SECT. 3] 



ESTUARIES, DELTAS, SHELF, SLOPE 



645 



deposits may be completely different at equal depths and 50 yd apart as a 

 result of local shelters. Conversely, rocky grounds exist in shelf areas where 

 current velocities do not seem to be especially high, for example on outer 

 shelves off Southern California (Revelle and Shepard, in Trask, 1939), and in 

 East Antarctica, where they often correspond to j^laces from which the inland 

 ice has retreated only recently (Lissitzin and Jivago, 1958). 



An interesting type of deposit on shelves consists of the sediments situated 

 in front of present or former glaciated areas in America, North Europe and 

 Antarctica. Those lying off More and Romsdal, Norway, have been studied by 

 Holtedahl (1955) and Berthois (1957) (Figs. 20 and 21). The samples include 

 stones, sand, silt and clay in variable percentages. "The medians show wide 



23 mm 



1480^ 360m 



0.1 M 



0.4% 



3 mm 



50 M 



Fig. 21. Cumulative curves of particle diameters of glacial marine sediments of continental 

 shelf and slope off More and Romsdal, West Norway. Figures on curves indicate the 

 depths of samples in fathoms. (From Berthois, 1957.) 



variations at the same depths, both sand and gravel being found in the de- 

 pressions as well as on the shallow banks." Only pebbles are dredged in some 

 places. Their roundness is not at all typical of a former coastal environment: on 

 the contrary, they closely resemble glacial pebbles, and the bulk of the sedi- 

 ments has the character of glacial tills. Some samples consisting of sand and 

 lying at 100 fm are fairly well sorted and might represent an old shoreline, but 

 generally the sorting is extremely poor and the size distribution curves show 

 such individual characteristics that it is difficult to classify them. The nature 

 of the pebbles covers a wide range of crystalline and metamorphic rocks; 

 limestones, sandstones and flints are sometimes present. These data point to a 

 Pleistocene glacial origin and a transportation to the shelf by ice, either through 

 glaciers extending over it or through icebergs dropping their material when they 

 melted. Transportation by drifting icebergs is most probable for flints, which 



