368 ME. H. B. MIL^EE OX THE [vol. Ixxviii^ 



observation to-day; the luocle of occurrence and the elevation of the 

 St. Erth deposits at 170 feet above sea-level both tend to suggest 

 a greater antiquity, and hence a probable Eocene age. On this 

 assumption, the belief is justified that the whole of South-Western 

 Cornwall, if not the greater part of the South- West of England. 



; stood then at a higher level than it does now, and the forces of 



denudation probably continued to act unchecked throughout Oligo- 

 cene and early Miocene times, and possibly later. Such a belief 

 can only be based on analogy with contiguous areas to the east, 

 since in Cornwall itself we have really no direct evidence of the 



I trend of geological events between Upper Palaeozoic and Pliocene 



times — an enormous gap in the stratigraphical record. 



With the renewal of folding along the ancient Armorican lines 

 in Miocene times, the effects of which are so marked in the Isle 

 of Wight and the Isle of Purbeck, it is possible that considerable 

 changes were wi'ought in the surface-relief of Southern Cornwall, 

 although it is by no means certain that these tectonic influences 

 extended so far westwards ; if they did, it would be expected that 

 the tendency Avas towards the production of asymmetrical fold- 

 features with a gentler dip southwards, as is the case in these 

 localities, though of far less severity in the west where the move- 

 m.ents were decreasing in intensity. In Cornwall the effect of 

 such deformation would probably only be an accentuation of the 

 chief Palaeogene physical features, and such drainage-courses as 

 the St. Erth valley with its southerly gradient would receive an 

 increased impetus in the work of discharging material into the 

 English Channel. Even if the oris'inal earlv Tertiarv drainage of 

 the St. Erth valley were in the opposite direction : that is, from 

 south to north (Avhich is extremely unlikely fi'om all that we can 

 gather of its history), the result of Miocene earth-movement 

 would still tend to produce drainage into the English Channel in 

 South- Western Cornwall, and at the close of Miocene times the 

 St. Erth valley would in any case be well differentiated. What- 

 ever happened during this pre-Pliocene period, two factors stand 

 out very clearly from the late Miocene records: namely, the general 

 subsidence of the area in early Pliocene times and the concomitant 

 accentuation of the 'l:00-foot platform' as a result of marine erosion.^^ 

 This Pliocene platform is one of the most characteristic physical 

 features of Cornwall, and it can be traced at heights varying from 

 370 to 420 feet above O.D., at intervals along both the southern 

 and the northern coasts of the county, extending from the coastal 

 margin well inland to the higher Paleozoic country Avhich consti- 

 tuted the ancient Pliocene land-area (shown in the accompanying 

 sketch-map, fig. 1, p. 350). It is difficult to estimate the precise 

 amount of this subsidence from the data at present available ; but, 

 taking into account the St. Erth deposits, the character of their 



I fauna, their lithology, and present topographical evidence, Clement 



1 See Mem. Gaol. Siirv. Sheets 3.51. 358. 346, 359. and H. Dewev. Q. J. G. S. 

 vol. Ixxii (1916-17) p. 63. 



