Oolite and Cornbrash

Jurassic

The rocks in Bedfordshire tell an amazing story - a story that starts around 190million years ago with the oldest rocks in the northwest of the county called the Inferior Oolite and Upper Lias.

The Upper Lias is the oldest rock of all in Bedfordshire, a series of silty muds and muddy limestones that were laid down in the sea around 190 million years ago. This sea was warm and teemed with life including abundant ammonites. Uplift and subsequent erosion means there is a gap between this and the overlying Inferior Oolite, so these rocks were deposited up to around 175 million years ago.

The little oolites are tiny balls of calcium that tell us they formed in a warm, shallow (c. 5 to 10m or so) sea. Waves rolled particles backwards and forwards on the sea floor, giving them an even coating of calcite. Oolitic limestones are very good building stones, although the Inferior Oolite is a bit too muddy for this purpose (mud means a poorly cemented limestone). Both these ancient formations lie at the surface on either side of the River Ouse, but in very patchy, tiny areas. This makes finding them difficult to predict.

The Great Oolite is a much harder rock, again an oolitic limestone formed in warm, shallow waters, but containing far less clay. Ammonites, sea urchins and bivalves are common fossils in this rock. This rock can be found lying at the surface in a thin strip of land on the valley sides of the River Ouse between Bromham, Oakley, Pavenham and Milton Ernest and also along the section starting at Harrold to Chellington southwards. The Great Oolite is a good building stone and the buildings of Harrold are excellent examples of its use.

The Cornbrash is a sequence of thin beds of rubbly limestone containing lots of completely smashed fossil shells, the result of strong wave action near a shoreline 170 million years ago. In Roman times it was occasionally used as roofing material, but it is not a good building stone. It does make a fertile, welldrained soil (hence the name Cornbrash; ‘a site where good corn is grown’.)