SECT. 2] CONTINENTAL SHELF AND SLOPE 291 



along the shores of Brittany, the rounding of pebbles is extremely slow as soon 

 as the low-tide level is reached. Guilcher, Adrian and Blanquart (1959) have also 

 shown that, in the same region, the roundness of pebbles is generally poor at 

 low-spring-tide level, whereas it is very high on the upper parts of the same 

 sections of the coast. This does not mean that the wave base is reached at low- 

 tide level, since sand and gravel are found to be stirred farther out to sea by 

 waves, but that the figures quoted above from American post-war authors seem 

 to the writer to be correct as an approximation. The wave base is generally 

 shallower in temperate and cold seas, where large seaweeds efficiently break 

 the energy of the surf at low tide, than it is in warm seas where no kelp beds are 

 found (McGill, 1958, see map). However, the contrary may sometimes be true ; 

 for instance in the vSubantarctic Islands, which are beaten by the exceptionally 

 heavy swell of the Southern Ocean. As a rule, wave action cannot be responsible 

 for wide and deep platforms if the sea-level and the land remain stable. 



During Pleistocene times, however, the sea-level underwent large variations 

 as a consequence of glaciations. This can account for a wider range of wave 

 action, extending down to several dozens of fathoms. On the other hand, a slow 

 subsidence of land can also allow waves to erode wider jDlatforms ; and both 

 factors may combine. Such has been the case off Southern California (Emery, 

 1958; Bradley, 1958), where submarine terraces have been recognized along 

 the mainland, around islands and on bank tops : the larger part of them is 

 too deep to be affected now to an appreciable extent by wave erosion. These 

 terraces are distributed into sets which are related to different low sea-levels ; 

 moreover, they have suffered a differential warping (see below). Other sub- 

 marine terraces are reported from the Persian Gulf off the Qatar Peninsula at 

 3-9 fm, 11-17 fm, 17-28 fm and 30-40 fm by Houbolt (1957), who assumes 

 that they represent stages in the post-glacial rise of sea-level (however, this 

 hypothesis makes one wonder whether the lapse of time has been sufficient). 

 Others are found off western Australia (Carrigy and Fairbridge, 1954). On the 

 West Florida shelf, the Florida Keys line is continued under water by a ridge 65 

 miles long, down to 90-100 fm in depth: this would be a Pleistocene barrier 

 spit, possibly consolidated in beach-rock which may have enabled it to persist 

 (Jordan and Stewart, 1959). 



In addition, it must be pointed out that wave abrasion in solid rock is not the 

 only force in action on the shores and continental shelves. In the intertidal 

 zone, weathering plays an essential part in cliff recession and platform lowering : 

 deep granular distintegration in granites, under tropical humid climates, 

 exposes the rotten rocks, which can then be more easily removed ; alternations of 

 wetting and drying, action of salt in the spray zone, borings of marine organisms 

 and especially of microscopic algae (Cyanophyceae) are very powerful in lime- 

 stones, where solution is probably also in progress, although there has been 

 much discussion about the processes of limestone solution on sea-shores. 

 Again, with volcanic rocks, weathering is often most efficient and leads to 

 spectacular features ; in cold regions, frost splitting in the ice-foot bordering 

 the cliffs carves benches into them (Tricart, 1957; Emery, 1946; Emery and 



