EVIDE^sCE FROM MAKGINAL BELT WEST OF NEW ENGLAND 313 



explaining the great depth of the channel, nearly 5,000 feet, at the outer 

 edge of the continental shelf. Upwarping to so great an amonnt is im- 

 probable ; some other cause must be sought to complete even a speculation 

 concerning the origin of the channel. According to Fuller, the excava- 

 tion of the deep rock gorge of the Hudson at and above New York City, 

 like that of the channel out to sea, is to be referred to an early stage of 

 the Glacial period. ^^ He thinks the cutting was essentially fluviatile, and 

 it is hard to credit any other cause with the original development of the 

 channel. But, taking together the gorge above New York City and the 

 whole channel in the continental shelf, we have a trough that has the look 

 of a rather typical fjord depression. Is it possible that the original ante- 

 cedent part of the valley of the formerly extended Hudson Eiver was 

 occupied by an effluent glacier as far as the edge of the shelf? Modern 

 glaciers are cutting deep troughs below sealevel, matching in principle 

 the fjords of Norway. If the Montauk or other pre-Wisconsin ice ex- 

 tended farther seaward than the Wisconsin moraine, such a tongue might 

 have reached so far out. Later filling by glacial outwash and by marine 

 agencies would tend to fill the old trough, especially along its inner part. 



The outer part would long escape aggradation. The shallow trench 

 inside the 100-fathom line might be explained as due to etching by the 

 Hudson when extended during the latest (Wisconsin) glaciation. 



The question as to whether the submarine channel may be used as a 

 test of the Jamieson hypothesis is seen to include many problems. Baffling 

 as these are, they are no more troublesome than the assumption of a 

 Pleistocene emergence of this part of North America to the extent of 

 5,000 feet or more. A combination of moderate crustal warping, glacial 

 excavation, and river-cutting is surely complex; but no simple theory is 

 likely to account for the remarkable channel of the Hudson. 



SUSQUEHANNA ''DEEPS" 



Another recent change of river regime in the peripheral belt is found 

 in the line of "deeps" along the channel of the Susquehanna Eiver near 

 the Maryland-Pennsylvania boundary. Mathews has given a good de- 

 scription of them.^^ Within a distance of about 25 miles the broad, flat 

 rock-floor of the river is interrupted by six long spoon-shaped depressions. 

 These "deeps" vary in length from 4,000 feet to nearly 2 miles and in 

 maximum depth from 50 to 130 feet, measured below the general floor of 

 the channel. The bottoms of five of the holes are 10 to 30 feet below sea- 

 level. The "deeps" usually show a "downstream sag in the bottom 



" M. Fuller : Professional I'aper 82, U. S. Geol. Survey, 1914, pp. 60, 218, 220. 

 " E. B. Mathews : Bull. Geol. Soc. Am., vol. 28, 1917, p. 335. 



