38 Upham — Review of the Quaternary £ra, 



fessor LeConte has shown, whereas now they are separated 

 from the mainland and from each other by channels twenty to 

 thirty miles wide and 600 to 1,000 feet deep. 



The general absence of marine Pliocene formations in North 

 America and northern Europe indicates that the areas on 

 which ice-sheets were accumulated in the Glacial period had 

 long stood higher than now ; and this elevation probably was 

 shared, in its culmination, by our entire area north of the 

 Gulf of Mexico, including even our southeast coast from the 

 Carolinas to Florida, where scanty Pliocene beds are found. 

 The altitude attained by this continent was about 3,000 to 

 3,500 feet, if not more, above its present level, as is known by 

 the valleys that were then eroded on the margin of the conti- 

 nental plateau but are now submerged. The greatest height 

 seems to have been reached in the early part of the Quaternary 

 era, causing snowfall to prevail, instead of rain, throughout 

 nearly the whole year in what are now temperate latitudes, and 

 thus inaugurating the Ice age. South of the glaciated area, 

 the Allegheny Mountains and all the southern part of the 

 United States received increased precipitation of snow in 

 winter, but it was melted by the warmth and rains of summer. 



If this view is true, that the great elevation of the land was 

 the principal cause of its glaciation, the height must have been 

 maintained during a considerable time, sufficient for the accu- 

 mulation of thousands of feet of ice ; and after an interval of 

 subsidence, at least for the southern part of the region, marked 

 by the interglacial epoch, we must suppose that another uplift 

 brought on the later ice-sheet. But the whole extent of the 

 time during which the land was held at great heights by the 

 successive uplifts could only have been a small part of the 

 Quaternary era ; for such elevation would evidently afford as 

 favorable opportunity for erosion by the rivers in their steep 

 descent on the margin of the continental plateau as has been 

 granted to the Colorado Kiver in the erosion of its Grand 

 Canon. We may therefore infer that the combined length of 

 the Glacial epochs bears somewhat the same ratio to the entire 

 Quaternary era as the extent of the short submarine fiord of 

 the Hudson bears to that of the long and more profound inner 

 canon of the Colorado. 



The uplift of North America in late Tertiary time brought 

 to the partially or almost wholly base-levelled basins of rivers 

 on the Atlantic slope and in the central United States a new 

 cycle of erosion, as Davis and Wood have shown in northern 

 New Jersey,* and President Chamberlin for the Upper Mis- 

 sissippi and its tributaries. f Tertiary erosion, probably most 



* Proceedings, Boston Society of Natural History, vol. xxiv, 1889, pp. 393, 412. 

 f U. S. Geol. Survey, Sixth Annual Report, pp. 222-4. Geology of Wisconsin, 

 vol. i, p. 255. 



