SECT. 3] ESTUAKIES, DELTAS, SHELF, SLOPE 649 



on the slope have proved that the coarse sediments occurring on the outer shelf 

 exist also at lower levels. Sand, gravel and pebbles have been dredged between 

 250 and 410 fm: the pebbles range from 23 to 60 mm in length and include a 

 variety of crystalline, metamorphic and sedimentary rocks. But another sample, 

 collected in a Kullenberg piston corer at 480 fm, consists of a much finer, 

 plastic sediment including a microfauna, which points to a gradual warming 

 of the environment from the base to the top. The core is 2.67 m long, and it 

 may be assumed that the rate of deposition has been relatively high here since 

 the end of the last glaciation. At another locality on the Biscay shelf, a Mesozoic 

 rock has been obtained. The Lamont Geological Observatory took photographs 

 from the upper part of the slope off Saint Nazaire, France, at 260 and 285 fm, 

 showing different kinds of ripples on a sedimentary bottom (Heezen, Tharp and 

 Ewing, 1959, pi. 11). More data are needed for an accurate picture of this area, 

 which appears to be complicated but promising. 



In the Beaufort and East Chukchi Seas (Carsola, 1954), clays have been 

 deposited on the slope, while poorly sorted sediments including pebbles have 

 been dredged on the shelf (Fig. 22). The decrease in grain size from the shelf to 

 the slope is due to the fact that the ice-rafts carrying coarse sediments outwards 

 do not reach the slope, as seen above, so that they do not disturb the "normal" 

 sedimentation. 



The north-west Gulf of Mexico (Greenman and LeBlanc, 1956) is another area 

 where the sediments are distinctly finer grained on the slope (clay) than on the 

 shelf (sand, silt, shells, with poor correlation from core to core). But the out- 

 standing feature on the Gulf slope is found further to the east: it consists of a 

 huge cone gently sloping into great depths from the Mississippi delta (Ewing, 

 Ericson and Heezen, 1958: see also Chapter 13). This cone is beheved to have 

 been built by the Mississippi River, whereas the slope on both sides of the cone 

 seems to have remained largely free from deposition. The building of the cone 

 must belong essentially to the Pleistocene, since warm water planktonic 

 Foraminifera are found only in an upper layer 2-3 ft thick; the lower layer, 

 consisting of a grey silty clay with occasional beds of sand and silt, generally 

 exceeds in thickness the length of the cores, which are 6 to 10 m long. The 

 abrupt change in sedimentation has been dated by radiocarbon as being about 

 11,000 years B.P. We meet here again the idea of a fundamental modification 

 and drastic decrease in deposition at the end of the Ice Age which has been 

 put forward concerning the continental shelf. 



As to the processes of deposition on the slope, httle will be said in this section, 

 because they involve a discussion of turbidity currents, submarine landslides 

 and other mechanisms which are considered elsewhere (see Chapters 27 and 

 28). Here it will be merely pointed out that, if turbidity currents and/or land- 

 slides are acting up to the present time, as shown by spectacular breaks of 

 submarine cables, they were probably much more efficient during the Pleisto- 

 cene, and may largely account for changes in deposition on the slope such as 

 those which have just been mentioned in the Gulf of Mexico. They may also 

 explain why soHd rock is so often found in dredgings on the slope. 



