312 W. A. E. Ussher — Pleistocene Geology oj Cornwall. 



banks, as in the West Green, have been shifted higher by winds ; a 

 tongue of sand occurs on the east of the Loo Pool similarly drifted. 



The surfaces of the planed Killas reefs, of which I have only 

 given a few examples, occupy in most cases a position intermediate 

 between the base of the several raised beaches in their vicinity and 

 high-water mark (3 c, d, e ; lb; 8e; 15). Mr. Godwin-Austen 

 attributed (Eep. Brit. Assoc, for 1850, Trans, of Sects, p. 71) then- 

 positions to a recent elevation (preceded by a subsidence) of not more 

 than 10 feet. In further proof of this he cites the mud beds of the 

 Exe and Sussex Ouse, containing estuarine shells at slight elevations 

 above the present sea-level. The occurrence of many of the rock 

 jDlatforms are explainable without invoking changes of level. The 

 comparatively recent subsidence by which the forest lands were sub- 

 merged would have brought again within the influence of the waves 

 such portions of the old platforms, upon which the raised beach 

 rested, as had survived the intervening subaerial waste, and, whilst 

 robbing them of whatever superimposed deposits might have existed, 

 would plane anew those more durable portions which came within 

 the influence of the waves, leaving others shorn of their deposits, 

 marking by the heights of their surfaces the seaward slope of the 

 old plane of marine denudation. Bearing in mind the very unequal 

 heights of old beaches of the same age, and the irregular levels of 

 their platforms in places at the base of the same cliff (in places, 

 as in Fistral Bay, the base of the raised beach occupies an almost 

 uniformly persistent level), except where great discrepancies in their 

 levels with reference to adjacent raised beaches occurred, the plat- 

 forms might be explained as above. Other phenomena, however, 

 whilst in no way interfering with the above explanation, would 

 appear to favour the idea that a pause in the downward movement, 

 after the submergence of the forests, was succeeded by a slight con- 

 trary movement. Such an oscillation might serve to explain the 

 river sediments gaining on the marine, in estuarine stream tin sec- 

 tions, and to enable them to continue pari passu with a resumption 

 of the subsiding movement. If, from the sections in Marazion Marsh 

 given by Messrs. Henwood (T. B. G. S. Corn. vol. v. p. 34) and Carrie 

 (Ibid. vol. vi. p. 230, etc.), we may place the top of the marine bed 

 at 2 or 3 feet above high- water, an oscillation would alone account 

 for its position. The formation of the Warren Sand Bank and 

 Northam Pebble Bidge might also be explained by a slight elevation, 

 whilst the rapid diminution of both would seem to indicate a return 

 to the previous contrary movement. The formation and diminution 

 of the West Green Sand Bank might be similarly explained. 



Mr. Edmonds (Edin. New Phil. Journ. for 1848), commenting on 

 the diminution of the bank, says, that 300 years ago, in Leland's 

 time, the causeway leading to St. Michael's Mount was uncovered 

 six hours out of twelve, and continued so for 220 years. The passage 

 to the Mount in 1848 was open four hours out of twelve, and often 

 during strong S.W. winds covered at neap tides for days together. 

 He ascribed these rapid changes (6 b) within 80 years to the removal 

 of sand, which supported the western side of the ridge, for ballast 



