498 Dr. A. Irvhuj — The Thames High-level Plateau Gravels. 



lines of drainage, they now cap the liiglier parts of the terrain of 

 the Bagshot country, having preserved those parts from degradation, 

 while the intervening portions have been removed by ordinary 

 agents of denudation to form the present southern upland valley- 

 system of the Thames Basin. 



This view, first propounded in 1883, was endorsed by the late 

 Sir Joseph Prestwich, F.E.S., in his papers on the Mundesley 

 and Westleton beds, which were given to the Geological Society 

 in the year 1890. The author is not aware that that view has been 

 seriously combated.^ 



In the present communication the term ' Plateau Gravels ' is 

 used in the same sense as in that earlier literature of the subject 

 by the author and other writers. It denotes a series of altogether 

 different age from that denoted by the term as it is used in the 

 Handbook to the Natural History of Cambridgeshire written for 

 the use of members of the British Association at the Cambridge 

 meeting of 1904. They are of pre-Quaternary age, and are overlain 

 by Boulder-clay. They may be regarded as the lower stretches 

 of the series which cap the higher ground of Herts and the adjoining 

 counties, which was doubtless a continuous sloping plateau before 

 the present trough-like valleys, such as those of the Stort and the 

 upper Lea, were incised upon it.^ The term ' plateau gravel,' 

 therefore, here, as in the case of the other series to the south of the 

 Thames, seems to be appropriate. The gravels themselves repose 

 upon the Tertiary strata of the district. In structure and in their 

 relation to the Eocene formations tliey correspond completely with 

 the high-level plateau-gravels to the south of the Thames, that 

 structure having been minutely described by the author in the 

 papers referred to.^ But the composition of these gravels on the north 

 side is totally different from that of those on the south side of the 

 Thames Valley. There everything points to the derivation of the 

 materials from the once much higher country formed by the Wealden 

 and Kingsclere axes of elevation ; here the materials are such as 

 could only have come from the north or north-west. Their most 

 marked characteristic is the abundance of quartzite pebbles, together 

 with quartz, chert, and Lydian stone, from the Bunter of the north 

 and west Midlands. Along with these rolled fragments of Millstone 

 Grit, Coal-measure Sandstone, Carboniferous Limestone, Magnesian 

 Limestone are of less common occurrence, while even pebbles of 

 felstone and gneiss, probably from the Malvern crystallines, may be 

 recognised. All these materials ma?/ have been derived immediately 

 from the Bunter Sandstone as the author is acquainted with it ; and 

 along with these the more durable rocks of the intermediate Jurassic 

 series of the Mercian area are represented, together with Belemnites 

 and Grypheas rolled into pebbles; telling us that the peneplane 



' This is uow generally recognised, yet in a referee's note to the original MS. in 

 1883 it was pronounced " new to geology and baseless " ! 



2 The vaDey in which Much Hadham lies affords a good example of this trough- 

 like form on a smaller scale. 



2 See especially "Note on the Plateau Gi'avels, etc." : Q.J.G.S,, Nov., 1890. 



