484 



STEATIGRAPHICAL GEOLOGY 



different facies, the deposits in Norfolk being mainly sands, while 

 those in Yorkshire are clays and those in Lincolnshire are sands r 

 clays, and limestones. In the two latter counties we have a more 

 complete succession of marine deposits of Lower Cretaceous age 

 than is found in any other part of England, and our detailed 

 knowledge of them is mainly due to the labours of Professor J. W. 

 Judd and Mr. G. W. Lamplugh. 11 Unfortunately the Norfolk 

 outcrop is separated from that of Lincolnshire by the breadth of 

 the Wash, and in Yorkshire the exposed areas near Speeton are 

 very small, their southerly continuation being concealed beneath 

 the overlap of the Upper Cretaceous (see map, Fig. 165). Con- 

 sequently correlation is rendered difficult and entirely dependent 

 on the discovery of fossils. The probable correlation is shown in 

 the following table : 



It will be convenient to take Yorkshire first, and in that 

 county the only good exposure is on the coast near Speeton Gap, 

 and hence the whole series, which has a total thickness of about 

 330 feet, is sometimes termed the Speeton Clay. 



The base of this clay is a layer of black phosphatic nodules- 

 embedded in stiff clay. The fossils are badly preserved, but 

 among them Professor Pavlow identifies Belemnites absolutus with 

 Ammonites resembling Holcostephanus Panderi and Hole, scythicus,. 

 species characteristic of beds in Russia which are regarded as 

 Portlandian. It is a question, however, whether these fossils 

 are contemporaneous or derived. This bed is succeeded by 

 clays which contain Belemnites lateralis, Holcostephanus subditus, 

 Hole. Lamplughi, Hoplites amblygomis, with Exogyra Couloni and 

 Astarte senecta. These clays are about 35 feet thick and were 

 considered to be of Purbeck age by Professor Pavlow, but the 

 continental zone of Hoplites Boissieri with which he correlated part 

 of these beds is now regarded as the base of the Valenginian, and 

 the natural base of the Cretaceous System both in England and 

 Germany is at the base of this zone of Bel. lateralis. 



The overlying clays form the zone of Actinocamax jaculum and 



