Geology of Gloucestershire - The Cotswolds

The Cotswolds

The Middle-Jurassic oolitic limestone series which forms the bulk of the Cotswolds hills contains the best-known of the county’s rocks on account of its extensive use throughout the area as a building stone. All from churches to humble cottages have been imbued with a mellow, warm character from the golden yellow colour of the stone - to which many ‘honey-pot’ Cotswold towns owe their modern popularity and prosperity. The series is divided into the Inferior and Great Oolite Groups. The Inferior Oolite, capping the main north-west facing escarpment, comprises up to 100m of mainly oolitic limestones including the Cheltenham freestone - quarried most extensively at Leckhampton and used widely in the distinctive Regency architecture of Cheltenham. The Great Oolite, forming much of the rather barren Cotswold plateau, encompasses a variety of oolitic, shelly, sandy or marly limestone beds, many of which are used for building purposes such as dry-stone walling and roof tiles.

The limestones were predominantly laid down in a shallow warm continental sea, similar to that off the present-day Bahamas, and are rich in fossils of hard-shelled sea creatures such as brachiopods and sea urchins. Clay bands (e.g. the Fuller's Earth Clay at the base of the Great Oolite Series) represent deeper water periods. As the Cotswold dip-slope is followed toward the south-eastern county extremities these clays and mudstones begin to predominate as the Upper Jurassic period Oxford Clay.

Most of the Cotswold rivers flow south-eastwards down the dip-slope, supplying the river Thames, whose headwaters lie at the bottom of the slope on the Upper Jurassic clays. Terrace deposits up to 6m thick of mainly local oolitic limestone gravels have been left by the river Thames in its upper course during glacial and peri-glacial periods. Downcutting and a gradual southerly migration of the river has left a succession of four terraces at varying heights, chiefly north of its present course.