.Solid Formations.

In Hertfordshire the oldest deposits occurring at the land surface are the Gault Clay and Chalk of Upper Cretaceous age, about 100 million years old. The Gault Clay occurs only in two north-western extremities of the county near Tring and Ashwell, but it forms a continuous area of low-lying ground all along the foot of the Chiltern Hills, and has been proved in boreholes to occur at depth beneath southeast Hertfordshire.  See cross section below. 

Both the Gault Clay and Chalk contain the fossilised shells of sea animals, such as ammonites and sea urchins, and were deposited on the sea floor. The Gault Clay is composed mainly of fine mud brought into the Upper Cretaceous sea from surrounding land areas by rivers. In contrast, the Chalk is composed almost entirely of calcium carbonate accumulated by microscopic marine organisms, especially planktonic algae. The sea in which it accumulated was much deeper and clearer, with almost no mud from the land.

In south-eastern Hertfordshire the Chalk is about 680 ft thick. It is divided into the Grey Chalk Subgroup (traditionally the Lower Chalk) and the White Chalk Subgroup (traditionally the Middle and Upper Chalk units) with a major boundary taken at an unconformity surface located at the base of the Plenus Marl.

The Grey Chalk (180 ft thick) often forms a bench projecting north-westwards from the base of the Chiltern scarp as seen near Whipsnade and Ivinghoe. The Westbury Formation (trad. Chalk Marl) is the lower unit overlain by the Zig Zag Formation (trad. Grey Chalk) and these two formations are separated by the Totternhoe Stone or Totternhoe Clunch Stone. This has been quarried at the village of Totternhoe, Bedfordshire, since the 13th Century as a building and ornamental stone. Examples of its use can be seen in St. Albans abbey gateway, the font in St. Stephens Church, St. Albans, and many other local churches.

The White Chalk (500 ft thick) is divided into a number of distinct formations, four of which are represented in the Hertfordshire Chilterns: – the Holywell Nodular Formation, the New Pit Formation, the Lewes Nodular Formation, and the Seaford Formation. No Chalk younger than the Seaford has been recorded in Hertfordshire.

Holywell Nodular Formation: this indurated (hardened) nodular chalk unit is flintless and is recorded as being approximately 45 feet thick across Hertfordshire. The base of the unit is formed by the Melbourne Rock, an extremely hard shelly unit.

New Pit Chalk: this is described as a “massively bedded, non-nodular chalk, with regularly developed marl seams and sporadic flints”. It is the presence of the marl seams which characterise this unit, as these argillaceous clay seams, frequently between 1 cm and 10 cm thick, are extremely widespread, being recognised across the whole of southern England and in a number of cases even across to Germany.

The Lewes Nodular Chalk: this unit comprises “a hard nodular chalk, with conspicuous regularly developed flints, thin marls and hardgrounds.” The hard nodular chalks occur as a series of condensed hardgrounds, principally the Chalk Rock and the slightly higher Top Rock, both of which occur along the top of the Chiltern escarpment. These beds are extremely hard and were often used as quarry floors in old quarry workings in the county. The hardness of the Chalk Rock unit where it occurs below the floor of the Colne Valley will provide a firm substrate into which piles can be driven, however in this area the Chalk Rock could be as deep as 40 – 50 metres below the surface.

The Lewes Formation has been described as “characteristically hard, nodular, locally iron stained and flinty. Marl seams, up to 0.1 m thick, occur throughout. Hardgrounds occur locally, and at least some of the thickness variation in the Lewes Chalk may be caused by condensed sequences or depositional breaks at these horizons. Layers of flints are regularly spaced throughout the succession. At some horizons these flints almost interlock to produce laterally continuous bands”.

Chalk stream valleys cutting through the Hertfordshire Chilterns may well be following old structural lines (faulting) which were exploited by erosion during the ice age. Research studies carried out by members of the Hertfordshire Geological Society, indicates that the “Tring Gap”, at the top of the Bulbourne valley, is most likely underlain by a series of northwest – southeast trending faults.

The Seaford Formation: The youngest chalk recognised in Hertfordshire is the Seaford Chalk Formation. This is the more typical white chalk with regular flint bands that most people would consider to be a ”normal” chalk sequence. “The Seaford Chalk is typically a soft, flinty chalk with local shell rich horizons” and “the base of the Seaford chalk is marked by the Upper East Cliff Marl.” This marl is a widespread clay seam recognised across much of southern England. The Seaford Formation comprises a uniform, very fine chalk with a relatively high microporosity, making it a major aquifer through southern England.

The Seaford Formation is very rarely exposed in the county and probably the best examples are at Anstey (NGR TL395329) and Codicote (NGR TL214171) quarries.

The White Chalk thickens south-eastwards from about 90 ft on the Chiltern crest near Tring to a maximum of 280 ft beneath south-east Hertfordshire (see cross section). This is because at the end of the Cretaceous Period the rocks were uplifted and tilted, and then eroded unevenly before the next sediments were laid down on the eroded surface in the Palaeocene Period about 66 million years ago. The junction of the Seaford Formation and the overlying Lambeth Group is clearly seen at Harefield Chalk pit (NGR TQ 049898) where an approximate 27 million time gap is represented by the intensely burrowed unconformity surface.

The Lambeth Group consists of the Upnor and Reading Formations. In the south-east of the county the underlying Thanet Formation appears but undifferentiated from the Lambeth Group. They are the main Palaeocene deposit in Hertfordshire, which consists of 30-50 ft of multicoloured clays with irregular patches of yellow sand and occasional bands of small (1-5 cm) black pebbles known as the Reading Beds. The Lambeth Group was deposited in a shallow sea deposit at the base, but upper parts were deposited by rivers flowing eastwards across the area. It occurs mainly in south-eastern parts of the county near Radlett and Essendon, but originally extended over the whole of the Chilterns. Patchy remnants or outliers of this earlier cover were left in north-western areas when most of it was eroded by rivers. Examples of Reading Beds outliers occur in northern parts of St. Albans, and at Ayot Green and Burnham Green. An interesting hard rock material sometimes found in the Reading Beds is the Hertfordshire Puddingstone , which was formed during the Palaeocene/Eocene Thermal Maximum (PETM) about 56.8 million years ago.

The London Clay was deposited after a cycle of uplift, tilting, erosion and then subsidence of the land beneath the sea. It overlies the Reading Beds in south-east Hertfordshire and on some of the outliers to the north-west. The London Clay also extends and thickens south-eastwards beneath London. It is a grey or brown clay often containing fossils of marine shells or land plants preserved in iron pyrites (FeS2), a brassy mineral sometimes known as fool’s gold.

The Red Crag, which is found mostly in Essex from sea level at Walton-on-the-Nase rising to about 80m OD on the Hertfordshire Border, was deposited around the Pliocene/Pleistocene boarder, 2.6 – 2.1million years ago. Evidence in Hertfordshire was found at Rothamsted (120m OD) in 1926 when blocks of sandstone were found with many Red Crag fossils. Excavation at Little Heath Pit SSSI (160 m OD) (NGR TL 01650825) of sand and gravel beds have been provisionally dated to the same time. This indicates another cycle of uplift in SE England occurred since 2.1 million years ago.

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