SECT. 2] SUBMARINE CANYONS 499 



end of the cable breaks. In the experience of the writer similar findings have 

 come from areas with a thick covering of fine sediment. Furthermore, an 

 examination by Ericson of a group of four Lamont cores taken in the zone 

 where Heezen and Ewing had estimated a speed of 55 knots showed nothing to 

 indicate that there had been any recent turbidity current of appreciable size. 

 Only one of the cores contained a layer at the surface with material coarser 

 than a silty clay. The investigations of the cable companies in the area where 

 the breaks occurred showed the sediment was a clayey material with a mixture 

 of stones (Kindle, 1931). This is entirely what would be expected without 

 turbidity currents since there are sufiicient icebergs drifting past the area to 

 introduce the scattered stones. It seems quite possible that the stones caused 

 the bending of Heezen's coring tubes and prevented recovery of cores. In all 

 fairness, however, it should be emphasized that the situation relative to 

 sediments in the cable-break area is still relatively unknown and detailed 

 exploration is required before more than tentative conclusions are justified. 



The turbidity- ciuTent deposits fomid associated with various submarine 

 canyons provide some evidence concerning cuiTent velocities. The vast majority 

 of turbidity-current sediments collected to date, both in the canyons and on the 

 fans beyond, are fine sands with a considerable amount of silt. Such material 

 can be moved by currents of velocity considerably less than one knot. Currents 

 of 15 knots or more should be capable of gouging out the rocks on the canyon 

 floors and spreading coarse gravel debris, even boulders, over the fans. Little 

 action of this sort has been found anywhere. The single cobble block reported 

 by Lamont scientists (Ericson et al., 1951) from the outer Hudson Channel 

 could have been introduced by flotation by an iceberg during glacial stages. 

 Gravel of smaU dimensions, however, is found in a number of places. 



Another troublesome feature about tiu'bidity currents as erosion agents is 

 the flnding in the axes of various submarine canyons, where turbidity currents 

 are now operating, that the dej^osits in cores alternate between sands representa- 

 tive of turbidity currents and muds which are normal to the present depths. 

 This alternation seems to indicate that the typical turbidity currents are not 

 capable of eroding even the accumulation of muddy sediments formed after 

 the occurrence of the previous turbidity current. As yet no case has been found 

 where the turbidity-current deposit directly overhes a rock floor, as might be 

 expected if the currents are eroding the rocky slopes where they are found. 



B. Turbidity Currents as the Cause of Channels 



Whether or not the turbidity currents can excavate the rock canyons there 

 seems little doubt that they are responsible for the channels found in the fans 

 on the outside of canj^ons. The depth of these channels is ordinarily only a 

 matter of 20-50 fm (37-92 m), but several places have been found where the 

 channels are well over 100 fm (183 m) deep at a considerable distance out along 

 the fans. Such a place is indicated in Fig. 4 where the fan channel beyond 

 Coronado Canyon shows an increased gradient in crossing a ridge leading down 



