Natural England - Hertfordshire


The rolling chalk hills of the Chilterns characterise the Hertfordshire landscape, the gentle slope running towards Welwyn contrasting with the much steeper and dramatic western facing slopes between Tring and Hitchin.

The shallower slope towards St Albans and Watford harbours a diverse landscape with a series of broad valleys containing the rivers Ver, Colne and Lea. The valley patterns and soil types of the Chiltern Hills and Vale of St. Albans are all strongly influenced by the geological history of the area.

The underlying geology of Hertfordshire is relatively simple, with the rocks forming part of the shallow London Basin, the beds tilted in a south-easterly direction. The most important rocks are the Cretaceous Chalk, which forms the high ground of the Chilterns in the north and west, and the Tertiary Reading Beds and London Clay which occupy the remaining southern part of the county.


The oldest rock exposed in Hertfordshire is the Gault Clay (120 million years old) that forms an area of low-lying land at the foot of the Chilterns, but only occurs in two small areas near Tring and Ashwell. This distinctive blue-grey clay was formed from fine mud transported by rivers and deposited in a shallow tropical sea. Later in the Cretaceous, as sea-levels rose, inputs from these rivers largely ceased and the almost pure limestone of the Chalk was deposited (between 70 and 100 million years ago).

The Chalk is divided into two units (the Grey Chalk and the White Chalk). A distinct bench at the base of the Chilterns near Whipsnade and Ivinghoe is formed by the Chalk Marl or Totternhoe Stone (within the Grey Chalk), which has been quarried as a building stone. The steepest part of the western face of the Chilterns (e.g. at Whipsnade) is formed in the softer overlying White Chalk. Much of the White Chalk contains layers of flint, which has been used as building stone, mainly for facing external walls of churches and houses throughout Hertfordshire. At the end of the Cretaceous (about 65 million years ago) the rocks were uplifted, tilted and eroded unevenly before further sediments were laid down on this surface during the Tertiary.

The Chalk escarpment of the Chilterns supports important areas of grassland vegetation noted for its wide range of plants including several species of orchids and beech woodland which, on the steep slopes of the valley sides, forms 'cathedral-like' hanger woods.

Tertiary (comprising Neogene and Palaeogene)

Tertiary sediments (65-2 million years old) form the central core to the London Basin and occur in Hertfordshire in a north-east to south-west trending swathe of land between Bishop’s Stortford and Watford. Here, the oldest Tertiary sediments are the Reading Beds, a mix of multicoloured clays, yellow sands and rounded pebbles which were laid down in an environment that shallowed from sea to river as the land was gradually uplifted during this time. The sediments of the Reading Beds are frequently hardened into masses made up of pebbles known locally as Hertfordshire Puddingstone.

Overlying the reading Beds in south-east Hertfordshire, the London Clay (30-50 million years old) represents the fine mud transported into a shallow semi-tropical sea that then covered the county. The London Clay extends and thickens south-eastwards beneath London and contains the fossils of marine shells and land plants. Where the London Clay does come to the surface it gives rise to heavy, slightly acidic soils which support significant areas of oak and hornbeam woodland.


Over the last two million years the climate of Britain has varied tremendously with periods of temperate climate interrupted by repeated advances and retreats of glaciers and ice sheets. Collectively these periods have become known as the Ice Age (we are still in one of the temperate phases) and the actions of the ice sheets have been instrumental in forming the landscape we see today.

The last ice sheet (the Anglian Glaciation), some 400,000 years ago, eroded much of the Chalk and overlying sediments and deposited large amounts of glacial till or boulder clay over much of the north-eastern half of Hertfordshire. The main part of this region was also shaped by the forerunner of the River Thames system, which was pushed southwards to its present course by glaciers during the Anglian Glaciation. This river system deposited large amounts of gravel, which now overlie the London Clay and Reading Beds. Other Ice Age influenced deposits include Clay-with-Flints (thought to be the weathered remains of the Reading Beds), which occurs on the higher parts of the Chilterns in the south-western part of the County, and the fine wind blown silt known as loess. This sediment was deposited in thin layers over the whole county and where it is thickest, in valleys such as the Lea south of Ware, it provides some of the most productive agricultural land in Hertfordshire.

Geological Highlights:

  • The Chalk Marl, or Totternhoe Stone, at the base of the Chalk succession, has been quarried in this region since the 13th Century as a building and ornamental stone. Examples of its use can be seen at St. Albans abbey gateway, the font in St. Stephens Church, St. Albans, and many other local churches. The Chalk is now quarried at one location for the production of lime.

  • The Reading beds and London clay have been exploited for brick making at Watford, Hertford and Hatfield. The scale of working for brick clay is now relatively small, the only extraction site being located in the west of the county.

  • In areas where the Chalk underlies a thin sequence of Reading Beds, slightly acidic water travelling through the Reading Beds has enlarged fissures in the Chalk leading to the production of small cave systems known as swallow holes. During the winter, large volumes of water may flow down these swallow holes through a network of subterranean channels to reappear in riverbeds, often many miles away. Examples of these features occur at Waterend and Northaw Great Wood.

  • The Hertfordshire Puddingstone has featured in the folklore of the County for many centuries and even lends its name to a local magazine for folk music! A giant block (megalith) of puddingstone, which may have marked a prehistoric burial site, has been set up at Standon and forms the focus of the May Day festivals every year.

  • During the Ice Age (the last 2 million years), ice sheets advanced across the County from the north-east. During the main Anglian glaciation, some 400,000 years ago, large volumes of gravels and clays were deposited, including material derived from many miles away (so-called erratics). One large example of an erratic is a large boulder of Upper Carboniferous Millstone Grit carried from North Yorkshire. This rock can be seen in Royston town centre where it has been mounted on a plinth.

  • Prior to the main ice-sheet advance, the original course of what is now the Thames (known as the proto-Thames) flowed through the Vale of St. Albans and entered the North Sea via the East Anglian coast (Norfolk-Suffolk). The advancing ice-sheet blocked this river valley, diverting the river into its present course. During this period large glacial lakes developed at Wheathampstead and St Albans.

  • The gravels deposited by the extensive proto-Thames river system have been exploited for some considerable time as a major source of building materials. The gravel pits produced by extraction are a prominent feature in the southern part of the County and, where flooded following the cessation of production, now provide important wetland habitat, particularly for waterfowl.

Local sites

The following localities represent, in part, the geology of this county. Each locality has a grid reference, a brief description of how to get there and a short summary of the geology you are likely to find. All the localities listed are openly accessible.