CHAPTER XT. 



GLACIAL DAMS, LAKES, AND WATERFALLS. 



No single cause has done more to diversify the surface of 

 the country, to add to the attractiveness of the scenery, and 

 to furnish the key by which the conditions of the Ice age 

 can be reproduced to the mind's eye, than glacial dams. To 

 them we owe the present existence of nearly all the water- 

 falls in North America, as well as nearly all the lakes, while 

 the shore-lines and other marks of temporary bodies of water 

 produced by ice-barriers are of the most instructive character. 



In the chapter upon "Glacial Erosion and Transportation" 

 we have already spoken at some length of rock-basins which 

 have been formed by glacial action. In the case of the so- 

 called cirques there described, there can be no question that 

 they have been produced by ice-action, the rocks being worn 

 deepest where the ice impinged upon them most directly and 

 for the longest period of time, leaving at the front a rocky 

 rim much higher than the bottom of the cirque. To what 

 extent the fiords, and the lakes which are some distance from 

 mountain declivities and have rock-rimmed basins, owe their 

 origin to the same cause, is an unsettled question ; and still 

 greater doubt appertains to such basins as are occupied by 

 the great lakes of the United States and British America. 

 Professor Newberry, whose opportunities for investigation 

 have been most ample, would attribute these lake-basins 

 largely to glacial erosion.* 



* " Notes on the Surface Geology of the Basin of the Great Lakes," " Bos- 

 ton Society of Natural History 1862" ; "Geological Survey of Ohio, Report of 



