Wood — Structure of the Thames Valley. 57 



(Kimmeridgian) genus of Saurian fishes, for whicli the term 

 Thlattodus is here proposed, the present species being designated 

 suchoides, from the crocodilian resemblance above noted. The 

 references in the figure of PL III. are explained in the text. 



in. — On the Stkucture of the Thames Valley and of its 



Contained Deposits. 



By Searles V. Wood, jun., F.G.S. 



THE great Valley of the Thames has long been known to contain 

 deposits of gravel intermixed with freshwater beds ; but the true 

 Geological age both of the valley and of its deposits was long in 

 doubt. Latterly the view expressed by ]\Jr. Prestwich (in which he 

 was of accord both with Professor Morris and Mr. Trimmer) , that the 

 whole was newer than the Boulder-clay, has met with acquiescence. 

 The general structure of the deposits in this valley, although they 

 form the most extensive of the Post-Glacial series in England, has 

 not however yet been shown in any comprehensive manner. 



Mr. Prestwich, speaking of the gravel which forms the principal 

 member of the deposits, says' that it stretches ''in a continuous and 

 uninterrupted sheet from the sea to Maidenhead." In his paper "On 

 the Loess of the Valleys of the South of England and of the Somme 

 and Seine,"* he identifies the brickearth of the Thames Valley with 

 the Loess of the Belgian plateau and with the brickearth of the 

 Valleys of the Seine and Somme ; but of the Thames Valley he says, 

 the structure is complicated by the existence, in addition to the high 

 and low-level valley-gravels, of a wide-spread set of marine hill- 

 gravels covering large tracts of country, but that, after eliminating 

 the foreign element, "there remains a set of valley and terrace- 

 gravels which, though not so marked or well characterized as in the 

 Seine Valley, are nevertheless of nearly similar order and age."^ 

 He adds that the Loess "is intimately associated with all the valley- 

 gravels and is contemporaneous with, and dependent upon, them 

 from the beginning to the end of the series," and he conceives it to 

 be "the result of river-floods commencing at the period of the 

 highest valley-gravels and continued down to the end of that of the 

 lowest valley-gravel."* 



Although the age of the Thames Valley and its deposits, relatively 

 to the Boulder-clay, has never been shown (Mr. Prestwich considering 

 that the exact relation of the deposits is nowhere clearly seen, and 

 that the question of relative age depends on a variety of collateral 

 evidence'), yet the proof is readily afforded by drawing a line from 

 the Hog-Market, Finchley, upon which hill exists an outlier of the 

 Boulder-clay or Upper-drift (underlaid by a thin bed of the Middle- 

 drift gravel), to the summit of Havering Hill, in Essex, upon which 

 is another outlier of the Boulder-clay ; this line will form a perfect 



' Quart. Journ. Geol. Soc, Tol. xii., p. 131. See also map ia paper in Phil. 

 Trans., 1864, Part II. 

 2 Phil. Trans., 1864, Part II. ' Ibid., p. 255. 



* Ibid., pp. 273-4. « Quart. Journ. Geol. Soc, Tol. lii., p. 133. 



