Z LAKES OF XOKTH AMERICA. 



expansion of the lakes than Avoukl appear if the land was barren. The 

 wealth of vegetation tends also to preserve the original barriers from 

 erosion. Abont the southern shore of Hudson bay there is another area 

 recently abandoned by the sea, on which there are lakes, but this region 

 is so little known that it cannot be pointed to with confidence as a case 

 in point. In the Great Basin, as the vast area of interior drainage 

 between the Sierra Nevada and Rocky mountains is termed, there are 

 many lakes, some of them of large size, which occupy depressions in the 

 surfaces of sedimentary deposits left exposed l^y the evaporation of much 

 larger Pleistocene water bodies. Great Salt lake and Sevier lake, Utah, 

 occupy the lowest depressions in valleys formerly flooded by the waters 

 of a great inland sea to which the name Lake Bonneville has been 

 applied. Pyramid, Walker and other lakes in Nevada, occur in valleys 

 which are deeply filled with the sediment of another ancient water body 

 named Lake Lahontan. In these instances, however, and in many others 

 of similar character throughout the Arid Region, the positions of the 

 present lakes on the approximately level floors of desert valleys have 

 been partially determined by recent movements of large blocks of the 

 earth's crust adjacent to lines of fracture, and liy the unequal deposition 

 of alluvial material swept out from mountain valleys and deposited on the 

 adjacent plain. These recent changes have modified the character of the 

 basins now occupied by lakes, but essentially they are depressions on new 

 land areas, and form the most typical examples of their class that can 

 be found in this country. 



There are nsw land areas about the borders of the Laurentian lakes, 

 which have been left exposed by the recession of still greater lakes that 

 occupied the same basin at a comparatively recent date, and also in the 

 region drained by Red river in Minnesota and Canada, formerly flooded a 

 vast lake named in honor of Louis Agassiz. Along some of our rivers, 

 also, which flow through ancient valleys now deeply filled, there are 

 narrow areas of new land, similar to the recently exposed borders of the 

 Laurentian lakes. In all of these instances, however, the lakes formed 

 in the inequalities of the surface are small and of little importance. 



Lakes on new land areas are surrounded by topographic forms 

 expressive of youth, and are themselves evidence of topographic im- 

 maturity. When drainage is established on such areas the basins are 

 soon emptied. The lives of lakes of this class, as is the case with all 

 terrestrial water bodies, depend largely on climatic conditions. They 

 may continue longer in one region than in another, but in the 



