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). 5
