36 Ujpham — Review of the Quaternary Era^ 



the rivers in erosion, transportation, and deposition has been 

 often increased by these changes, with the result that abundant 

 fluvial deposits have been spread upon the land, along the 

 valleys and in great alluvial plains. At the same time an im- 

 mense tribute of river silt has been borne forward to the sea 

 and forms a part of the submerged margins of the continental 

 plateaus. If the proportion of the deposits made by the 

 rivers before reaching the sea appears to be greater than in 

 previous eras, the contrast is probably attributable to the more 

 general preservation of marine than of fresh-water formations, 

 the latter being liable to repeated erosion until they finally are 

 carried beneath the level of the ocean. 



Foremost among the Quaternary changes affecting river 

 action have been oscillations of the land, both of continental 

 areas and of mountain ranges. Fiords and submerged valleys 

 indicate that the glaciated areas of North America and Europe 

 have been elevated 2,500 to 3,000 feet above their present 

 level. The district intersected by the Grand Canon of the 

 Colorado has been uplifted, according to Powell and Dutton, 

 not less than 6,000 feet, causing the river to erode to this depth, 

 during Pliocene and Quaternary time ; and the latest compar- 

 atively rapid elevation, amounting to about 3,000 feet, the 

 depth of the inner or lower part of the canon, is referred to 

 the early Quaternary. In this epoch, also, great uplifts and 

 subsidences by faulting have taken place, as shown by Powell, 

 LeConte, and Diller, in the Great Basin region, with its mono- 

 clinal mountain ranges consisting of blocks of faulted but not 

 usually flexed or folded strata ; while the lofty Sierra Nevada 

 has the same structure, and attained its present prominence at 

 the beginning of the Quaternary era. In South America the 

 observations of Alexander Agassiz show that the Peruvian 

 Andes have experienced Quaternary uplifts of 2,900 feet, or 

 perhaps much more ; and Medlicott, Blanford, and other ex- 

 plorers find that much of the mountain-building of the Hima- 

 layan range, and the formation of the table-land of Thibet, 

 belong to the same latest era of geologic time, and that large 

 tracts of central and northwestern Asia have contemporane- 

 ously risen out of the sea. These upward movements, gener- 

 ally followed in glaciated regions by depression while the land 

 was ice-covered, but mainly permanent or only partly counter- 

 acted by subsequent sinking in the mountain ranges, have 

 given to the rivers temporarily or permanently steep gradients 

 and consequent increase of power to erode and transport ma- 

 terial, which has been often deposited in very extensive gently 

 sloping plains along their lower courses, on account of the 

 decrease there in the rate of their descent and the slackening 

 of their velocity. 



