Geology of Hertfordshire

[1][2] On the northern boundary and just inside the county, at the foot of the chalk Chiltern Hills, near Tring and Ashwell, there is a small strip of exposed Cretaceous Gault Clay and Upper Greensand.

[1] Above these beds, the Lower and Middle Chalk, without flints, rise up sharply to form the steepest part of the Dunstable Downs, which are the easterly continuation of the Chiltern Hills.

[1][3] The Palaeocene Reading beds consist of mottled and yellow clays and sands, the latter are frequently hardened into masses made up of pebbles in a siliceous cement, known locally as Hertfordshire puddingstone.

[1][2] About 478,000 to 424,000 years ago during the ice age period known as the Anglian Stage, glaciers approached from the North Sea and reached as far south-west as Bricket Wood.

Glacial gravels and boulder clays cover a great deal of the whole area to the north east of the county and the Upper Chalk itself has been disturbed at Reed and Barley by glaciation.

[1][2] At the retreat of the glaciers, wind blown powdered rock known as loess was deposited over the whole county, forming thin layers under a metre thick.

Soil map of Eastern Hertfordshire - by G. A Dean, 1864
Barkway Chalk Pit , a nature reserve in Barkway in Hertfordshire managed by the Herts and Middlesex Wildlife Trust. The exposed chalk is a block displaced by an ice sheet which once covered the area
Tyttenhanger: Gravel pit workings. Flooded gravel pits to the left with a conveyor belt bringing material into a central batching plant from more distant active workings, the River Colne to the right. The gravel was deposited here by the primordial River Thames as it flowed through Hertfordshire.