horizontal (see the many recent papers on continental sheh-es and 

 submarine terraces). These observations have been based mainly on a 

 study of the more recent hydrographic charts (many of which are based 

 on very accurate sonic sounding). Notice that emphasis is placed on 

 the phrases " most usual " and " more stable," because the following 

 remarks hardly apply either to entirely new shores, such as those of a 

 freshly erupted volcanic island, or to the rapidly emerging shores of 

 formerly glaciated areas which had been isostatically depressed, or again 

 to highly mobile shores in the grip of active tectonic movement. In 

 short, I am thinking of the normal coasts of the stable parts of the 

 Continents of Australia, Africa, America, &c., but, nevertheless, the 

 conclusions may also be seen to apply to many of the younger shores 

 as well. 



The general consensus of opinion now indicates that most continental 

 shelves were, at least superficially, sculptured by shore-line erosion during 

 the Pleistocene into series of platforms at various levels. The question 

 of the degree to which glacial lowering of sea-level or tectonic oscillation 

 of the continental border contributed this eventual result need not con- 

 cern us for the present (c/. Lewis, 1937 ; Novak, 1938 ; Bourcart, 1938 ; 

 Umbgrove, 1946 ; Fairbridge, 1946, 1947a, 1948). Thus, for example, 

 the continental shelf off Western Australia appears to consist of a series 

 of terraces at 3-5, 10-15, 25-30, 55-60, and at about 100 fathoms below 

 sea-level. Other stable coasts exhibit very similar features. 



We must conclude that no such thing as a mature offshore profile 

 of equilibrium exists to-day, since major changes of sea-level, ranging 

 through the Tertiary and Pleistocene, right up till to-day, have kept the 

 profile in a continuous state of alternate rejuvenation and drowning. 



It is apparent that the few thousand years, which have elapsed since 

 the last major submergence of the continental shelf have not been a 

 sufficient span of time to permit mechanical erosion at wave base to 

 reduce the ruggedness of these submarine terraces to a smooth profile ; 

 sedimentation may be helping to fill up some of the depressions and 

 round off the contours, but the essential sharpness is still there. The 

 power of waves and currents to sort and distribute loose sediment across 

 the shelf is undoubted, but a serious question is raised as to the strength 

 of waves and currents to rapidly erode hard rock outcrops at depths of, 

 say, 10-100 fathoms. 



Horizontal Platforms 



My problems really started when I noticed that the average " wave- 

 <:ut " platform of to-day is generally steep-to on the outside edge and is 

 essentially horizontal on the surface. This means that the platform does 

 not slope gently outwards to merge into the off-shore profile that descends 

 to 100 fathoms or so, as we would have to believe from text-book figures 

 {e.g., Johnson, 1919-38, Fig. 37). A soft, sandy, or muddy coast may well 

 be endowed with a gently sloping off-shore profile, but not a rocky coast ; 

 in fact, I have the impression that the cliff foot on the average rocky 

 ■coast is being eaten back more rapidly than is the outer margin of the 

 platform (see, for example. Gill, 1949) ; the older the coast {i.e. the longer 

 it remains stable in relation to the sea-level) the wider the platform 

 will become. Such a conclusion is hardly in conformity with the 

 ■" traditional " teachings of the Johnson school of marine erosion. 



348 



