52 GEOLOGY OF THE ISLE OF WIGHT. 



CHAPTEE V. 



LOWER GREENSAND— conimwe^. 



THE CARSTONE. 



Introduction. 



This name has been given to a coarse and highly ferruginous 

 grit, which may be traced continuously at the base of the Gault 

 through the Isle of Wight. Wherever fully exposed the Carstone 

 is seen to pass up into the Gault ; on the other hand a fairly sharp 

 line at its base separates it from the Sand-rock Beds, with an 

 appearance even of slight erosion at times, though we have no 

 evidence of an actual unconformity. The feature produced by 

 this comparatively hard grit, capping the soft sands of the Sand- 

 rock Beds, is especially prominent where the beds are nearly 

 horizontal. It is most marked at Marvel Wood, near Shide, and 

 in the neighbourhood of Godshill. 



The Carstone varies considerably in thickness within the Island. 

 From 6 feet at Compton Bay it expands to 12 feet near Blackgang, 

 to 30 feet near Bonchurch, and to no less than 72 feet at Red 

 Cliff. At Punfield, on the other hand, it seems to be represented 

 by a few inches only of pebbly grit, but is not seen there in place. 

 The Carstone, therefore, thickens towards the north-east, while the 

 other subdivisions of the Lower Greensand increase towards the 

 south. 



The Carstone corresponds to the upper part of Fitton's 

 Group XVI. The present name* has been adopted on account 

 of the resemblance the rock bears to the Carstone of Lincolnshire 

 and Norfolk, of which there is reason to suppose it to be the 

 stratigraphical equivalent. For the Carstone of those Counties 

 passes up into the Red Chalk, which there occupies the position 

 of, and partly represents the Gault. Moreover, further south we 

 find that the Gault when it makes its appearance passes down 

 into a grey clay with phosphatic nodules, which in its turn shades 

 into a lower light brown sand with phosphatic concretions and 

 numerous fossils, f 



These fossils, as pointed out by Mr, Teall, are found in the 

 south of England to occur in the Gault, and in the Ammonites 

 mcmimillaris zone, which lies next below the Gault. Ho infers, 

 therefore, that "• the Norfolk Neocomians [Carstone] are found to 

 resemble both stratigraphically and palasontologically the Folke- 

 stone Beds of the South" {ojk cit., p. 22). But we have already 

 pointed out that the Folkestone Beds as a whole are comparable 

 to the Sand- rock Scries, It remains to be seen whether any sub- 



* The name is applied locally iu thu Weald to the portions of the Folkcstoue 

 Beds, which have been cemented by brown iron oxide into a hard rock. 



f The Pottou and WJcken Phosphatic Deposits (Sedgwick Prize Essay for 1873) 

 by Mr, J. J, H. Teall. Cambridge, 8vo., 1875, p. 20. 



