296 GUILCHER [chap. 13 



Rigby, 1957). During the Pleistocene, the eustatic movements of sea-level have 

 played a part in the geomor])hological evolution of the banks, and the sub- 

 mergence after the melting of glaciers accounts for the drowning of a karst 

 surface over them. Nevertheless, however conspicuous the effects of eustatism 

 may be on the surface, the thickness of the strata they have affected is almost 

 negligible when compared with what had been deposited during the long- 

 continued subsidence before the eustatic loAvering, The Bahama Banks afford 

 an excellent example of the respective importance of Pleistocene events and 

 former phenomena in the present features of a constructional, subsiding 

 platform. 



Another well-known constructional shelf is the so-called Gulf Coast geo- 

 syncline in and off South Louisiana. All geologists do not agree on the use of 

 the term "geosyncline" when applied to such a structure (Aubouin, 1959), but 

 the feature in itself is outside discussion. In this area R.J. Russell and R. D. 

 Russell (in Trask, 1939) state "deltaic sedimentation by the Mississippi and 

 its ancestral streams has been an essentially continuous process since at least 

 the beginning of the Tertiary". Subsidence has been going on as an isostatic 

 response to the deposition of a huge pile of sediments since the Cretaceous, and 

 the aggregate thickness of post-Oligocene deposits, established from records of 

 several thousand wells, is about 62,000 ft in southern Louisiana (Crouch, 1959). 

 The axis of the trough migrated southward during the Tertiary, but not ap- 

 preciably since early Pleistocene times. The Pleistocene terraces are distinctly 

 tilted seawards and uplifted landwards, a hinge line limiting the zone of iso- 

 static dowmvarp and the zone of uplift (Fisk, 1939); at the ]3resent day, 

 subsidence is maximum in the active sub-delta, where natural levees are 

 already sinking below sea-level in some areas (Welder, 1959). In older post- 

 glacial sub-deltas the drowning is more pronounced, as it is, for example, to 

 the north of the present mouths. Off the Mississippi delta, the fact that the 

 continental slope is much more gentle than on either side (see above) is ex- 

 plained by Ewing, Ericson and Heezen (1959) by the building of a huge sub- 

 marine cone by the river during the Pleistocene, which should have buried the 

 steep scarjis existing elsewhere in the lower parts of the slope around the Gulf 

 of Mexico. In Ewing's interpretation, these scarps should thus be relatively old, 

 and no important recent faulting has occurred in this area where seismicity is 

 low; while, on the contrary, Greenman and LeBlanc (1956) have suggested 

 that the Gulf was downfaulted to its present depth in late Pleistocene or early 

 Recent time. On the other hand, the change in form of the West Florida scarp 

 at 27°N is explained by Jordan and Stewart (1959) as a result of a change from 

 clastic underlying rocks and a thick blanket of unconsolidated sediments in the 

 north to carbonate rocks with only a veneer of sediments at most in the south. 

 In North-west Euro])e, Dangeard (1928), W. B. R. King (1948, 1954), 

 Bourcart and Marie (1951), Day (1958), Curry (1960), and Boillot (1961) have 

 collected a great deal of geological data concerning the English Channel 

 and the Celtic Sea, and knowledge has also been largely improved by seismic 

 prospecting in the same areas (Bullard and Gaskell, 1941 ; Hill and King, 



