294 OUR COMMON BRITISH FOSSILS. 



deeper water, and also by the greater preponderance 

 of arctic or northern forms. After the formation of 

 the Norwich Crag, the depression continued so as to 

 bring the sea over its site, and it was along its floor 

 that this Upper Norwich Crag was deposited. Hence 

 it obtains a more extensive geographical distribution 

 than the lower, or true " Fluvio-marine " Crag. In 

 Norfolk it extends to about eight miles beyond 

 Norwich ; and in Suffolk it is so well displayed at 

 Chillesford that it sometimes goes by the name of the 

 Chillesford Crag. At Aldeby, near Beccles, this crag 

 is exposed in some brick-pits, and such shells as Mya 

 arenaria are found abundantly, with both valves 

 attached, standing upright in the sands as when they 

 were alive. At Chillesford, in the stackyard near Mr. 

 Crisp's house, this crag is seen to its best advantage, 

 full of shells, many of them of a northern type, such 

 as Mya truncata, Cardium Greenlandicum, Astarte 

 borealis, etc. There can be no doubt that much con- 

 fusion still prevails in the catalogue of the Norwich 

 Crag shells, owing to the manner in which both the 

 upper and lower crags were formerly confounded. In 

 the pits on Thorpe and Sizewell Commons, near 

 Aldborough, the ordinary Norwich Crag fossils are 

 abundant ; whilst the brick-pits at Aldeby yield 

 most beautifully preserved Upper Norwich, or Chilles- 

 ford Crag. 



The Norwich Crag is seen resting on the Chalk at 

 Thorpe, near Norwich, where the shell bed is about 



