SPEED OF THE COLD BOTTOM WATER 215 



A combination of several physical and topographical phenomena is required to produce a turbidity 

 current. I quote from Heezen (1959). 



Turbidity currents first of all require sediment, just as rivers require water, without it they do not exist. Turbidity 

 currents require a trigger mechanism ; this can be supplied by earthquakes, hurricanes impinging on the shore, high 

 bedload discharge of rivers; and lacking a trigger mechanism, turbidity currents probably can occur after long 

 continued deposition simply by slope failure resulting from over-steepening of a depositional slope. Finally, turbidity 

 currents require a slope. The optimum conditions for the generation of a turbidity current probably would be a 

 large body of fine, well-sorted sand triggered by an earthquake on a steep slope. 



Some of these requirements exist at the head of the Weddell Sea. There is a wide shelf covered by 

 an ultra-fine terriginous deposit, the hurricanes are there, there is a steep slope, and above all there 

 is continuous, high and probably rapid bedload discharge (Lisitzin, i960) from the ice-girt land that 

 could well, it seems, lead to a slope failure such as Heezen describes. 



CAUSES OF PATCHINESS 



Some early writers on whales and whaling have sought to link the accumulation of the feed into 

 surface patches with deliberate action on the part of the whales themselves. Perhaps the earliest 

 and most fanciful story of such action is recorded by Sagard-Theodat (1632) in his account of 

 the whales of the Gulf of St Lawrence. I quote from True's translation (1904). 'I was very much 

 astonished by a Gibar which with its fin or its tail (for I could not well discern or recognize which it 

 was) struck so terribly hard on the water, that one could hear it for a long distance, and I was told 

 that it was to surprise and mass together the fish, in order afterwards to swallow them '. Or again, 

 to quote Dudley (1725), fin whales, 'with a short turn, cause an Eddy or Whirlpool by the Force of 

 which, the small Fish are brought together into a Cluster; so that the whale with open Mouth, will 

 take in some Hundreds of them at a Time'. Later writers (Buchet, 1895; Racovitza, 1903; Millais, 

 1906) repeat this story, Millais stating that whales when feeding make 'one or two subsidiary circles 

 to drive their prey together '. Close observers of the habits of whales in more recent times however 

 do not record this practice and it must be regarded as a fanciful belief which probably arose as Gunther 

 (1949) suggests as a fisherman's explanation of the patches and their frequent association with 

 ' boltering '^ whales. In any case, as Gunther points out, pronounced and widespread patchiness has 

 been observed in areas marked by a conspicuous absence of whales, which need not, therefore, be 

 instrumental in bringing it about. 



Although it may have nothing to do with the formation of patches, circling it seems is in fact 

 occasionally practised by feeding whales, the following passage suggesting that in existing patches it 

 could cause still denser crowding of already densely congregating krill. I quote from Ingebrigtsen 

 (1929) on the humpback. 



It employed two methods of capturing 'krill' when the latter was on the surface of the water. One was to lie on its 

 side on the surface and swim round in a circle at great speed, while it lashed the sea into a foam with flukes and tail 

 and so formed a ring of foam. The frightened ' krill ' gathered together in the circle. This done the humpback dived 

 under the foam-ring and a moment later came up in the centre to fill its open mouth with krill and water, after which 

 it lay on its side, closed its mouth, and the catch was completed. 



The other method was to go a short distance below the surface of the water, swimming in a ring while at the same 

 time it blew off'. The air rose to the surface like a thick wall of air bubbles and these formed the ' net '. The ' krill ' saw 

 this wall of air bubbles, were frightened into the centre, and then the manoeuvre of the first method was repeated.^ 



'^ ' Boltering. ' A name given by Norwegian whalers to describe the habit, peculiar to some whales, of swimming or turning 

 on their sides when feeding. Andrews (1909) gives a particularly good description. 



- This appeared in the reputable Rapport et proces-verbaux des reunions, and although to many it may seem a remarkable 

 and somewhat fanciful account, no one it seems has yet questioned its authenticity. 



