Bedfordshire and Luton geology

Overview

Like the adjacent counties of Hertfordshire and Buckinghamshire, the geology of Bedfordshire is relatively simple and is reflected in observable changes in the landscape from north to south. Bedfordshire is located on the northern edge of the London Basin, with the oldest rocks being found in the north of the county and progressively younger rocks to the south.

Jurassic

The oldest rocks in the county belong to the Cornbrash, a Middle Jurassic (170 million years old) limestone, which often has an orange colour due to iron staining. It was formed from sediments deposited in a shallow tropical sea, as can be seen by the wide variety of fossil sea creatures (e.g. ammonites and bivalves) that the rock contains.

Across the centre of the County the overlying Upper Jurassic Oxford and Kimmeridge Clays form the broad sweeping claylands plateau on which Bedford itself is located. The plateau is dissected by a number of shallow valleys, including the rivers Great Ouse and Ivel. To the east there is a distinct boundary between the gently undulating lowland landscape of the claylands and the level fenlands of Cambridgeshire. The Oxford Clay formed at the bottom of a shallow Jurassic sea about 160 million years ago. The Clay has been extracted for the brick making industry, particularly in the Marston Vale area, where the many former pits, lakes and brickwork chimneys attest to the extensive nature of the industry.

The outcrop of the succeeding Ampthill Clay marks a transition in the landscape between the southern edge of the clay plateau and a narrow, but raised ridge, known as the Bedfordshire Greensand Ridge. The Ampthill Clay was deposited about 155 million years ago and is named after the town of Ampthill where the clay was first discovered and described.

Cretaceous

The Bedfordshire Greensand Ridge, which runs between Woburn and Potton is formed from the outcrop of the Greensand. The Greensand, as its name suggests, comprise a series of sands, often stained green (due to the presence of the mineral glauconite), yellow, orange or brown as a result of staining by iron oxide. In the east of Bedfordshire these sands are known as the Potton Sands and further west, towards Buckinghamshire, as the Woburn Sands. The sands were formed at the bottom of a shallow sea during the Lower Cretaceous period 120 million years ago. Beds of decomposed volcanic ash, known locally as “Fullers’ Earth“, are found at some locations in the Greensand succession. The distinctive geological character of the Greensand Ridge is also reflected by the habitats and vegetation that it supports. In contrast to the surrounding clay vales, the acidic soils of the ridge support areas of heathland and mixed woodland.

Resting on top and to the south of the Greensand Ridge is the Gault Clay. Laid down about 100 million years ago the Gault Clay weathers to reveal a diverse and rich fauna of marine animals such as ammonites, bivalves and fish. The clay has formerly been used for the manufacture of pipes and tiles.

Cretaceous Chalk rises to the south of the Gault Clay Vale to form the high ground around Dunstable and Luton at the north-eastern extremity of the Chiltern Hills and marks the establishment of a shallow, tropical sea that covered Britain up until the end of the Cretaceous. Towards the base of the Chalk is the Totternhoe Stone, which, on account of its great hardness, usually stands out as a well-marked feature.

Quaternary

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 left large amounts of glacial till or boulder clay, sands and gravels over much of Bedfordshire. The Greensand, being made up of more resistant sands, was not eroded as much as the surrounding softer clays and when the ice retreated, the Greensand Ridge was left as a prominent feature on the landscape. Gravel deposits laid down in the valley of the River Ouse during the Ice Age have yielded many interesting mammal fossils including rhinoceros and mammoth and have also provided evidence of occupation by early man in the form of flint tools such as axe heads.

Geological Highlights:

  • The Oxford Clay brickpits in the Marston Vale area have, over the years, produced the remains of many marine reptiles that inhabited the shallow tropical seas of the Middle Jurassic. These include Pliosaurs, such as Liopleurodon, which at up to 20m in length was the largest carnivore that swam in the Jurassic seas. Two types of crocodile have been recorded from the Oxford Clay brickpits and a number of Ichthyosaurs, marine-going reptiles, which looked remarkably like modern-day dolphins, Specimens of many of the reptiles found from the brickpits can be seen at Bedford Museum.
  • Phosphate-rich nodules at the base of the Greensand used to be quarried around Leighton Buzzard, as a source of phosphate for agricultural fertiliser, which was once a flourishing industry in the area.
  • The layers of Fuller’s Earth found in the Greensand are the product of a volcanic explosion when ash produced by volcanic eruptions settled in the shallow waters of the sea that covered what is now central and southern England. Originally, this material was quarried and used to extract the grease from sheep’s wool, a process called fulling, hence the name. Fuller’s Earth is now used for a variety of chemical processes.
  • Totternhoe Stone, at the base of the Chalk, 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.

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.

Totternhoe Stone Pit (SSSI)

Greensand Ridge Walk