138 



PEOE. J. PRESTAVICH ON THE RELATIOIST OP THE 



Pig. 11. — Section from Hatfield Brick-jnt to the G. N. Railway near 

 Digswell Junction. 



a. Post-Glacial beds. 



h. Boulder-clay. 



h'. Light-coloured sands and ochreous gravel. 



c. Westleton Shingle. 



T. Lower Tertiary strata. 



C. Ohalk. 



Speaking of the characters of these gravels, Prof. T. M*^K. Hughes * 

 says : " The gravel of the Upper Plain consists chiefly of pebbles ; of 

 these, about fifty per cent, are of quartz, about ten per cent, of 

 quartzite, about five per cent, various (such as jasper and a 

 conglomerate of quartz pebbles in quartzite), and the rest flint." 

 These gravels have also been noticed by Mr. Whitaker and Mr. 

 S. Y. Wood in the papers before referred to. 



This Hertfordshire shingle is remarkable for the large proportion 

 of pebbles of white quartz and of Lower- Grreensand debris f. A 

 specimen from Brickenden Hill yielded broadly : — 



per cent. 



1. Flint-pebbles m 



2. White quartz-pebbles 2S 



3. Subangalar flints — 2 stained brown, and 3 not stained 17 



4. Subangular fragments of red and brown chert and of white 



ragstone 18 



5. Pebbles of Lydian stone &c 3 



100 



Further to the south-west similar beds are met with at Shenley 

 Hill and again a little northward at Bernard's Heath near St. 

 Albans, where they form a bed from 8 to 10 ft. thick, capping Lower 

 Tertiary Sands and Clays, at an altitude of 408 feet. They there 

 consist of: — 1. Flint-pebbles ; 2. White quartz-pebbles; 3. Sub- 

 angular flints, not stained ; 4. Subangalar cherty ragstone ; 5. 

 Pebbles of Lydian stone, yellow quartzite, &c.; imbedded in a 

 loamy red and yellow mottled sandy clay, disturbed at top. 



Notwithstanding the extreme denudation which the high Chalk- 

 plain, extending from Hertfordshire into Bedfordshire, Buckingham- 

 shire, and Oxfordshire, has undergone, a few small outliers of Lower 

 Tertiary strata still remain, rising above the general level of the 

 Chalk-plateau with its scanty Glacial Drifts and its " Red Clay with 

 flints " 



These little isolated hills are frequently capped by a gravel- or 

 shingle-bed, which I believe to be of Westleton age. They are 



* Op. cit p. 285. 



t The chert and ragstone debris doss noi seem to have been hitherto 

 recognized, or is mentioned under some other name. 



