100 THE FLOOR OF THE OCEAN 



From the beach out to the line where the depth of water meas- 

 ures 50 to 100 fathoms (300 to 600 feet), the broad top of each 

 terrace is inclined gently toward the deep ocean. This nearly 

 horizontal part of the surface of the terrace is called the conti- 

 nental shelf. At its outer limit, given by the depth just stated, 

 begins the fall-off, or break-of-slope, at the beginning of the 

 continental slope, which extends to the foot of the terrace, 2000 

 to 2500 fathoms below sealevel. See Figure 2. 



Most of each continental shelf lies some fathoms deeper 

 than it would lie if the land-derived sediments were now being 

 moved in a continuous sheet of detritus all the way from the 

 beach to the continental slope. By actual dredging Dr. H. C. 

 Stetson has found that the seaward limit of even the fine muds 

 which are now being added in clearly observable quantity to 

 the Atlantic terrace off Massachusetts is tens of miles inside 

 the fall-off. Out to the limit of these muds the shelf is being 

 raised by sedimentation. As the slow shallowing continues, the 

 waves and currents will be able to push that outer limit for the 

 continuous layer of sediment farther and farther to seaward. 

 When the new layer shall have been extended to the fall-off, 

 the surface of the shelf will be something like 10 fathoms or 

 60 feet higher than it is at present. It will then have reached 

 a profile of equilibrium. Then the level of the fall-off will be 

 at wave-base, to use the technical term meaning the level down 

 to which the ocean waves can effectively stir and transport 

 detritus. At the present time, then, it appears that the outer 

 half of each broad continental shelf is too deep to represent a 

 profile in equilibrium with the forces responsible for the build- 

 ing of the continental terrace. The observation of Dr. Stetson 

 suggests that in comparatively recent time a top layer had been 

 removed from the terrace, and that an equivalent amount of 

 sediment is being slowly restored to the broad continental 

 shelf. Why the original profile of equilibrium, which had 



