Professor Spencer — Continental Elevation. 35 



Submarine Channels off the Eastern Coast of America. 



The submerged valleys, whioh are best developed among the 

 Bahamas and oif the adjacent portions of the continent, provide the 

 key for interpreting the submarine features of other regions. The 

 broad subcoastal plain off the south-eastern States becomes narrowed 

 to a few miles east of Cape Hatteras ; but northward it broadens 

 again, and eventually reaches a width of nearly' 300 miles south-east 

 of New England, and more than that across the submarine plateau 

 which forms the Newfoundland banks. East of Labrador it has 

 a considerable breadth, but the soundings there are too scanty for its 

 delineation. In drawing the contours at a considerable distance 

 apart, the same forms of indentation are repeated in the borders of 

 the plateau as those observed farther south ; but where the contours 

 are drawn close together (even where the soundings are not as 

 numerous as is desirable), the deep valleys are found to be con- 

 tinuations of existing rivers. Thus, LindenkohP traces the Hudsou 

 Eiver channel to a depth of 2,832 feet, and the Great Egg Harbour 

 channel to 2,334 feet, where the plateau is submerged only 600 feet. 

 The Delaware and the Susquehanna valleys are also recognizable on 

 the subcoastal plain to depths of about 3,000 feet. 



la 1889, the writer showed how the Laurentian valley was 

 submerged for a distance of 800 miles, beneath the waters of the 

 Gulf of St. Lawrence, with the channel from 1,200 to 1,800 feet 

 below the surface of the sea; but near the edge of the drowned 

 plateau it descends abruptly to a depth of 3,666 feet.* The same is 

 true of the valleys crossing the New England, Nova Scotia, and the 

 Newfoundland banks. From the edge of the continental shelf, the 

 Susquehanna valley descends precipitously to a depth of more than 

 9,000 feet, with its valley recognizable to 12,000 feet. The Delaware 

 descends abruptly to 6,066 feet, and is plainly traceable to 11,256 feet, 

 and to greater depths beyond. The same is true of the Hudson and 

 its tributaries from Connecticut, being recognizable to depths of 

 more than 12,000 feet. From the borders of Massachusetts, Nova 

 Scotia, and the Newfoundland banks the valleys descend pre- 

 cipitously into amphitheatres 6,000 or 7,000 feet below the surface, 

 and continue to depths of 12,000 feet, and in some cases to even 

 15,000 feet. 



While to an unknown extent the drowned plateaux are covered 

 with Tertiary formations, still the submerged valleys must, to 

 a considerable extent, have been excavated out of hai'd Palasozoic 

 and older strata, thus producing variations in the lengths of the 

 deeper channels, and forming a contrast with some of those of the 

 Antillean region. 



From analogy with land valleys, the channels crossing the sub- 

 marine coastal plains of a few hundred feet, afterwards of perhaps 

 3,000 feet, i-epresent a long period of elevation. Then followed the 



^ American Journal of Science, vol. xli, p. 490, 1891. 



"^ J.W. Spencer, " High Continental Elevation preceding the Pleistocene Period" : 

 Bull. Geol. Soc. Amer., vol. i, p. 66, 1889. 



