EPICONTINENTAL SEDIMENTATION 487 



development. Rates of sedimentation and nature of the foundation 

 beyond the reach of coring instruments are matters of interest to 

 the general geologist. Depth of rock channels and their fill are in- 

 valuable indications of lowered sea level during the Quaternary. 

 It is hard to say whether echo sounding is straightforward geo- 

 physics, but we certainly owe a great improvement in our tech- 

 niques to the geophysicists. The value of improved bottom charts 

 for marine geology is obvious. The charting of subbottom surfaces 

 by echo sounders of various kinds is proving useful in ascertaining 

 rates of recent sedimentation and in finding water-covered land 

 surfaces of the Pleistocene age, and other types of stratification. 



Having noted how important the shapes of the solid crust are 

 to the conditions under which sediments collect, the sedimentolo- 

 gist is bound to ask how these topographic forms are created and 

 how they develop. The answer must be based mainly on geophysics. 



Some inland seas, such as Hudson Bay and the Baltic, are closely 

 linked with glacial isostasy, whereas others, such as the North 

 Sea, Lake Maracaibo, Gulf of Paria, and the Persian Gulf, are 

 surface expressions of ancient areas of subsidence. Deeper basins 

 like the Mediterranean, the Black Sea, the Caspian Sea, and the 

 Moluccan basins are of more Recent origin and must represent 

 areas of very swift subsidence. The geophysical nature of the floors 

 either oceanic or continental, the isostatic conditions, and the 

 thickness of sedimentary cover are among the obvious urgent 

 questions asked of the geophyslcist. 



Continental Terrace 



A problem, not to say an enigma, of even wdder scope is that of 

 the origin of the continental terrace. This is the most ubiquitous 

 and uniform of all major topographic shapes of the earth's crust, 

 and yet the composition is fundamentally different in different 

 areas. There are shelves that are most readily accounted for by 

 erosion because there Is so much rocky bottom close to the edge. 

 Whether subaerial or marine erosion should be invoked and in 

 what depths erosion has taken place, and how much time is re- 

 quired to produce the observed width are much debated questions 



