422 CRETACEOUS. 



is broken up, and in part re-arranged, the flint layers are disrupted, and all 

 symptoms of stratification destroyed. Thus in a pit at Costessey, near Norwich, 

 the flints were disturbed to a depth of 20 feet. The Chalk when in this condition 

 is known as "Glaciated Chalk," and to this reference will subsequently be made. 

 The surface of the Chalk frequently exhibits great hollows or channels filled with 

 material from the superincumbent deposit. These hollows, called "pipes" or 

 "sand-galls," are caused by the dissolving away of the Chalk, and are filled by 

 the overlying sand or gravel ; sometimes these hollows give way suddenly, causing 

 subsidence of the ground. (See also notes on reconstructed Chalk, p. 429.) 



