440 STRATIGRAPHICAL GEOLOGY 



more continuous through Oxford, Berkshire, and Buckinghamshire, 

 but the limestones die out in the last county, and only the clays 

 remain through Bedford, Cambridge, Huntingdon, and Lincoln. 

 In South Yorkshire they are entirely concealed by the Cretaceous 

 overstep, but in North Yorkshire there is a fine development of 

 the Oxfordiaii, Corallian, and Kimeridgian Groups. In Scotland 

 the Oxford Clay exists on the west coast, and the complete series is 

 found on the coast of Sutherland. 



With regard to the eastward extension of these rocks beneath the 

 newer systems in the south-east of England we have the testimony 

 of four important borings. The Sub-Wealden boring near Battle 

 traversed all the higher groups, and a large part of the Oxfordian, 

 terminating at a depth of 1905 feet from the surface. In the 

 Dover boring all the members of the series were found to be 

 greatly reduced in thickness, the combined thickness of the 

 Portlandian, Kimeridgian, Corallian, and Oxfordian being only 

 452 feet. A boring at Chatham proved the existence of Oxford 

 Clay immediately below the Lower Greensand, a fact which makes 

 it probable that there was considerable erosion of the Upper 

 Jurassic Series before the deposition of the Lower Cretaceous 

 sands in that district. There is a similar gap and unconformity 

 at Richmond, where a boring found the Lower Greensand resting 

 directly on Great Oolite. It is probable that the Oxfordian, 

 Corallian, and Kimeridgian Groups originally spread over the 

 area which underlies the Thames valley, but no traces of them 

 have been found in any of the deep borings in Essex, Suffolk, or 

 Norfolk, where Cretaceous rocks everywhere rest directly on a 

 plateau of Palaeozoic rocks. 



In dealing with the stratigraphy of this series, the simplest 

 plan will be to take each stage separately, and indicate its variations 

 from what may be regarded as its typical development in Dorset as 

 it is followed northwards along the line of outcrop. 



1. The Oxford Clay (Oxfordian).- Lithologically this is 

 divisible into two unequal parts (1) the Kellaways Beds, a set of 

 sands and clays with beds of shelly sandstone or sandy limestone ; 

 and (2) the Oxford Clay proper, including over 400 feet of bluish 

 clays with occasional layers of earthy limestone. Palajontologically, 

 however, it can be divided into at least three zones, 17 viz. : 



Zone of Cardioceras cordatum Blue clays with septaria. 

 ,, Cosmoceras ornatum Grey clays and shales. 

 ,, Kepplerites calloviensis= Kellaways Beds. 



Part of the Oxford Clay is well exposed near Weymouth, where 

 the highest zone is about 250 feet, the central beds probably about 



