562 KEV. A. IRVIXG ON THE PLATEAU-GRAVELS 



§ 4. Other evidences of glaciation may be seen (a) in an unstra- 

 tified clay with pebbles standing erect in it, upon an older angular 

 and sanely gravel, which lies upon an eroded surface of London 

 Clay in the Brick-and-Tile Company's Brickyard north of Bracknell ; 

 (6) in what appears to be a lacustrine deposit, as if in an extra- 

 morainic lake, above the Bagshot beds in the brick-pits at Warfield *, 

 wrongly described (as I venture to think) as a part of the Bagshot 

 formation ; (c) in the California clay-pits at Einchampstead t, where 

 a similar lacustrine deposit probably occurs above the Middle Bagshot 

 Clay, at about 230 ft. (o.d.). 



The phenomena described in §§ 1, 2,3, I take to be due to the 

 mechanical work of floating ice, as pack-ice was formed from time 

 to time in the old Thames Straits under the influence of high 

 winds J. 



Conclusions. 



(1) Looking at the distribution of the Plateau-gravels of the 

 district under consideration, their roughly- stratified structure, their 

 constituent materials, the entire absence in them of contemporaneous 

 organic remains, the frequent occurrence in them (especially near 

 their base) of blocks of concretionary sandstone § (in many cases 

 unworn, in others smoothed on the upper surface by the scouring 

 action of sand and water), the frequent reconstruction and rough 

 stratification of the immediately subjacent Upper Bagshot Sands (on 

 which for the most part these gravels rest), and the marked erosion 

 of these sands in many places beneath the gravels, — it is diflicult to 

 resist the conclusion that they had a fluviatile origin, the materials 

 having been derived from the south, 



(2) The only conception I can form which harmonizes with all 

 the evidence available, is that of an elevated district of the older 

 Tertiaries, subjected to subaerial waste, and having a general slope 

 to the north towards what may have been an estuarine arm of the 

 ancient Anglo-Germanic Sea, on the western margin of which the 

 Crag was being deposited at levels not very diff'erent from those at 

 which it is found to-day ; the present gravels marking the drainage- 

 courses of the later Tertiary valleys, while the present valley-system 

 of the district occupies those portions which then formed higher 

 ground, and has been initiated owing to the absence of the gravel- 

 capping to which the present hills owe their preservation. 



* Compare Section F, p. 386, vol. xliii. Quart. Journ. Geol. Soc. 1887. 



t Ibid. vol. xliv. p. 172, Section Q. 



I In all such cases the piling up of the ice takes place too quickly for defor- 

 mation of the ice-masses to proceed ^an pass?^ with the increase of pi-essure. 

 We thus get a mechanical agency brought into play quite different from anything 

 which we can rationally postulate from the slow movement of a glacier. See 

 my paper on the " Mechanics of Glaciers," Quart. Journ. Geol. Soc. vol. xxxix.; 

 also the published work of Prof. Spencer, Geol, Mag. April 1887, and ' American 

 Naturahst,' vol. xxii. 1888. 



§ In the Proc. Geol. Assoc. 1883, I gave reasons for rejecting the Sarsens as 

 evidence of a glacial origin of the gravels, in and near the base of which they 

 usually occur. 



