Upper Greensand - Mid-Cretaceous Building Stone (Lead Author WGT)
The Upper Greensand Formation (Selborne Group) is mainly Late Albian to earliest Cenomanian in age, embracing the boundary between Lower and Upper Cretaceous (hence “Mid-Cretaceous” here). It is a mixture of fine to medium quartz sand, bioclastic sand and green glauconite grains deposited in a shallow sea during some 7 million years (c.105-98my). Fossil bivalves and tube worms are common, as are burrows (trace fossils). The Upper Greensand (UGS) has long been quarried for building stone in two main areas (a) NW Dorset: Shaftesbury to Okeford Fitzpaine, (b) SW Dorset - SE Devon.
SW Dorset - SE Devon (max 60m thick)
In this area, the UGS is divided into three Members, from base upwards: Foxmould, Whitecliff Chert, Bindon Sandstone. Most building stone was extracted in the Branscombe area of SE Devon but used extensively in SE Devon (e.g. Exeter Cathedral) and West Dorset. In addition, the orange-brown siliceous chert (qv) was used for walling, knapped and unknapped, much like the grey-black flint (qv) from the Chalk.The Eggardon Grit, a bioclastic limestone at the very top of the succession, is a thin local unit of the Bindon Sandstone occasionally used in walls near outcrops but also burnt for lime near Abbotsbury.
Text by WGT & JT. Photos by JT Feb 2017
NW Dorset (max 60m thick)
In this area, the Upper Greensand is divided into four Members, from base upwards: Cann Sand, Shaftesbury Sandstone, Boyne Hollow Chert, Melbury Sandstone. The Shaftesbury Sandstone, hard and well-cemented by calcite, has been used for building around Shaftesbury and down the Stour valley as far as Blandford and Crawford Bridge, where the Tarrant joins the Stour. It was particularly useful for bridges, or the foundations of water mills, as well as the foundations of domestic buildings and churches. The UGS has been quarried recently on Manor Farm, Melbury Abbas, for new buildings in Shaftesbury. Melbury Sandstone, albeit younger, is virtually identical to Shaftesbury Sandstone.