GLACIAL DAMS, LAKES, AND WATERFALLS. 359 



the surface deposits. The great area of excavation in which 

 the lakes lie was probably at this time filled to the brim with 

 ice-cold fresh water ; and this, flowing outward through all 

 the channels open to it, may have been sufficient to prevent 

 the entrance of the arctic marine mollusks, of which the re- 

 mains are so abundant in the Champlain clays of the St. Law- 

 rence Valley and the Champlain basin.* 



Lakes caused by glacial dams are of two classes : 1. Those 

 produced by the irregular deposition of moraine material ; 

 2. Those caused by the ice itself during the period of its 

 continuance. The first of these classes may also be profit- 

 ably subdivided into — (1) Those caused by deposits which 

 have closed up old water-courses ; (2) Those caused by de- 

 posits producing complete inclosures of the nature of kettle- 

 holes. 



Making the last class the first subject of consideration, 

 we note that by far the larger number of the small lakes 

 which diversify the glaciated region occupy the basins of 

 kettle-holes. When treating of terminal moraines and 

 katnes, kettle-holes were spoken of as one of the prominent 

 features characterizing these deposits, and the origin of the 

 smaller ones at least was said to be due to the melting away 

 of masses of ice which had from time to time been covered 

 by the earthy debris which accumulates near the front and 

 along all the great lines of drainage of extensive glaciers. 

 Protected for a while by this debris from melting, these 

 masses of ice first begin to disappear around the exposed 

 edges of their sides, allowing the sand, gravel, and pebbles 

 to be heaped up about their bases, so that, upon the final 

 disappearance of the ice, an inclosure is left, varying in size, 

 shape, and depth according to the extent of the ice-mass 

 inclosed. Many of these kettle-holes are dry throughout the 

 larger part of the year, since they are above the general 

 level ; and the surrounding material of the rim is so coarse, 

 that, even after long-continued rains, the water remains in 



* " Geology of Ohio," vol, ii, pp. 77-79. 



