XI. 



THE POST-TERTIARY DEPOSITS OF HERTFORDSHIRE. 

 By J. Vincent Elsden, B. Sc. (Lond.), F.C.S. 

 Read at Watford, \&th March, 1880. 



TJndek this title I propose to consider those superficial accumu- 

 lations of gravels, sand, and clay which cover up nearly the whole 

 of our county. 



The systematic mapping and description of these deposits being 

 perhaps the most important geological work our Society can under- 

 take,* it has occurred to me that it would be useful, as a preliminary 

 step, to lay before it some account of the views generally held 

 concerning their age, origin, and general influence. 



It will be convenient to divide the subject into three parts, 

 viz. : — (1) General description, (2) Mode of formation, (3) Econo- 

 mic importance. 



I. General Description. 



The deposits under consideration are usually subdivided as 

 follows : — 



i Sub-aerial. 

 Fluviatile. 

 Marine. 



! Upper Glacial. 

 Middle Glacial. 

 Lower Glacial. 



It will be endeavoured, as far as possible, to keep to this 

 arrangement, though in ascending order. 



Lower Glacial. — There may be seen at Queen Hoo Hall, Bright's 

 Hill Wood, Hertford Heath, Brickendon Green, Bayford, Little 

 Berkhamstead, Essendon, and Hatfield Park, generally capping 

 the Tertiary hills, a pebble-gravel, composed chiefiy of flint and 

 quartz pebbles, which has been referred to Pre-glacial times. 

 From its position on the higher ground Professor T. McK. 

 Hughes has called it the Gravel of the Upper Plain. f It is very 

 distinct in its composition from the gravels in the lower grounds, 

 and is presumed to be of marine origin, from its great extent, 

 persistent character, and uniform level. (See Fig. 1.) 



Mr. S. y. Wood, jun., is inclined to class these beds with the 

 Middle Glacial, thinking they are not older than the gravels of the 

 Lower Plain, but intermediate between them and the Boulder- 

 clay. J Mr. ^VTiitaker suggests the possibility of these gravels 



* For suggestions as to a method see "A scheme for the Examination of the 

 Glacial Deposits of the Midland Counties of England," by W. J. Harrison. 

 'Midland Naturalist,' Sept. 1878. 



t * Quart. Journ. Geol. Soc.,' vol. xxiv, p. 283. 



X ' Mem. Geol. Survey on Sheet 47,' and "Whitaker's ' Guide to the Geology of 

 London,' p. 51. 



