. Gilbert, of the LTnited States 

 Geological Survey, informs me that, from the vicinity of 

 Oneida Lake toward the northeast, the ridges rise rapidly 

 with reference to the lake-level, and that to a less extent 

 they rise in a westerly direction, showing that if they were 

 w r ater-deposits, there has been considerable oscillation of the 

 land both northeast and southwest of an axis following the 

 line of the Mohawk Valley. 



That the ridges on Lake Erie mark temporary shore-lines 



* " Geological Survey of Ohio," vol. ii, p. 207. 



