SECT. 2] ABYSSAL PLAINS 317 



seamoimts. However, even here no hills are found. In the strip extending be- 

 tween 30'' and 37 °N, the east and west boundaries of the plain are formed by 

 scarps 200 to SOO fm high. The Iwundary of the north side of the Bermuda 

 Rise is formed by a distinct shallowing but no scarp is seen. There is some justifi- 

 cation for separating the northwest arm of the Sohm Abyssal Plain north of the 

 Bermuda Rise from the rest of the Sohm Abyssal Plain, since a mid-ocean 

 canyon runs from the northwest to the central portions of the plain. This north- 

 west portion of the Sohm Abyssal Plain has sometimes been called the Nova 

 Scotia Abyssal Plain. A sill might lie northeast of the northeast tip of the 

 Bermuda Rise. Here the bottom gradient changes slightly and an extremely 

 large magnetic anomaly is associated with the region. However, the plain 

 narrows only slightly at this point and, since we cannot tell how many in- 

 dividual basins were filled to form any individual abyssal plain, we will consider 

 tliat the Sohm Abyssal Plain now includes the whole area. South of Newfound- 

 land the continental rise merges with the abyssal plain with no scarp or line of 

 hills. Here the boundary is taken at the point where the gradient is gentler than 

 1 : 1000. This is not an entirely arbitrary definition since an appreciable change 

 in gradient generally occurs near this point. South of the boundary the gradient 

 gradually decreases to 1 : 3000 at 37°N, beyond which it decreases even more 

 gradually and reaches 1 : 5000 at the southern edge of the plain. The Sohm 

 Abyssal Plain covers 350,000 mi^. 



b. Hatteras, Blake-Bahama and Silver Abyssal Plains 



The Hatteras and Sohm Abyssal Plains are separated either by abyssal 

 hills or quite possibly by an irregular gap. The northwest margin of the 

 Hatteras Abyssal Plain is abruptly formed by the lower continental rise hills. 

 The eastern boundary is generally distinct and in places is marked by a 

 50-100-fm scarp. From 25-44°N the western margin of the plain gently laps 

 the smooth Blake-Bahama Outer Ridge. Its southern margin laps the Antilles 

 Outer Ridge. Only two or three small hills have been found within the 

 Hatteras Plain. 



The narrow Blake-Bahama Al)yssal Plain which lies between the Blake- 

 Bahama Outer Ridge and the Blake Plateau-Bahama Island region connects 

 through C'at Gap at 24.5°N, 74.5°W with the Hatteras Abyssal Plain (see Fig. 

 14, Chapter 12, p. 247). Judging from width and regional slope this plain 

 appears to have been fed largely from the Providence Channel. 



The Silver Abyssal Plain which lies northeast of Silver Bank is bounded 

 on the north by the Antilles Outer Ridge which separates it from the Nares 

 and Hatteras Abyssal Plains. Only recently discovered, it appears to be fed 

 from the southern Bahamas. 



c. Nares Abyssal Plain 



South and southeast of the Bermuda Rise lies the Nares Abyssal Plain, a 

 37,000 mi2 abyssal plain that slopes eastward toward the Mid-Atlantic Ridge 



