210 J. W. SPENCER — SUBMARINE VALLEYS 



of the sea. Farther on, where the coastal plain is submerged 90 feet, 

 the channel is still 90 feet deeper. At 53 miles the channel reaches the 

 depth of 180 feet, with a breadth of two or three miles between the 

 banks, which are here also covered by 90 feet of water. The coastal 

 plain is more and more submerged, until at 91 miles from the Hook its 

 depth is 234 feet; but the channel of the Hudson here is still deeper 

 than above, although it shows only a depth of 48 feet beneath the banks, 

 probably due to partial filling. At 97 miles it begins to assume the form 

 of a canyon, which continues 23 miles farther, to the apparent edge of 

 the continental shelf. The average breadth of the river channel is one 

 and a quarter miles and of the gorge about three miles, with a depth 

 reaching to 2,844 feet, where the coastal plain is submerged only 420 feet. 

 The bottom of the channel and the can5^on is covered by a bluish slate- 

 colored mud, with fine sandy grit. At this point Mr Lindenkohl stopped 

 his inquiries. The continuation of the valley beyond, down the conti- 

 nental slope for 140 miles, will be noticed in its proper place (page 214). 



Susquehanna and Delaware Channels 



The surface of the submerged plain in this region has been so leveled 

 by the shifting sands that Mr Lindenkohl was unable to trace the chan- 

 nels across it, as in the case of the Hudson, but inside of the ba3's he 

 found them. Thus in the Chesapeake the channel was observed for 35 

 miles and found to be from one to two miles wide, with a depth from 42 

 to 108 feet at points where the adjacent banks were submerged to a 

 depth of 48 feet. Below it is filled by sand bars. So also he found the 

 drowned channel of the Delaware from 54 to 108 feet below sealevel, or 

 from 30 to 84 feet deeper than the shoals. The obstruction of the chan- 

 nels by the sands appears to have been due rather to the action of cur- 

 rents in shallow water during epochs of slight changes of level than to 

 their filling in deeper water at a greater distance from the shore. Fur- 

 ther evidence of the existence of channels deepl}^ incising the coastal 

 plains, whether above the sealevel or below it, is found by borings which 

 reveal them and show that the modern streams in their lower reaches 

 are flowing over channels, now silted up to depths of 250 feet or more, 

 the most notable being not in this region, but near the mouth of the 

 Mississippi, whose original bed is a thousand feet below sealevel at New 

 Orleans, though for some distance beyond the coastline it is obscured 

 like the channels of the Chesapeake and Delaware. The former courses 

 of these great river valleys again become apparent in the deep coves or 

 canyons incising the edge of the continental slope, which is the principal 



