at Graveley, near Huntingdon. 183 



it is possible that the surface of the Oxford clay may sink down- 

 wards in that direction, forming a shallow basin so as to hold up 

 water in the base of the Boulder clay, which there seems to be 75 feet 

 thick. There is at Yelling a natural chalybeate spring called 

 " Nill well," which rises at about 100 feet above Ordnance 

 datum. 



I have hitherto been inclined to think that the Boulder clay 

 of East Anglia has been deposited from floating ice, having 

 formed my opinion mainly from what I have seen in the contorted 

 drift of the Norfolk cliffs. But the Boulder clay at Graveley has 

 a different character; and it is difficult to account for the rounded 

 and scratched lumps of hardened chalk, of Kimmeridge shale, and 

 other rocks impacted in it, except on the hypothesis that it has 

 been shoved about in all directions in mass. The smallest en- 

 closures bear scratches pointing in every direction upon their 

 surfaces. This is very noticeable on the scraps of Kimmeridge 

 shale. 



A somewhat corroborative piece of evidence is that the 

 Boulder clay reposed upon an indurated layer in the Oxford 

 clay, the Boulder clay having apparently sheared off the softer 

 clay till it was arrested by the harder rock. 



13—2 



