GEOLOGY 



through either Lower Greensand or Portland Sand, most probably the 

 latter, as it comes to the surface at Hulcot in Bucks, four miles to the 

 north-west. Below this the Kimeridge Clay is described as ' dark clay, 

 with waterstones ' [probably septaria], but the search for water was futile. 1 

 The Kimeridge Clay is a marine formation, as are the Silurian and 

 Devonian rocks reached so far beneath the surface in the east of the 

 county, but there is evidence, in its lignites and in the presence of coni- 

 ferous wood in considerable quantity, of the proximity of land. The 

 Portland Beds are also of marine origin ; but immediately above them, 

 and coming to the surface at Liscomb Park near Soulbury, thirteen miles 

 north of Puttenham, are Purbeck Beds, which are ofestuarine and fresh- 

 water origin. It therefore seems probable that towards the close of the 

 long interval unrepresented in our county after the Devonian beds became 

 dry land perhaps many millions of years a submergence here took 

 place, and rivers brought down from a not far-distant land-surface the 

 mud of which the Kimeridge Clay consists ; that by the gradual eleva- 

 tion of this land-surface the sea became shallower, the sands of the Port- 

 land series then being deposited ; and that, the elevation still continuing, 

 estuarine and fresh-water conditions prevailed, these being characteristic 

 of the Purbeck Beds. The three formations here mentioned the Kim- 

 eridge Clay and the Portland and Purbeck Beds form the Upper 

 Oolites, the highest division of the Jurassic rocks. 



Within twenty miles from Puttenham, in a north-north-westerly 

 direction, the whole of the lower divisions of the Jurassic series are met 

 with the Middle Oolites, the Lower Oolites, and the Lias the axis of 

 elevation having thus been on the north or north-west. After the beds 

 were raised from their original horizontal position, so as to dip towards 

 Hertfordshire away from this axis, they were planed down by denudation, 

 the edges of the strata thus successively cropping out. It is this tilting- 

 up which brings the older and originally lower rocks to the surface so 

 that they crop out from underneath the newer rocks which have been de- 

 posited upon them. When the tilted-up edges of the newer rocks offer 

 a greater resistance to denudation than those underneath them they 

 terminate in an escarpment such as that of the Chalk ; when a less re- 

 sistance, in a valley, which may be extended into a plain such as that of 

 the Gault. 



We now come to the third great division of the Secondary rocks, the 

 Cretaceous System. Its lowest member represented in Hertfordshire is 

 the Lower Greensand. Whilst the Hastings Sands and Wealden Beds 

 were being deposited in the south-east of England, there was probably 

 dry land here, but this was gradually submerged, and the Lower Green- 

 sand was deposited over the Kimeridge Clay with a slight uncon- 

 formity, its phosphatic-nodule bed at Potton, just outside our county 

 boundary, showing, in the numerous water-worn fossils derived from the 

 Jurassic rocks, what a great amount of denudation they must have 



1 Whitaker, ' Hertfordshire Well-sections,' and paper, Trans. Herts Nat. Hist. Sac., vol. 

 vi. p. 60 (1890). 



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