ORIGIN OF LAKE BASINS. 17 



gravel with hollows of the character just described, scattered over their 

 surfaces, are known as " pitted plains," and find their most acceptable 

 explanation in the hypothesis just suggested. 



Lakes Walden and Cochituate, JNIassachusetts, are believed to be 

 examples of the class of lakes here referred to, and to owe their origin to 

 the melting of ice masses that were either partially or wholly l)uried in 

 gravel and sand.^ Lakes of similar character in southern ^licliigan, 

 where glacial deposits are unusually abundant, might also be cited in this 

 connection. These lakes occupy crater-shaped depressions in the surfaces 

 of gravel and sand plains, of the character that would be expected to 

 result from the burial and subsequent melting of ice masses, in the 

 manner outlined above. 



From this 1)rief account of the action of ice in oljstructing drainage, 

 it will appear that lake basins are formed not only on account of the 

 damming of streams by the glaciers themselves, but by glacial erosion 

 and glacial deposition; and in still other ways, in connection with the 

 deposits made by streams. 



Basins due to volcanic ag-encics. — Inequalities on the surfaces of 

 lava sheets sometimes give rise to lakes in much the same manner as 

 lakes are formed on the surface of glaciers. Examples of such basins in 

 various stages of extinction, by drainage and sedimentation, occur on 

 portions of the lava plains of Washington and Idaho. 



A lava stream may cross a valley so as to obstruct its drainage and 

 cause a lake to form above it, in much the same way as glaciers dam 

 lateral valleys. A large lake was formed in this manner, probably in 

 Pleistocene times, on the Yukon river, Alaska, where it is joined ])y Pelly 

 river. A series of lava flows there filled the river valley from side to side 

 to a depth of several hundred feet, and formed a dam which retained 

 the waters of the Yukon, and gave origin to a broad water-body known 

 as Lake Yukon.- The obstruction has since been cut through along the 

 southern margin of the old channel, leaving a series of basaltic precipices 

 on the right bank of the river. 



1 Warren Upham, Boston Soc. Nat. Hist., Proc, vol. 25, pp. 228-242. 



- W. M. Dawson, " Report on an exploration in the Yukon district," Canadian Geol. 

 Nat. Hist. Surv., Ann. Rep., 1887-88, p. 132 B. 



I. C. IJussell, "Notes on the .surface geolop;y of Alaska," Geol. Soc. Am., Bull. vol. 1. 

 1800, pp. 14(i-148. 



C. W. Hayes, "An expedition through the Yukon district;" National Geog. Mag., 

 vol. 4, 18!»2, p. loO. 



