18 GEOLOGY or THE ISLE OF WIGHT. 



CHAPTER III. 



LOWER GREENSAND OR UPPER NEOCOMIAN. 



Introduction. 



This formation occupies the greater part of the southern or 

 Cretaceous area of the Isle of Wight, and forms important escarp- 

 ments, such as that which runs from Corapton Bay by Brook, 

 Mottistone, and Brixton, or the succession of bold shoulders which 

 dominate the upper parts of the Medina and Yar valleys, and on 

 one of which Godshill is situated. But the most complete sections 

 are to be obtained in the four coast-sections of Compton Bay, 

 Atherfield, Shanklin, and Redcliff at the east end of Sandown 

 Bay. 



At Redcliff the thickness of the Lower Greensand is about 

 600 feet ; at Atherfield it has increased to over 800, but at 

 Compton Bay, about 16 miles west of Bedcliff, the thickness is 

 reduced to 399 feet. Lastly, at Punfield, 20 miles west of 

 Compton Bay, it is no more than 198 feet. It would seem then 

 that the direction in which the strata thicken most rapidly lies a 

 little east of south. 



The Lower Greensand of Atherfield was made the subject of 

 the most exhaustive examination by Dr. Fitton in the years 

 1824-47. The results of his work were embodied in a laro-e 

 number of papers, but chiefly in a paper read before the Geological 

 Society in 1845.*" Not only was the thickness at Atherfield found 

 to be greater than elsewhere in the Island, but fossils were very 

 much more abundant. The rich collection made by Dr. Fitton 

 showed that the fauna of the Lower Greensand was both distinct 

 from that of the Upper Cretaceous Rocks above, and possessed 

 nothing in common with the Wealden Shales below, there being in 

 fact a complete palseontological break at the base of the formation. 

 This is the more noticeable in that the lower beds of the Lower 

 Greensand are, like the Wealden Shales, of a clayey character. 



Later observations have shown that this complete contrast in 

 the fauna was caused by an abrupt change in the physical geo- 

 graphy of the area in which the Lower Greensand was distributed, 

 and was preceded by some erosion of the Wealden Beds. The 

 abruptness of the change is indicated by the following evidence: — 



(L) The division of the Lower Greensand from the Wealden 

 Shales is everywhere absolutely sharp, so much so that 

 the two can be cleanly separated by a knife-blade. 



* Quart. Journ. Geol. Sue, vol. iii. p. 289. 1847. Kefereuccs to his other 

 papers will bo found in the Bibliography at the end of this book. 



I 



( 



