160 PBOF. J. PRESTWTCn 0^ A SOUTHERN DRIFT IN THE THAMES 



matrix of bright yellow and red sands. The Downs at this spot are 

 from 400 to 500 feet high. The Hog's Back, which rises to the height 

 of 500 feet between Guildford and Farnham, is bare of any Drift. 



To the north of these Downs are the extensive heath-covered hills 

 of the Bagshot, Chobham, and Primley ridges, from 350 to 415 feet 

 high. These are capped by the Southern Drift, with a certain 

 proportion of ragstone and chert, imbedded in a coarse ochreous and 

 ferruginous sand. The gravel is from 6 to 12 feet thick, roughly 

 stratified, and becomes white on exposure. Large blocks of Tertiary 

 sandstone * occur not unfrequently, and in places there are a few 

 small quartz-pebbles (PL VII. fig. 2). 



BerTcshire. — The lower part of the valley of the Thames imme-, 

 diately adjacent to the river exhibits only Glacial and Post-Glacial 

 Drifts. The higher hills which range from Windsor Forest to 

 Sandhurst are covered in great part by the Southern Drift. 



The most striking instance I know of is that exhibited on Cherry 

 Down (named Burleigh in the new 1-inch Ordnance Maps), near 

 Ascot. Subangular fragments of chert and ragstone are there so 

 abundant that they almost equal in number the subangular flints, 

 which, with flint-pebbles and a few fragments of iron-sandstone, 

 constitute the other portion of this gravel. These are imbedded 

 in a yellow quartzose sand with some black grains and quartz- 

 grit : in the gravel there are a few intercalated grey and red argil- 

 laceous seams as at J^orwood. This hill is about 300 feet above 

 sea-level, and 220 to 240 feet above the plain of the Thames a^^^ 

 Windsor and Slough (see PL YII. fig. 2). 



Further westward the ridges are less persistent, and isolated 

 hills more frequent. Several of these are capped with this gravel. 

 On one of them, called " Gravel Hill " on the map, 400 feet high, 

 and 1 mile east of Caesar's Camp Hill, near Easthampstead, the gravel 

 consists of : — 



per cent. 



1 . Subangular fragments of flint, part not stained, and part stained 



yellow and brown 40 



2. Tertiary flint-pebbles, of which about one fifth part ai'e broken. 32 



3. Much-woi'n fragments of light-coloured ragstone and yellow 



chert, a nd ironstone-grit 18 



4. Small pebbles of white quartz 4 



5. Pebbles of white sandstone and subangular fragments of Sarsen 



stone (Tertiary) 6 



100 



Long exposure has bleached the upper part of the gravel, which 

 is roughly stratified, as seen in the following section : — 



* Some of these are 20 tons in weight. They mostly occur deep down at 

 the base of the gravel, and are derived from the Upper Eagshot Sands on 

 which the gravel rests. 



