ON THE PLEISTOCENE DEPOSITS OF DEVON. 449 



29. The Chronological Value of the Pleistocene Deposits of 

 Devon. By W. A. E. Ussher, Esq., F.G.S., of H.M. Geological 

 Survey, (liead December 19, 1877.) 



[Communicated to the Geological Society by permission of the Director- 

 G-eneral of the Geological Survey of the United Kingdom.] 



Part 1. Deposits. 



A. (a) The high tableland of the Cretaceous area of the Black- 

 down Hills and the summit of its outlying landmark, Haldon Hill, 

 are capped by an accumulation of brown, yellowish, or grey clay full 

 of broken unworn fragments of flint and chert. The clay rests alike 

 on Chalk and Greensand in pipes *. Its character in Chalk districts 

 is almost identical with that of the clay with flints in Kent ; 

 but near the westernmost extension of the Chalk (to the east of 

 Sidmouth) the contained flints are seldom whole. In many places 

 the matrix is thickly crowded with small flint chips and powder, 

 giving it a grey colour and gritty texture, as on Haldon and Peak 

 Hill, near Sidmouth. The clay with flints is confined to tableland 

 areas : where the contour has been subsequently modified, it is only 

 found on such heights as exhibit the original surface of the old plain 

 from which they were isolated. The clay with flint and chert varies 

 in thickness up to nearly 50 feet. 



That it has suffered from denuding agencies is testified by the 

 presence of 



(/3) Waterworn fragments of flint on its surface, bearing some- 

 what the same relation to the clay with flint and chert that is pre- 

 sented by the brick-earth to its representative in the south-eastern 

 counties. This waterworn material is of very local occurrence on 

 the Cretaceous tableland ; upon Haldon it has been described by 

 Mr. Godwin-Austen f and others. 



(y) A patch of flint- and quartz-gravel rests on the Triassic breccia 

 near Colford in the Crediton valley. 



(3) Near Staple Hill, on the north summit of the Blackdowns, I 

 found a patch of worn flint- and quartz-gravel, containing also frag- 

 ments of that dark slaty grit noticeable in the scattered waterworn 

 materials on Haldon. This gravel appears to rest directly upon 

 Greensand. 



The Cretaceous materials of Orleigh Court, described by De la 



* In chapter 13 of De la Beche's ' Report on the Geology of Cornwall' &c, 

 and in Mr. Godwin-Austen's "Geology of South-east Devon" (Trans. Geol. 

 Soc. ser. 2, vol. vi. p. 440 &c), the most general accounts of the Pleistocene 

 deposits are given. In Mr. Pengelly's numerous papers on the cavern deposits, 

 raised beaches, and submerged forests of Devon, to be found in most of the 

 yearly volumes of the 'Transactions of the Devon Association,' a partial sequence 

 of the events transpiring during the later stages of the Pleistocene epoch has 

 frequently been put forward. >^ 



t Trans. Geol. Soc. ser. 2, vol. vi. p. 447. 



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