Douglas Johnson (1919/1938) states bluntly : " The vertical limit of 

 marine denudation is a surface so low that wave action is no longer 

 retarded by it " (a depth of 600 ft. or so in his opinion), and he finds the 

 corollary that " there is no horizontal limit of marine erosion " (p. 234). 



Returning, however, to the English physiographers of the last century, 

 in specific reply to the wave enthusiasts, G. Maw (1866) expressed the 

 opposite with precision and clarity. He was struck first by " the level 

 surfaces of sea-coast reefs," indicating the base-level of rapid coast erosion 

 at sea-level ; the logical conclusion, of course, was that " the sea does 

 no material work below the tidal range," except, of course, in certain 

 very exposed places. The general tendency of marine erosion is towards 

 a straightening of a coast-hne, while drowning institutes indentations 

 formerly occupied by subaerially formed vaUeys. Maw's accurate 

 description of the normal contemporary horizontal bench or off-shore 

 reef in England is a model which to-day could hardty be bettered 

 elsewhere : "If we examine the sea-bed between high and low water 

 mark, on any cliff-girt shore it is impossible not to be struck with the 

 singularly level disposition of the reef surfaces extending seaward, 

 which once formed the foundations of the old cliffs. Their general height 

 would be a trifle above that of low tide, and any irregularities of surface 

 will not exceed one or two feet. This well-marked lower limit to the 

 erosive action of the sea is not confined to hard rockj' coasts, but will 

 be found to hold good in the softest strata." 



The same thing was realized, too, by W. Whitaker (1867), who con- 

 cluded : " The sea, therefore, does not by itself destroy the land, but 

 is largely helped by atmospheric actions." Mellard Reade (1885) agreed, 

 but claimed in fact that this was thoroughly understood by Hutton 

 already a century before. On a previous occasion (1877) Reade had 

 outlined his chemical experiments and calculations, concluding : " The 

 chemical agency decomposes the matrix. . . . The ocean acts 

 merely as a mechanical distributor of matter. . . . The action of 

 subaerial erosion is . . . unlimited, except by sea-level." 



Again, it is interesting to quote A. H. Green (1882) : " The sea to a 

 very lai-ge extent only finishes the work begun for it by subaerial denu- 

 dating agents" (p. 206), and "It must be noted that the destructive 

 action of the sea is confined almost entirely to the belt between high 

 and low water mark. Within that space the rise and fall of the tides and 

 the forces of the breakers grind down any loose matter exposed to their 

 action. These agencies, however, cease to have any effect on the bottom 

 covered by a moderate depth of water, and hence very nearly all the 

 denuding work of the sea is coast denudation. The drifting of the rough 

 sediment over the bottom by under-currents may produce some abrasion, 

 but its amount cannot be very much " (p. 207). And yet again, Jukes- 

 Browne (1893, p. 135) : " Sea-waves can only act along one plane, and 

 its currents, though able to erode soft sands and clays, can make but little 

 impression on more solid rocks." And earlier, in more detail, he suggested 

 that low-water mark was the lowest limit of subaerial forces ; it was in 

 fact " a line of non-erosion " below which accumulation would normally 

 begin. " On a rocky shore the tendency of these conditions is to produce 

 a horizontal or gently sloping platform, the outer edge of which corres- 

 ponds to the line of non-erosion, so that its surface is bare at low water " 

 (1884, p. 120). 



In America, too, the same concept was expressed. J. D. Dana (1880, 

 p. 676) says clearly : " The lower limit of erosion is above low-tide 

 level." And, " The wearing action of waves on a coast is mainly confined 



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