Geological Maps of Bedfordshire

Geological maps show the nature, extent and relative stratigraphical age of the different rocks and superficial deposits in a given area. They are based primarily on surface information, but may incorporate subsurface data derived from temporary excavations, boreholes and geophysical analyses. Geological maps are typically produced in a variety of formats.

BEDROCK MAP

Previously known as a ‘Solid’ map, this map shows how the rocks would appear if the superficial deposits were removed. In other words, they show only those rocks that predate the usually unconsolidated glacial and post-glacial deposits that are less than about two million years old.

The map key shows all the rock units represented on the map and indicates their relative stratigraphic age. The succession is not complete, either because the rocks were not deposited locally or were deposited and have since been eroded. These time gaps, known as unconformities, are indicated by the uncoloured parts of the key above the wavy lines.

The approximate ages of the rock units are shown in millions of years ago (Ma) in the left hand column of the key, along with the corresponding periods of geological time. It is evident that Bedfordshire’s bedrock is dominated by Jurassic and Cretaceous strata of sedimentary origin.

In the right hand column of the key the basic mapped unit, known as a formation, is shown. These are typically named after places where similar rocks are best exposed (e.g. Oxford Clay and Ampthill Clay), although sometimes the name reflects the predominant rock type in that particular formation (e.g. Great Oolite and Middle Chalk). Indicative thicknesses are also provided for each formation.

Notice that the key is not drawn to scale and be aware that some of the detail that can be shown on a larger scale map has been omitted here. For example, it is common to subdivide formations into formal or informal units known as members and beds, particularly where they are easily identified in the field. This is rarely possible in the poorly exposed clay vales of Bedfordshire but it has been achieved with some success in the Upper Cretaceous.

The Chalk stratigraphy has been simplified for clarity, but several hard bands are used to help subdivide the succession. They are the Totternhoe Stone (T) within the Lower Chalk, the Melbourn Rock (M) at the base of the Middle Chalk, and the Chalk Rock (C) at the base of the Upper Chalk.

SUPERFICIAL MAP

Previously known as a ‘Drift’ map, this map shows the distribution of glacial and post-glacial deposits that formed during the Quaternary, the period from about two million years ago to the present day that is characterised by alternating cycles of cold and temperate climate. These maps may also show areas where the landscape has been modified by natural processes such as landslips, and artificial processes such as gravel extraction, quarrying or land restoration.

The superficial Quaternary deposits shown in the map key are not necessarily listed in stratigraphic order and their relationships are better shown in the schematic cross-sections. Uncoloured areas on the map are where the superficial deposits are thin and/or not mapped in detail.

By combining both types of map a complete picture of the geology is achieved. The resultant Bedrock & Superficial Deposits map is therefore the single most useful map to have, but they are usually complicated to reproduce and difficult to read. For the purpose of this website we have deliberately chosen to keep the two expressions of Bedfordshire’s geology separate.