in level were carried out in a very short time (probably less than a century 

 for a 5 ft. drop). The sea-level appears to have suffered short, sharp 

 drops (Fairbridge, 1949) ; there seems to be little likelihood of any oscilla- 

 ting up and down below the present sea-level in these post-Glacial move- 

 ments. Several factors may be combined to explain the remarkable 

 development and, consequently, the discovery of these benches. 



First, there is a very small tidal range on much of our Western 

 Austrahan coast-hne, and likewise in the East Indies and elsewhere in 

 the Indian Ocean. A neap rise and fall of less than 12 in. is normal at 

 Fremantle. This means that at any rate the terraces at 5-6 ft. and 

 10-11 ft. above datum (low-water springs, which is the level of the 

 contemporary bench) are generally clear of actual marine erosion and 

 are only being slowly attacked by spray and rain. Shells in the 

 associated raised beaches often still maintain much of their original 

 colour. 



Secondly, off-shore reefs and wide fringing reefs protect much of our 

 coast-line from storms. Hurricanes are not experienced. The benches 

 are, of course, found best preserved in protected bays, inlets, and lagoons. 

 Thus, in the Abrolhos Islands, the broad lagoon is flanked by low cHffs, 

 in which here and there may be found all four levels rising in steps. 

 On Rottnest a series of inlets, which were open to the sea until a few 

 centuries ago, have now been sealed off by advancing sand-dunes to form 

 a series of salt-lakes. The cliff ed margins of these lakes now exhibit 

 the most beautiful series of undercut notches, benches, and raised 

 shell-beds which it has ever been my good fortune to see. Such perfect 

 preservation is, of course, not found everywhere ; it would hardly be 

 reasonable to expect to find a 2-3 ft. terrace preserved from the present 

 attack of the sea in any but exceptional places. Normal^ this low terrace 

 is completely destroyed or reduced to a sloping ramp which leads up 

 into the 5 ft. notch. In exposed spots the contemporary 2 ft. and 

 5 ft. notches are all smoothly graded into one and only the 10 ft. bench 

 stands out like a visor above the contemporary terrace. 



The third factor of importance in making these West Australian 

 benches so striking is the fact that they are all cut in limestone. In 

 places it is a coral limestone, or a massive lagoon limestone, but 

 generally it is a sandy calcareous eolianite. All are of Pleistocene age, 

 but generally extremely indurated and hard. The nature of limestone, 

 as I have tried to indicate already (Fairbridge, 1948, 1949), is particularly 

 favourable to the formation of a very massive, yet clean-cut, marine 

 platform at exactly the height of low- water springs. This level is the 

 critical lowest limit of subae'rial erosion and also is close to lowest limit 

 of super-aerated surf. The latter is rendered temporarily acid in its 

 reactions to limestone under certain bio-chemical and physico-chemical 

 conditions (extemely variable temperatures, CO 2 and pH ranges). 

 With the sharp changes experienced in this zone limestone is alternatively 

 dissolved (tending to reduce the level of the land to a horizontal plane 

 coinciding with low- water springs), and alternately precipitated (pro- 

 ducing a travertine filling in all the interstices of the limestone and 

 rendering the rock so massive and hard that it rings to a hammer blow 

 and was even used by primitive man for " flint " artifacts). The result 

 is a horizontal bench of extreme resistance. It is realized that purely 

 biological and mechanical factors are also highly active in this same 



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