ORIGIN OF LAKE BASINS. 15 



extremities of the existing glaciers of this country, from the High Sierra, 

 California, northward to Alaska. These are retained hy moraines, from 

 Avhich the ice has receded within a few 3'ears, thus leaving not even the 

 shadow of a douht as to their mode of origin. 



^lany of the lakes of Scandinavia and of Switzerland are retained by 

 ancient moraines, as are also in part, the long, deep lakes on the Italian 

 side of the Alps, and draining to the Po. The most striking example of 

 the type of lake here described, however, which has been studied b}" the 

 writer, is Lake Wakatipu, on the east side of the Southern Alps, New 

 Zealand. This magnificent water body, surrounded on all sides by lofty 

 snoA\-clad peaks, has many of the characteristic features of lakes Como 

 and ]\Iaggiore, and is not second to them in majesty and beauty. 



The drainage of mountain valleys, in which moraine-dammed lakes 

 have been formed, is frequently so abundant that stream channels are cut 

 tln-ough the obstructions, and the lakes chained. When this occurs, 

 beautiful grass-covered vales or " parks," as they are called in the Rocky 

 mountains, are formed. These charming valleys are quite as beautiful 

 and frequently furnish as great a contrast to the ruggedness of the sur- 

 rounding scenery, as did the gem-like lakes that preceded them. 



In most instances the deep mountain valleys of North America, 

 now occupied by moraine-dammed lakes, were excavated by streams 

 previous to being glaciated, and only served temporarily as avenues for 

 ice drainage. Their main topogra[)hic features are due to stream erosion 

 and weathering. Only minor changes such as the smoothing and round- 

 ing of their bottom contours, can be ascribed to glacial abrasion. 



The general sheets of debris left after the retreat of continental glaciers 

 and Ijy the melting of the expanded extremities of large Al})ine glaciers, 

 are usually uneven on account of the manner of their deposition, and 

 abounds in depressions Avliich may hold water. In many instances the 

 lakes origfinatino- in this manner are without surface outlets, their sur- 

 plus water escaping by percolation. 



On the formerly ice-covered portion of northeastern North America, 

 the lakes occupying depressions in the general covering of superficial 

 material are so numerous that the position of the southern boundary of 

 tlieold ice sheet may be ap])ro\imately tiace(l on a drainage map of the 

 region l)y noting the southern limit of the lake-strewn portion. The (»ld 

 land sui'face south of tlie glacial Ixtundary, is almost entirely free from 

 uiuhained basins ; and in this, as well as in other respects, presents a 

 striking contrast to the rejuvenated surface of the land to llic north. 



