Dorset is world-famous for the dry stone walls of Purbeck Limestone on the Isle of Purbeck and along the South Dorset Ridgeway. However, the rich geodiversity of the county provides a wide range of local walling stones. These are listed below, from the geologically oldest, to the youngest.
The Blue Lias Formation consists of alternating layers of limestone and mudstone. Blue Lias limestone was historically quarried in south-west Dorset from the cliff faces and foreshore at Lyme Regis and Charmouth. Also, from inland quarries in south-east Devon at Uplyme. Currently, it is available from quarries in south Somerset around Glastonbury and the Polden Hills.
The blue-grey colour is due to fine particles of pyrite (iron sulphide) within the stone. Some beds of Blue Lias, with high clay and pyrite content, are prone to weathering, in part due to oxidation of the pyrite to ochre minerals. This problem is exacerbated by the use of impermeable cement-based mortars which result in increased water-penetration and frost-damage to the stone surface.
Dry stone construction without mortar, or the traditional use of permeable lime-based mortars, allows Blue Lias stonework to naturally dry-out, and makes it less prone to weathering.
The Thorncombe Sand exposed in the cliffs near Eype Mouth shows three lines of ‘doggers’, rounded boulders of sandstone naturally cemented with calcium carbonate (calcrete). These doggers are also visible in sunken lanes around Symondsbury, where they were quarried and used locally in buildings and walls.
The Beacon Limestone Formation is exposed on the cliff-face of Thorncombe Beacon, near Eype Mouth. It is also known as the Junction Bed as it occurs at the junction between the Middle and Upper Lias. It is visible in sunken lanes around Symondsbury and was used in some rubble walling. It was also quarried and used in north Dorset, around Nether Compton, Trent and Sandford Orcas. In south Somerset, many old buildings in Yeovil are constructed of Beacon Limestone, locally known as Yeovil Stone.
The Bridport Sand Formation contains distinctive resilient calcium carbonate-cemented bands exposed in the cliffs to the east of West Bay and sunken lanes to the north of the county. The freshly exposed stone has a bluish colour due to the presence of iron minerals, though the surface oxidises to give the characteristic ochre colour.
Blocks of this calcium carbonate-cemented sandstone are used for harbour buildings and walls at West Bay and the stone is locally used in rubble walls, elsewhere in the County.
Ham Hill Limestone
The Ham Hill Limestone Member is a very localised limestone deposit within the Bridport Sand Formation, only found on the Ham Hill plateau near Montacute in Somerset.
Ham Hill Limestone is a fine grained freestone that takes detailed carving. It is used as an architectural stone throughout the region. Many Dorset churches are largely built from local stone but have carved doorways and windows of Ham Hill Limestone.
The Inferior Oolite is exposed on the foreshore at West Bay and at the top of the cliff at Burton Bradstock.
Inland, it was quarried as Burton Limestone along its outcrop from Burton Bradstock, through Bridport, Loders, Mapperton and Beaminster to Broadwindsor.
To the north around Nether Compton, Sherborne and Oborne the inferior Oolite forms a thick deposit of building stone, Sherborne Limestone. Further into Somerset, Inferior Oolite is still quarried as Hadspen Stone, near Castle Carey, and Doulting Stone, near Shepton Mallet.
Fuller’s Earth Rock
In north Dorset, the clay of the Fuller’s Earth Formation contains a limestone member, the Fuller’s Earth Rock. This forms a scarp to the south and east of Sherborne and is used locally as a building stone and in rubble walls.
The lower part of the Upper Greensand Formation comprises the relatively unconsolidated sands of the Foxmould Member. However, the Foxmould does contain some large rounded calcrete doggers, locally known as Cowstones. These have had some use as building stones and make up the core of the Cobb harbour wall at Lyme Regis