240 



HEEZEN AND MENABD 



[chap. 12 



youthful relief of the Pacific borders all suggest a recent origin. In contrast, the 

 non-seismic, non-volcanic character, as well as the lower relief of the Indian 

 and Atlantic margins, suggests a greater age. Thus, on the old, stable continental 

 margins, the deposition of sediments derived from the land has filled the margi- 

 nal trenches and produced the continental rise. The local relative relief on the 

 continental margin rarely exceeds 20 fm, with the major exceptions of submarine 

 canyons and occasional seamounts. 



200 



w 



h. 



? 300- 



■Z 



It 



u 



o 



400 



Fig. 9. Sonoprobe record on the continental shelf off San Diego, showing buried bedrock 

 surfaces. Note the smooth bottom -sediment profile and the easily detected, irregular, 

 sub-bottom rock profile, illustrating a buried beach on the continental shelf. (After 

 Moore, 1960.) 



b. Topographic forms of the continental shelf (see Chapter 13) 



Submerged marine beach terraces have been identified on the continental 

 shelf throughout the world. These terraces represent various Pleistocene low 

 levels and are preserved in varying degrees of completeness. In some areas 

 subsequent deposition has badly obscured these ancient beaches (Fig. 9). In 

 general, the continental shelf has a very low monotonous relief. But off major 

 rivers, such as the Hudson, partially buried ancient river channels can be 

 identified, and in certain well-surveyed areas, what appear to be ancient beach 

 ridges can be recognized. 



"The redistribution of sediments by tidal streams has been demonstrated in 



