and Deposits of Flooded Rivers. 37 



Before considering these river deposits, however, we shall 

 need to inquire somewhat more in detail what movements of 

 elevation and subsidence have affected our own continent, and 

 what have been the influences of Quaternary climatic condi- 

 tions, and of the ice-sheets, upon the volume of our rivers and 

 upon the supply of detritus which they have transported. 



The evidences of great uplift of JSTorth America preceding 

 the Pleistocene period have been impressively stated by Prof. 

 J. W. Spencer. * The submarine border of the continent is 

 cut by valleys or channels, which are doubtless river-courses 

 that were eroded while the land stood higher than now ; and 

 its subsidence evidently took place in a late geologic period, 

 else these channels would have become filled with sediments. 

 According to the United States Coast Survey charts, as noted 

 by Spencer, the bottom of such a submerged valley just out- 

 side the delta of the Mississippi is found by soundings at the 

 depth of 3,000 feet. The continuation of the Hudson River 

 valley has been traced by detailed hydrographic surveys to the 

 edge of the steep continental slope, at a distance of about 105 

 miles from Sandy Hook. Its outermost twenty-five miles are 

 a submarine fiord three miles wide and from 900 to 2,250 feet 

 in vertical depth, measured from the crests of its banks, which 

 with the adjacent flat area decline from 300 to 600 feet below 

 the present sea level, f Again, the Coast Survey and British 

 Admiralty charts, as Spencer states, record submerged fiord 

 outlets from the Gulf of Maine, the Gulf of Saint Lawrence 

 and Hudson Bay, respectively at depths of 2,664 feet, 3,666 

 feet and 2,040 feet. In California, too, Prof. George David- 

 son, also cited by Spencer, reports three submarine valleys 

 about twenty-five, twelve, and six miles south of Cape Mendo- 

 cino, sinking respectively to 2,400, 3,120 and 2,700 feet below 

 the sea level where they cross the 100 fathom line of the mar- 

 ginal plateau. 



Preglacial elevation of this continent is further attested, but 

 without supplying measurements, like the foregoing, of its 

 great extent, by the eroded areas of Pamlico and Albemarle 

 sounds, and of Chesapeake and Delaware Bays ; by the fiords 

 of Maine, of all the coast northward to the Arctic regions, 

 and of the Pacific Coast south to Vancouver Island and the 

 Columbia Biver; by the Golden Gate, which has a maximum 

 depth of 414 feet ; and by the islands south of Santa Barbara 

 and Los Angeles, which were united with the mainland dur- 

 ing the later Pliocene and early Quaternary periods, as Pro- 



* Bulletin, G-eol. Soc. of America, vol. i. 1889, pp. 65-70 ; also, Geol. Magazine, 

 III, vol. vii, pp. 208-213, May, 1890. 



f See Prof. J. D. Dana's recent discussion of the submarine Hudson river chan- 

 nel, with map reduced from a chart of the U. S. Coast Survey, this Journal, III, 

 vol. xl, pp. 425-437, Dec. 1890. 



