788 EMERY [CHAP. 28 



and Hayasaka, 1954) and on tidal rivers of Australia (Wolf, 1960), but they 

 probably do not occur on the open sea where waves roughen the surface. 

 Flotation of stones as large as 2.5 cm in diameter has been observed (Norden- 

 skiold, 1900) in a Patagonian bay, probably because the local shale was im- 

 pregnated with petroleum. The drifting ashore of buoyant tar from offshore 

 seeps and from ships is commonplace; however, after some oxidation the tar 

 becomes denser than sea- water and sinks to the bottom. Pieces have been taken 

 from cores in basins off southern California at depths corresponding to de- 

 position about 10,000 years ago and they are dead with respect to radio-carbon 

 dating. 



Widespread at high latitudes are deposits of sediment rafted by ice in the 

 form of floes and bergs. These sediments range from clay to boulder size, with 

 the boulders being most easily recognized as of ice origin by their mixed 

 exotic lithologies, angularity, occasional striations, sometimes adhering rock 

 flour and associated poorly sorted sediment (Carsola, 1954; Lisitsin, 1958). 

 Bottom photographs in the Arctic Sea also show many small pits in the mud 

 surface, presumably made by falling ice-rafted pebbles (Hunkins, Ewing, 

 Heezen and Menzies, 1960). Sediments are frozen into the bottom of ice floes 

 in shallow water or are deposited by melt-water streams on the tops of floes and 

 on glaciers which later become water-borne. During the travels of the floating 

 ice the sediment is released mostly by melting of the ice raft. Because of their 

 larger masses, icebergs drift into lower latitudes than floes before melting 

 away; but only floes occur in the North Pacific and eastern North Atlantic 

 Oceans. At present, bergs and floes reach farther equatorward in the Northern 

 Hemisphere than in the Southern, according to charts of Dietrich and Kalle 

 (1957, p. 172). However, glacial marine sediments in the North Pacific (Menard, 

 1953; Kuno, Fisher and Nasu, 1956) and in the North Atlantic (Peach, 1912; 

 Bramlette and Bradley, 1942) occur far south of the present limits of floating 

 ice (Fig. 2). These deposits are doubtlessly of Pleistocene age. In the Southern 

 Hemisphere the hmits of glacial marine sediments recognized by Hough (1956), 

 Lisitsin (1960), Needham (1962), and others are more restricted than is 

 present-day floating ice. The diflference between the limits of floating ice and 

 limits of ice-rafted bottom sediment in the Southern and the Northern Hemi- 

 spheres is probably a result of the relative lack of access of ice to rock on 

 Antarctica (leading to small loads of sediment) as compared to the great ease of 

 access on the continents of the Northern Hemisphere. In any event, ice-rafted 

 sediment is widespread at high latitudes both at the sediment surface and 

 beneath thin layers of post-glacial marine sediment. 



Coarse-grained sediments are also transported beneath the ocean in the form 

 of turbidity currents. Field studies have shown widespread deposits of sand on 

 the floors of the Atlantic (Ericson, Ewing and Heezen, 1952) and the Pacific 

 Oceans (Menard, 1955). Associated with the sand, but more restricted to the 

 base of steep slopes, are accumulations of gravel. Pebbles and cobbles have been 

 obtained even from the deep floor of the Philippine Trench by Bruun (1951). 

 Detailed studies of gravels and sands from the basins off southern CaUfornia 



