342 " ENDEAVOUR " SCIENTIFIC RESULTS. 



ever, does not now appear as simple as that, for the edge of 

 the shelf is frequently found to be remarkably hard with 

 sharp and high rocks protruding. It is obvious that erosion 

 must take place particularly within the tidal range where the 

 greatest number of waves are at work, but erosion due partly 

 to friction must also occur down to whatever depth the wave 

 action may extend. It is common knowledge amongst 

 fishermen that the nature of the bottom may alter frequently 

 and quickly in depths of 30 and 40 fathoms as the result of 

 severe gales ; and in our own experience it has happened 

 that at a depth of 40 fathoms an area became unworkable 

 after a severe storm through the previously covered-up rocks 

 being temporarily exposed. Scottish crab fishermen will 

 frequently find a considerable proportion of their pots actually 

 smashed after a gale when fishing at a depth of 20 to 30 

 fathoms in the open sea, so there is no doubt as to the 

 efficiency of the waves at that depth. How far this action 

 may be traceable is hard to say, but as along the outer ledge 

 the fine deposits are in depths of 55 to 80 fathoms it ma}' be 

 assumed the influence of the waves seldom reaches these 

 depths, if at all. The inner or shallow ledge down to perhaps 

 40 fathoms may be considered an excavation produced by 

 erosion and twisted by the sweeping currents produced 

 locally and temporarily l^y each succeeding gale. The surface 

 of this portion of the shelf is frequently hard, the bare and 

 ragged rocks being exposed, but at times extensive areas of 

 coarse sand are met with ; fine (and therefore more portable) 

 sediment is generally removed by the more constant currents 

 along the coast. The nature of these currents will shortly 

 be referred to ; at present it is sufficient to observe that from 

 the eastward a fairly strong stream strikes the Australian 

 coast about the latitude of Sandy Cape in Queensland. .\ 

 principal arm is diverted southwards along the coast, and is 

 instrumental not only in removing the fine deposits from 

 inshore, including river erosions, but has also an important 

 effect upon the formation and maintenance of the '" outer 

 ledge." This, it will be remembered, is covered with fine silt, 

 mud and clay, in varying admixtures, and the stream flowing 

 parallel to the coast carries with it those sediments from a 

 northern origin to a southern destination. The south-flowing 

 current acts like a soft broom along the shelf ; it is weak 

 enough along its inner margin to permit sediment deposition 

 in more than 60 fathoms, yet strong enough along the edge 

 of the bank to prevent excessive accumulation in that part. 

 Examination of this current has shown that it extends to a 

 depth of several hundred fathoms and is frequently accentu- 

 ated along the curvation of the edge, where for the first time 



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