COASTAL GEOGRAPHY AND BOTTOM TOPOGRAPHY 145 



Orange river, for the great underwater promontory sweeping seawards from the river mouth suggests 

 a considerable alluvial deposition from the great drainage system. 



Although more data are still needed, the representation of such major features of the sea-bed seems 

 adequate for our present purpose. The chart (Fig. 4) has been constructed from : 



1 . The most recent Admiralty Charts of the region. 



2. Carte Bathymetrique Internationale. A IV (International Hydrographic Bureau, Monaco, 1938). 



3. 'Meteor' expedition bathymetric charts (Stocks, 1941). 



4. Soundings made by the 'William Scoresby'. 



JFMAMJ J ASOND 



JFMAMJJASOND 



PORT NOILOTH 



J FMAMJ J A S O N D 



DASSEN 



ISLAND 



LUDERITZ BAY 



Fig. 3. Mean annual rainfall in inches, at five points on the South-west African coast: Mossamedes (15 12' S.), Walvis 

 Bay (22 56' S.), Luderitz Bay (26 39' S.), Port Nolloth (29 14' S.), and Dassen Island (33° 26' S.). (From data in Royal 

 Naval Meteorological Service and South African Air Force, 1944.) 



Over the greater part of the area covered by our surveys, the continental shelf, defined approxi- 

 mately by the 200 m. contour, is about 40 miles broad, widening to about 90 miles off the Orange 

 river mouth, and to 70 miles off Walvis Bay. Off Concepcion Bay, in 24 S., there is a sharp indenta- 

 tion on the shelf edge, and as at Luderitz Bay the shelf is less than 20 miles broad. 



North of 20 S. the shelf narrows, and between 16 and 13 S. is almost non-existent, the slope 

 falling straightway from the coast into the depths of the Angola Basin. The shelf edge off Bahia dos 

 Tigres is much dissected into deep valleys which extend to depths of 1000 m. 



The bottom slopes away fairly steeply from the coast to the 100-m. contour along most of the coast- 

 line, and then more gradually to the shelf-edge, forming virtually a submarine plain, a feature most 

 pronounced off the Orange river. From the shelf-edge there is a fairly gradual and constant slope to 

 3000 m., interrupted only off the Cape Peninsula where deep canyons are numerous between 500 and 

 3000 m., and in 20 S. where the continental slope is very gradual and at 1000 m. grows into a buttress 

 forming the northern end of the Walvis Ridge. 



The Walvis Ridge leaves this buttress in a westerly direction at 3000 m., and runs southwards and 

 then to the west again to link up with the central Atlantic Ridge. In 25 S., 6° E. there is a prominent 



