Geology and Physiography 7 



lower zones of the Oxford Clay are, however, worked in the well-known 

 brick pits around Peterborough. 



The relation of the Oxford Clay to the Corallian requires further 

 research. Dr Morley Davies has recognised a non-sequence just over the 

 boundary at Sandy (Beds), where Corallian Exogyra nana beds rest on 

 the renggeri-zone. The Elsworth Rock Series at Upware rest on the mariae- 

 zone, as does the Corallian Limestone at Warboys. 



CORALLIAN 

 The correlation table given below is based on the work of Dr Arkell. 



The Elsworth Rock is a hard ferruginous calcareous mudstone, blue in 

 colour when fresh, and brown when weathered. It is associated with clays 

 and sandy beds, and all the rock-types contain limonite ooliths. It is 

 diachronic. At Upware, it is 16 ft. thick, rests on Oxford Clay {tnariae- 

 zone), and belongs to the cor datum-zone, corresponding in age to the 

 iron-shot clays which form the lower part of the series at Elsworth. The 

 upper part of the series at Elsworth is about 12 ft. thick ; from the abundance 

 of ammonites, the masses of Serpulae and encrusting oysters, and the 

 ironshot lithology, it is usually regarded as a condensed deposit. To the 

 north, at Warboys, the base of the Corallian is represented by 3 ft. of hard 

 shelly limestone with Exogyra nana, and a few, as yet undescribed ammo- 

 nites ; this is probably equivalent to part of the Elsworth Rock Series. To 

 the south, at Sandy, the series is replaced by clays with limestone bands. 

 The Ampthill Clay is not well exposed. The pits at Gamlingay are 

 overgrown; the excavation at Long Stanton, described by Dr Arkell, was 

 temporary. New pits have been opened near Manea and Mepal (where 

 the clay includes a limestone band, possibly the Boxworth Rock). Ampthill 

 Clay fossils have been collected by the Geological Survey at Horseway on 

 the Forty-Foot Drain, and at Honey Bridge on the Sixteen-Foot Drain. 

 The Ampthill Clay is darker than the Oxford Clay, and contains phosphatic 



