320 HEEZEN AND L,AUGHTON [CHAP. 14 



a system of narrow inter]ilain cliannels leading through Thcta Ciap to the 

 Iberia Plain (cf. Section 5, pages 342-345). 



c. Iberia Abyssal Plain 



The Iberia Plain is roughly rectangular, covering 30,000 mi-, and near its 

 western boundary is extremely flat, the gradient being 1 : 4000 from the north. 

 The sediments are derived partly from the Biscay Plain via the Theta Gap 

 interplain channels and partly from the SE from the large canyons off the coast 

 of Portugal. The increase in the number of seamounts and small hills projecting 

 from the plain toward the west suggests that the thickness of sediments 

 steadily diminishes until the bm-ied topography totally emerges. The depth of 

 the plain at its northern end is 2850 fm, 100 fm deeper than the SW end of the 

 Biscay Plain, and its deepest part is 2930 fm. 



d. Tagus Abyssal Plain 



West of southern Portugal lies an oval plain of 6000 mi 2 of depth varying 

 from 2750 to 2800 fm sloping down to the SW. It is fed principally from the two 

 large canyons SW of the River Tagus. To the north of this plain are tw^o smaller 

 and shallower plains (2680 and 2670 fm) which appear to be filled from locally 

 derived sediments. 



e. Horseshoe Abyssal Plain 



A small abyssal plain of 5400 mi^ lies in the center of a horseshoe-shajied ring 

 of seamounts west of Gibraltar. There are insufficient soundings to establish the 

 extent of this plain. 



f. Seine Abyssal Plain 



To the east of Seine Bank an oval plain covers 15,000 mi^. While part of the 

 sediment has come from the NE, most of it derives from the SW where there is 

 a very smooth col 50 fm above the level of the plain and separating it from 

 the Madeira Plain. This source appears to be shared with the Madeira Plain. 



g. Madeira and Cape Verde Abyssal Plains 



From Gibraltar to Cape Verde a vast abyssal plain of some 200,000 mi^ 

 parallels the wide continental rise of Northwest Africa. A northern finger 

 extends to the west of the Madeira Rise. The lower continental rise and the 

 Madeira Rise, both relatively smooth (Fig. 1), merge with the plain in a smooth 

 transition and a distinct boundary is lacking. Northeast of the Canary Islands, 

 continental rise gradients of greater than 1:1000 reach the abyssal hills and 

 provide a natural boundary between the Cape Verde and Madeira Abyssal 

 Plains. 



The more southerly part differs in character from the northerly part. To the 

 west of Madeira, the sediment is apparently derived from the hills and seamounts 

 of the Azores-Gibraltar Ridge, and from the region between Madeira and the 

 Canary Islands. The w est ward boundary of the plain here is determined by the 



