534 THE PLEISTOCENE EPOCH 



In California raised beaches occur as high as 1500 feet above the 

 present sea-level, which were probably due to the same submer- 

 gence, though they may be older. Pleistocene movements con- 

 tinued, it may be, into the Recent epoch, increased the height 

 of the Sierra Nevada, Wasatch, and Basin Ranges, and of the 

 high Plateaus of Utah and Arizona. 



The late Pleistocene was a time of ameliorated climate and 

 heavy rainfall, when the flooded rivers moved sluggishly, owing to 

 the diminished slope, and spread sheets of sands, gravels, and 

 clays over their flood plains and in their estuaries, through which 

 they have subsequently cut terraces, when elevation had given 

 them renewed power. 



The events of the Glacial epoch, and the diastrophic movements 

 which accompanied and followed it, have had the most important 

 and wide-spread effects upon the topography of the glaciated 

 regions. The sheets of drift, stratified and unstratified, have com- 

 pletely changed the surface of the country, and by filling up the 

 pre-Glacial valleys, have revolutionized the drainage, only the 

 largest streams being able to regain their old courses. Innumer- 

 able lakes, large and small, were formed in depressions, rock basins, 

 and behind morainic dams, the contrast between the glaciated and 

 non-glaciated regions in regard to the number of lakes in each 

 being very striking. The events which led up to the formation 

 of the great Laurentian lakes are connected with the successive 

 phases of the Glacial epoch, but they are very complex and the 

 phenomena are differently interpreted by different observers, so 

 that only the briefest outline of them can be attempted in this 

 place. 



When the Laurentide glacier was retreating across the area now 

 occupied by or tributary to the Great Lakes, its front acted as a 

 great dam holding back the waters from descending the valley 

 of the St. Lawrence, and forming ice-dammed lakes, whose form 

 and size varied with the position of the ice-front. These lakes 

 discharged their waters southward at various points where the 

 divides were lowest. At one time, for example, when the eastern 

 half of Lake Ontario was filled by the ice, a long lake extended 



