148 PKOr. J. PRESTWICH ON THE RELATION OF THE 



the broad valley between Portland and Upway, above six miles 

 wide and from 400 to 500 feet deep, since the deposition of that 

 high-level Drift. 



Dr. Mourlon has also recently recorded * the occurrence, at 

 Ixelles near Brussels, of a bed of sand and gravel with Mammalian 

 remains, which he thinks may be of the age of the Porest-bed ; but 

 the evidence is yet uncertain, owing to the fragmentary condition 

 of the specimens. Amongst the species on which Dr. Mourlon 

 relies, but which are determined with doubt, are ElepJias cmtiquus, 

 Equus ^licidens, Cervus canadensis, and Bison priscus. 



Conclusion'. 



It is clear from its uniformity and its marine origin that the 

 Westleton Shingle must originally have formed a comparatively level 

 sea-floor (or broad coast-line) throughout the area over which its 

 outliers extend, and that all the inequalities of the surface below 

 that level have been formed since it was deposited. If this con- 

 clusion be correct, then it follows that all the Tertiary strata, which 

 spread originally over all the Chalk Downs of Hertfordshire, Buck- 

 inghamshire, Oxfordshire, Berkshire, Wiltshire, and some adjacent 

 districts, have been removed subsequently to this early Pleistocene or 

 so-called Pre-Glacial period ; also that the gorge of the Thames at 

 Pangbourne and Goring has been formed since then ; and that most 

 of the Pre-Glacial valleys in the district, to which no date, except 

 that they were Pre-Glacial, had been assigned, are of the same geo- 

 logical age. It follows further that before this date the Thames 

 had no existence ; to this point we shall revert on another occasion. 



During the Westleton period, we have to imagine a sea, with a 

 coast-line extending from Belgium to the West of England, bounded 

 on the south by the anticlinal range of the Wealden, and open to a 

 yet uncertain distance to the north. This area then underwent an 

 elevation from east to west, and from south to north, whereby it 

 was raised at the extreme points from 500 to 600 feet or more above 

 the sea-level, whereas it remained nearly at its original level and 

 comparatively undisturbed at its other or eastern extremity ; so that 

 while on the east the chronological orders of succession of the strata 

 continued unbroken, to the west they became discordant, in conse- 

 quence of the physiographical changes which intervened between 

 these successive stages. 



These changes and the action of meteorological agencies on the up- 

 raised land, determined the plotting-out of its hills and valleys during 

 the early Glacial epo(3h. Of the first stage we have now, in some 

 cases, an exact measure. Thus, to take a few examples, it is obvious 

 that at the gorge of the Thames at Goring the Westleton Shingle 

 and Tertiary Strata were continuous from side to side of the valley 

 until the last elevation of the land. As they emerged, and a 

 land-surface was formed, these beds, together with the underlying 

 Chalk, were first denuded in the line of the present Thames valley 



* Bull. Acad. Roy. Belgique, 3" ser. vol. xvii. p. 131 (1889). 



