646 GUILCHER [chap. 24 



may have come from Denmark where Cretaceous rocks exist. Such deposits are 

 known as glacial marine sediments. Off Brittany, the pebbles dredged on the 

 shelf may also come in part from the melting of Pleistocene ice-rafts, as sug- 

 gested long ago by French and British geologists; others may be drowned 

 strand lines. 



The Russian expeditions to East Antarctica during the International Geo- 

 physical Year have added to our knowledge of the glacial marine sediments 

 from the shelves of the Southern Continent. These had, however, been reported 

 before, especially by the scientists of the German South Polar Expedition at 

 the beginning of this century. Here we are concerned with present glacial 

 deposition. Lissitzin and Jivago (1958) have clearly defined this environment, 

 which is, at the present day, unique in the world for such a wide area. Run-off 

 is practically absent on land (whereas it exists even in Greenland) ; no chemical 

 weathering occurs, so that the material is affected only by mechanical dis- 

 integration; wave abrasion is hindered by the pack and the ice-foot; thermal 

 conditions prevent calcium carbonate precipitation; the ice-cap and diatoms 

 are essential in the supply of mineral and biogenic particles; the rocky bottom, 

 on which the sediments settle, is often very uneven and rugged (see Chapter 13); 

 icebergs sometimes drift for years before melting completely, so that their 

 material becomes widely scattered. Nevertheless, the greater part of the ice- 

 berg deposits falls to the bottom in front of areas in which ice calving is more 

 important and the rocky content in ice is higher. Between 50° and 70° E, where 

 land ice supply is comparatively poor, the iceberg deposits are restricted to a 

 narrower area than elsewhere. Several mineralogical provinces have been dis- 

 tinguished on the shelf, corresponding probably to differences in the structure 

 of the adjacent parts of the continent, which does not consist only of a Pre- 

 Cambrian shield, since Mesozoic and Tertiary stones have been collected on the 

 shelf. The sorting is typically poor and stones are numerous near the continent, 

 although fine-grained muds occur in some depressions. The fine fraction in- 

 cludes, along with terrigenous particles, an important skeletal content con- 

 sisting of diatoms and siliceous spicules of sponges. Carbonates are practically 

 absent. The rate of deposition seems to be high in some areas, since a core 14.5 m 

 long failed to reach the basement of the glacial marine sediments. It may, 

 however, have encountered Pleistocene deposits. 



The continental shelf in the Beaufort Sea and the East Chukchi Sea, off 

 north Alaska and north-west Canada, represents another environment in very 

 cold water (Carsola, 1954; and Fig. 22). Poorly sorted sediments consisting of 

 mixtures of cobbles, jjebbles, sand and mud an; deposited on the shelf, but only 

 sea-ice or river-ice rafts and not icebergs contribute to this sedimentation, since 

 no ice-cap exists on the adjacent continent. The situation appears to have been 

 the same during the Pleistocene. The dirty ice does not drift very far from land, 

 so that these "glacial" marine sediments, which would be more adequately 

 described as ice-borne marine sediments, extend to a much narrower strip than 

 in areas where they derive from land-ice. 



The recent results summarized above lead to a general observation. It appears 



