CONTINENTAL TERRACES AND SUBMARINE VALLEYS II5 



Hudson "Channel" which crosses the shelf and is itself en axe 

 with the Hudson River at New York City. 



A few years of sonic sounding increased the number of 

 known submarine canyons from a half dozen to more than one 

 hundred. This is already a large number, though less than one 

 per cent of the total length of shelf or slope off continental 

 land has been adequately sounded. 



Off a 400-mile stretch of the coast of eastern United States, 

 more than a score of canyons, or one for an average 'longshore 

 interval of 15 to 20 miles, have been mapped. But the wonder 

 of their disclosure grew with the simultaneous discovery of 

 deep and general trenching of the areas separated by the can- 

 yons. There the trenches, greatly outnumbering the canyons, 

 differ from these by heading at or below the fall-off at the head 

 of the continental slope. To distinguish these many shorter 

 valleys, we shall call them "slope furrows" or simply "fur- 

 rows." Figure 62 shows the canyons and principal furrows of 

 the long belt. Barbed arrows indicate their axial extent down 

 to the 1000-fathom level; they reach down much farther than 

 that. Between each pair of adjacent furrows there is a long 

 strip of higher ground; each strip may be called an "inter- 

 furrow ridge" or simply "ridge." Like the furrows and can- 

 yons, the ridges run in the general direction of the continental 

 slope, that is, approximately at right angles to the line of fall- 

 off or break of slope of the continental terrace. 



The first maps to illustrate well the association of canyon, 

 furrow, and ridge were made by the United States Coast and 

 Geodetic Survey. Its exploring vessels have sounded in great 

 detail the inshore waters from Georges Bank off Maine to 

 Cape Henry of Virginia. From the soundings of 1930 to 1932 

 the region of Georges Bank was mapped by Lieutenant P. A. 

 Smith. He indicated the submarine topography with isobathic 

 lines, an isobath being one that joins points of equal depth of 



