190 DE. C. T. TEECHMA.K'N OX INTERGLACIAL [vol. lxXYV 



The evidence for the Preglacial age of the Sewerby sub-Boulder- 

 Clay beach near Bridlington is by no means conclusive, and the 

 same remark applies to the 2o-foot raised beach of the South- West 

 of England and Wales. 1 The presence of CorMcula fluminalis in 

 the Kelsey and Holderness deposits which are associated with the 

 purple clay bearing Cheviot erratics also requires further explana- 

 tion than many of the reasons that have been put forward for its- 

 presence there. 



(h) — The valleys which have been eroded on the eastward- 

 sloping surface of the Magnesian Limestone of the coastal area, 

 especially those of Hesleden, Castle Eden, and Hawthorn Denes* 

 have been described as Preglacial. 2 They are certainly Preglacial 

 in the sense that they were eroded previously to the deposition, 

 of the main Cheviot and Scottish drift which now fills them. 



I am, however, inclined to think that they may have been 

 formed after the retreat of the Scandinavian ice-sheet from the 

 coast, and that they are in reality Interglacial. 



It is difficult to see how the Scandinavian ice-sheet filled up the 

 small Preglacial valley of Warren-House Grill and yet failed to fill 

 up the neighbouring but wider and deeper valley of Castle-Eden. 

 and Hesleden Denes if the latter were in existence previously to its 

 oncoming. Certainly a Norwegian erratic was found some years- 

 ago in a deep excavation for the railway- viaduct near the entrance 

 of Castle-Eden Dene, but it appears to have been a stray derived 

 boulder similar to. but much smaller than, two now lying on the 

 shore about 2| miles away to the south-east, which seem to have- 

 fallen from the Cheviot Drift that caps the cliffs. 



No certain or direct evidence is, however, yet available on this- 

 point, as the drifts that occup}^ the bottom of the deeper of these 

 gorges are buried out of sight ; but there is a possibility that some 

 of the valleys of the Magnesian-Limestone area of Durham were 

 formed in the period intervening between the retreat of the 

 Scandinavian ice and the oncoming of the local glaciation. They 

 would therefore correspond in age with many of the Interglacial 

 Chalk valleys of East Anglia described by Mr. P. W. Harmer. 3 



The Postglacial gorges of the coastal area are in some cases 

 more or less definitely superimposed on the above-mentioned Pre- 

 or Interglacial valleys, and are eroded through the Cheviot Drift, 

 and some of them cut down into the underlying Magnesian Lime- 

 stone for part of their course. 



The present gorge of the Wear across and through the 

 Magnesian-Limestone escarpment at Sunderland is a now familiar 

 example. It has long been known that previously the Wear was a 

 tributary of the Tyne, and that its old valley leading to that river 

 is filled up with Glacial debris. 



1 A. L. Leacli, ' On the Relation of the Glacial Drift to the Raised Beach 

 near Porth Clais, St. Davids ' Geol. Mag. dec. 5, vol. viii (1911) p. 465. 



2 D. Woolacott, Q. J. G. S. vol. lxi (1905) p. 64. 



3 ' The Glacial Deposits of Norfolk & Suffolk : Trans. Norfolk & Norwich 

 Naturalists' Society, vol. ix (1910) p. 130. 



