The Ocean 



21 



shelf has been estimated by Kossinna as less than 100 m and is probably between 50 m 

 and 70 m. 



Table 6. The shelf-areas (0-200 m) of the continents and oceans respectively 

 (10" km^, according to Kossinna, 1921) 



Sum 25-99 + rather distant islands 1-50 



Sum 27-49 x IC km% 7-6 % of the sea surface 



The shelf near the continental slope, often at a considerable distance from the coast, 

 shows remarkable canyon-like troughs stretching over the bottom of the shelf and the 

 adjacent continental slope. While previously only a few of these remarkable structures 

 were known it has been shown recently, especially by the work of the United States 

 Coast and Geodetic Survey, that they are of wide occurrence. Their topography can 

 be rapidly and accurately determined by echo sounding. They were first thought to be 

 drowned, sunken valleys, but it has been shown that they probably have a different 

 origin. Two trough forms are found : submarine valleys in areas which have at some 

 time been strongly glaciates (for instance around Iceland) and submarine canyons in 

 regions which have remained unglaciated. The latter are usually found only at the 

 edge of the shelf in the area of transition to the continental slope; these reach large 

 depths (2000-3000 m) and often have little apparent connection with the topography 

 of the neighbouring coastal area. Several series of these submarine canyons have been 

 found and accurately charted: on the continental shelf and the edge of the shelf along 

 the North American coast north of Cape Hatteras among which is the long-known 

 submarine valley of the Hudson mouth (Fig. 1 3), along the coast of Cahfornia and along 

 the coast of Washington and Vancouver Island (Smith, 1939). Individual submarine 

 valleys are known along the east coast of Korea, along both coasts of Japan and on the 

 eastern and southern coasts of Formosa. Submarine valleys frequently occur at the 

 mouths of large rivers, such as the Ganges, the Indus (Fig. 14), the Congo (Fig. 15), 

 the Ogowe and the Niger. They are also present in different parts of the European and 

 American mediterranean seas. Some parts of the continental shelf are free from these 

 canyons, for example the North American coast south of Cape Hatteras or the 

 eastern coast of Asia south of the Yellow Sea. A summary of the distribution of can- 

 yons in all oceans and the possible nature of their origin was recently given by 

 Shepard (1948). 



The walls of these subm.arine canyons are usually very steep on both sides, often 

 with a slope of 5-10° and sometimes 20-35° or even more. These canyon walls 

 must be made of hard rock since thick layers of soft loose sediments could not be 



