48 CORNISH POST-TERTIARY GEOLOGY. 



present time, that morasses had been formed, at different times and 

 in different places, from alterations in river courses or stoppages of 

 drainage, and had in some cases perhaps been temporarily converted 

 into a soil favourable for the growth of underwood, as may be 

 inferred from bed (c) in the Fowey Valley Section, 



Mr. Godwin-Austen commented on the occurrence of rock reefs 

 with planed surfaces occupying an intermediate position between 

 high-water mark and the height above it of the neighbouring raised 

 beaches. Numerous examples of such reefs occur, their surfaces 

 ranging from spring-tide high- water to 4 or 5 feet above it ; in veiy 

 many cases they may be regarded as relics of the old beach plane 

 sloping seaward, but in others Mr. Godwin-Austen's idea of a 

 temporary oscillation of a few feet in the later stages, of the sub- 

 sidence which led to the submergence of the forests may be admiss- 

 ible. Such an oscillation would to a great extent explain the 

 formation of such sand and gravel bars as those of the Loo, Swan 

 Pool, Par, and Pentuan ; and supposing its cessation and the resump- 

 tion of the downward movement to have taken place, the destruction 

 of the West Green Sand Banks (near Marazion), and perhaps the 

 entire insulation of St. Michael's Mount, can be readily conceived. 



CONCLUSIOX. 



In reviewing the evidence, one cannot avoid being struck with 

 the paucity of Pleistocene deposits in Cornwall, for, notwithstanding 

 their completeness, the stream-tin sections represent but a very 

 small portion of the denudation to which the valleys have been 

 subjected in Post-Tertiary times. Whether we regard the deposits 

 of Crousa Down, Crowan, and St. Agnes as of Tertiar}'^ or early 

 Pleistocene age, their positions indicate a configuration vastly 

 different from the present contour, that they were formed at a time 

 when denudation and deposition took place upon tracts of land 

 which have been for ages sparingly acted on by the one, and wholly 

 unmodified by the other. 



No direct evidence of glacial action in Cornwall has been obtained. 

 Had the county been invaded by a foreign ice-sheet, or submerged 

 beneath the waters of an Arctic sea, this could not have been the 



