The Chalk Marl is superimposed upon the Gault and Upper Greensand beds, which are confined to the western portion of the county. Its upper layer passes into a sandy limestone, known as Totternhoe stone, which has furnished materials for many churches in the shire. Ashwell, Pirton and Tring may be named as neighbourhoods where this stratum may be traced.
The Lower Chalk is devoid of flints, and rests, in somewhat steeply sloping beds, upon the Totternhoe stone. It forms the western slopes of the Dunstable Downs, and of the Chiltern Hills. It is fossiliferous, one of the commonest of its shells being the Terebratula.
The Middle Chalk, of resonant hardness, is laminated, and has at its base the Melbourn Rock and at its summit the Chalk Rock. Nodules of flint, greenish in appearance, and (rarely) arranged in layers, occur sparsely in the Middle Chalk, which may be traced in the neighbourhood of Boxmoor, Berkhampstead and Baldock, and also in a few other districts.
The Upper Chalk.—Although, as has been stated, the configuration of Hertfordshire is very undulating, we are able to discern a general trend in certain districts. Thus, there is a gradual slope to the S. from the N.W. and central hills, a slope which comprises the larger part of the county. This slope is formed of the Upper Chalk, a formation abounding in layers of black flints. The chalk is whiter than that of the lower beds, and very much softer. Fossil sponges, sea-urchins, etc., are abundant in this formation.
Tertiary.—Many of the chalk hills of Hertfordshire are strewn with outlying more recent deposits which prove that the lower Tertiary beds were more extensive in remote ages. The beds of sand and clay, of such frequent occurrence in the S.E. districts, contain fossils so distinct from those of the Upper Chalk that an immense interval must have elapsed before those Tertiary deposits were in turn laid down.
The Eocene Formation.—The Thanet Beds, of light-coloured sands, present in some other parts of the London Basin, notably in Kent, are wanting in Hertfordshire. There are, however, some widespread deposits of loamy sands which may possibly be rearranged material from the Thanet Beds.
The lowest Eocene deposits in the county are the Reading Beds. These rest directly upon the Chalk and have an average thickness of, say, 25 feet. They may be traced E. to S.W. from the brickfields near Hertford to Hatfield Park; thence to the kilns on Watford Heath and at Bushey; they may also be traced from Watford to Harefield Park. These beds contain flints, usually found close to the Chalk, and consist chiefly of mottled clays, sands, and pebble-beds. Fossils are but rarely found. From the Woolwich and Reading Beds come those conglomerate masses of flint pebbles commonly called Hertfordshire plum-pudding stone. These have usually a silicious matrix and were often used by the Romans and others for making querns for corn-grinding. It is, perhaps, not impertinent to mention here the opinion of geologists that during the Eocene Period a considerable portion of the land usually spoken of as S.E. England was covered by the ocean.
Resting upon the Reading Beds we find that well-known stratum called the London Clay, which is of bluish hue when dug at any considerable depth. It is found in some of the same districts as the Woolwich and Reading Beds, and from Hertford and Watford it extends to N.E. and S.W. respectively until it leaves Hertfordshire. Its direction may be approximately traced by a series of hills, none of which are of any great height.
The Drift.—In Hertfordshire, as elsewhere, the strata whose names are so familiar to geologists do not form the existing surface of the ground. For the origin of this we go back to a comparatively recent period, when disintegration was busily working upon the solid rocks, and glaciers were moving southwards, leaving stones and much loose débris in their wake. Rivers, some of which, as in the Harpenden valley, have long ceased to run, separated the flints from the chalk, forming a gravel which is found in quantities at Harpenden, Wheathampstead and St. Albans, and is, indeed, present in all valley-bottoms, even where no river now runs. Gravel, together with clays, sand, and alluvial loams, forms, for the most part, the actual surface of the county.
The Rivers of Hertfordshire are many, if we include several so small as hardly to deserve the name. They are the Ash, Beane, Bulbourne, Chess, Colne, Gade, Hiz, Ivel, Lea, Maran, Purwell, Quin, Rhee, Rib, Stort and Ver.