LATER TERTIARY GEOLOGY OF EAST ANGLIA. 77 



beneath are penetrated alike by sand-pipes, which cut through the 

 stratification of the sands and the shelly Crag beneath, as shown in 

 a detached portion of section I. It is obvious that the percolation 

 producing this pipe must have been posterior to the abstraction of 

 the calcareous material and restratification of the sands through 

 which it passes ; and indeed the sharply distinct line of denudation 

 which occurs between these seemingly stratified sands and the 

 Middle Glacial gravel overlying them suggests that the abstraction 

 of the shelly material and restratification of the sandy residuum 

 preceded the deposit of this gravel. 



If we rightly gather the view of Mr. Prestwich from the sections 

 which accompany his paper already referred to, and from the re- 

 marks at (and following) page 333 of that paper, he regards these 

 unfossiliferous sands as belonging to his upper or Chillesford division 

 of the Red Crag ; but in this we do not agree, not merely for the 

 reasons we have quoted from our " Introduction," but because the 

 shells of which the ironstone casts have been preserved militate 

 against that view. So far as we are aware, none of the shells 

 characteristic of the newer (or Butley) portion of the Red Crag or 

 of the Chillesford bed, such as Scrobicularia piperata, Leda oblon- 

 goicles, L. lanceolata, Nucula Cobboldice, Mya truncata dfc, are in- 

 dicated by these casts ; while, on the other hand, there occur among 

 them numerous well-preserved impressions of such shells as Cardium 

 angustatum, and the gigantic variety of Mactra ovalis. Now Car- 

 dium angustatum is an extinct form peculiarly characteristic of that 

 portion of the Red Crag which remains unaltered beneath the sands 

 containing these casts, and in which it abounds, and is unknown in 

 the Chillesford bed, and, with the exception of a solitary specimen 

 mentioned by Mr. A. Bell from Thorpe by Aldbro', is unknown also 

 in the fluvio-marine crag. Mactra ovalis is common enough in the 

 Chillesford bed, as it is also in Glacial and Postglacial beds ; but the 

 gigantic variety of which impressions occur among these casts is 

 peculiar to the Red Crag itself, and is unknown both in the Chilles- 

 ford bed and the fluvio-marine crag. If therefore, Mr. Whitaker's 

 view that these unfossiliferous sands represent what was once shelly 

 crag be correct, that crag can, we think, have been none other than 

 the same as that which still remains unchanged beneath such sands. 



The uniform absence of the Chillesford Clay over the Red Crag, 

 wherever this is exposed between the Butley creek and the Stour, 

 and its replacement by the sands and gravels of Middle Glacial age, 

 as exhibited in section III. (p. 81), show, when coupled with its pre- 

 sence to the south of the Stour at Walton Naze, that this clay suf- 

 fered a considerable denudation before the deposit of those sands and 

 gravels ; but, as we shall see in examining the evidences of the 

 break between the Lower and Middle Glacial, the Chillesford Clay is 

 not the only bed which was removed from this area between the 

 close of the Crag and the deposition of the Middle Glacial ; for 

 during this interval the Contorted Drift which once overspread the 

 Red Crag has also been denuded from it, as outliers of that forma- 

 tion remaining in considerable thickness prove. Whether both the 



