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.