LONDON CLAY. 97 



Some caution is needed in estimating the true thickness of the 

 Keading Beds in the Isle of Wight ; for it must not be forgotten 

 that the strata are nearly vertical and have been subjected to 

 violent pressure, varying in direction and amount according to their 

 proximity to the sharp monoclinal curve which forms such a con- 

 spicuous feature in the geology of the Island. Where the Chalk 

 is thrust northward, beyond the ordinary line of the Downs, the 

 compression of these lower Tertiary strata is also greatly exagge- 

 rated, but where the Downs recede slightly to the southward the 

 thickness of the Reading Beds increases considerably. Allowing 

 for this compression, and taking into account the measurements 

 obtained on the mainland, it seems probable that the thickness 

 we might expect to find in wells sunk beyond the limits of the 

 most violent disturbance would be from 100 to 120 feet. 



The only fossils this series has yet yielded in the Isle of Wight 

 are fragments of plants ; and though the beds are probably in the 

 main of freshwater origin, there is little direct evidence in the 

 district. On the mainland the principal fossils found in Reading 

 Beds of this type consist of leaves of plants and other vegetable 

 remains, showing, according to Sir J. Hooker and Mr. J. Starkie 

 Gardner, a temperate climate. In similar beds at Lancing, 

 however, the mottled clays are not entirely freshwater, for 

 they contain a line of ironstone nodules with casts of marine 

 shells. 



London Clay 



Like the Reading Beds, the London Clay forms a narrow belt 

 extending across the Island, between the west and the east coast, 

 from Alum Bay to Whitecliff. In consequence of the highly 

 inclined position of the strata between these points, the width of 

 the out-crop of the London Clay, or the space occupied by it at 

 the surface, is frequently very little more than the actual thickness 

 of the formation. The only places where it can be thoroughly 

 examined are on the coast. 



The junction of the Reading Beds and the London Clay is 

 sharp and well defined. In Alum and Whitecliff Bays the highest 

 part of the older deposit consists of red mottled clays, while the 

 base of the newer one is ferruginous or blue sandy clay. At both 

 localities the division between the two formations is indicated by 

 a band of flint pebbles, sometimes mixed with pebbles of the 

 underlying red clay, representing the Basement Bed of Professor 

 Prestwich. In Alum Bay, however, this seam of pebbles is not 

 perfectly continuous. Inland, the Basement Bed is better repre- 

 sented by an impersistent bed of fine sand, seen in the road 

 cuttings between Calbourne and Swainstone, and dug near Ashey 

 Chalk Pit and close to Ryde Waterworks. This sand appears 

 nowhere to exceed 10 or 12 feet in thickness. There is nothing 

 especially characteristic in the fauna of these basement beds in the 

 Isle of Wight, all the species being also found in higher zones. 



E 56786. G 



