140 PEOF. J. PRESTWICH ON THE RELATION OF THE 



The gravel rests on an uneven bed of London Clay. Lower 

 Greensand debris is comparatively scarce, and the debris of Tertiary 

 rock more abundant, while pebbles of the older rocks are rare. 



There are several Tertiary outliers on the high Chalk-plateau 

 between the Misbourne and the Wye. One of these, to the west of 

 the village of Penn, near Beaconsfield, rises to the height of about 

 600 feet, and is capped by a well-marked bed of Westleton gravel 

 like that on Tiler's Hill, while another gravel (Glacial ?), derived in 

 part from the Westleton Shingle, lies on the Chalk-plain at Penn (547 

 feet) and Penn Common. At Lane End (600 feet), four miles west 

 of High Wycombe, there is also an outlier of Lower Tertiary sands 

 and clays, capped by a similar gravel of flint- and white quartz- 

 pebbles, subangular flints weathered white, with a few old-rock- 

 pebbles (see PI. VII. fig. 1). 



South Oxfordshire. — From the borders of Buckinghamshire to the 

 Thames between Pangbourne and Walliugford there are but few 

 Tertiary outliers. The most conspicuous of these is that at 

 Nettlebed hill *. The Westleton Shingle (?) there attains its highest 

 level of about 650 feet. It is but a small patch, and presents a less 

 definite composition than the others, as might be expected from its 

 distance from the main body. 



The shingle, which reposes upon a very uneven surface of ihe 

 Lower Tertiaries, consists approximately of : — 



per cent. 



1. Tertiary flint-pebbles 54 



2. Small pebbles of white quartz 14 



3. Subangular flints, not stained 20 



4. Chert &c? 4 



5. Pebbles of liornstone (?), veinstone, and Sarsen stone 8 



100 

 imbedded in a matrix of light quartzose sand. 



Five miles S.W. of Nettlebed, and close to the edge of the Chalk 

 escarpment overlooking the plains of Oxfordshire, a thin outlier of 

 Lower Tertiaries (the mottled clays of the Reading Beds) overlies 

 the Chalk at Greenmoor hill (560 to 600 feet) and Woodcote 

 Common, about 3 miles east of Goring. It is capped by a well- 

 Pig. 12. — Section on the hill above the Thames, near Goring. 



c. Westleton Shingle. I T. Lower Tertiaries. 



h'. Glacial Gravel. | C. Chalk. 



defined bed of Westleton Shingle, which is in marked contrast with 

 the Glacial gravel, with its JN'ew-Iled-Sandstone quartzites, which sets 



* Described in Quart. Journ. Geol. Soc. vol. x. p. 89 (1854). 



