Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/115

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