504 THE GLACIAL LAKE AGASSIZ. 



wide and GOO to 1,000 feet deep, were still a part of the luainlaud during 

 the late Pliocene and early Pleistocene periods.^ 



In northern California Prof. George Davidson, of the United States Coast 

 Survey, as cited h}" Spencer, reports tlu-ee submarine valleys aljout 25, 12, 

 and G miles south of Cape Mendocino, sinking respectively 2,400, 3,120 and 

 2,700 feet below the sea-level where they cross the 100-fathom line of the 

 marginal plateau.' If the land here were to rise 1,000 feet, these valleys 

 would be fjords with sides towering high above the water, but still descend- 

 ing beneath it to profound depths. The time of great elevation permitting 

 erosion of these and a large number of other submerged valleys of the 

 Californian coast is shown by Le Conte to have been the Pliocene period, 

 with culmination of the uplift in the early part of the Pleistocene.^ 



Farthei' to the north, Puget Sound and the series of sheltered channels 

 and sounds through which the steamboat passage is made to Glacier Bay, 

 Alaska, are submerged valleys of erosion, now filled by the sea, but sepa- 

 rated from the open ocean by thousands of islands, the continuation of the 

 Coast Range of mountains. From the depths of the channels and fjords 

 Dr. G. M. Dawson concludes that this area had a preglacial elevation at 

 least aliout 1)00 feet above the present sea-level during part or the whole 

 of the Pliocene period.* 



The general absence of Pliocene formations along both the Atlantic 

 and Pacific coasts of North America indicates, as pointed out by Prof. C. 

 H. Hitchcock, that during this long period all of the continent north of the 

 Gulf of Mexico held a greater altitude, which from the evidence of these 

 submarine valleys is known to have culminated in an elevation at least 

 3,000 feet higher than that of the present time. Such plateau-like uplift 

 of the continent appears to have exerted so great influence on its meteoro- 

 logic conditions, bringing a cooler climate throughout the year, that it 

 finally became enveloped by ice-sheets to the southern limit of the glacial 

 striae, till, and moraines, stretching from Nantucket and Cape Cod to New 

 York City, Cincinnati, St. Louis, Bismarck, and thence westward to the 



'Bulletin of the California Academy of Sciences, Vol. II, 1887, pp. 515-520. 



2 Ibid., Vol. II, pp. 265-268. 



"Bulletin, G. S. A., Vol. II, 1891, pp. .323-330. 



< Canadian Naturalist, new series. Vol. VIII, pp. 241-248, April, 1877. 



