Elsworth Rock 13 



Corallian. 



Lying comformably on the top of the Oxford Clay over 

 most of the district is a somewhat remarkable bed which has 

 been correlated by Mr T. Roberts and others with the Lower 

 Calcareous Grit of Yorkshire. This bed varies somewhat from 

 place to place along its outcrop which is much obscured by 

 drift. It has been named Elsworth Rock and St Ives Rock. 

 It is a hard calcareous or ferruginous mudstone, but differs 

 from the so-called limestone bands of the Oxford and Ampthill 

 Clays in that it contains iron-shot oolitic grains which are ap- 

 parently composed of chalybite. When fresh, as in well sections, 

 the bulk of the rock is blue, but the oolitic grains are always 

 brown. When weathered the whole rock becomes rusty brown 

 with occasional creamy patches where oolite grains are not too 

 abundant. The rock is almost always exceedingly fossili- 

 ferous. 



The best and, at present, the only workable exposure, is in 

 the village street at Elsworth and in the stream which runs 

 along that street. Until recently good material could be 

 obtained from one of the farmyards at the south end of the 

 village, but now the erection of a building has obscured that 

 exposure. Mr Wedd in mapping the country on the six inch 

 scale has been able to trace the outcrop of the Elsworth Rock 

 over a very considerable distance, from Meadow Farm, south of 

 Croxton, by Yelling and Papworth Everard to Elsworth and on 

 to Red Hill Farm. He also noted a probable exposure between 

 Fen Drayton and Swavesey, and north of the Ouse found a 

 fairly continuous outcrop from Holy well by Lindsell's Farm 

 and the old St Ives brickyards as far west as Houghton 

 Hall. 



About Elsworth the rock may be divided into a lower 

 limestone, four to seven feet thick ; a middle black but rusty- 

 clay, five feet ; and an upper limestone, one and a half feet. 

 It is the upper limestone which is most constantly oolitic, the 

 lower is often concretionary and may be hard and creamy. 



