Heathstone, Carstone, Ironstone (Lead author JT)
The dark brown sandstones used for buildings in a wide area of south and east Dorset are colloquially known as Heathstone, Carstone or Ironstone, and are cemented into a strong building stone by their iron mineral content. They all come from local sand and clay sequences that were deposited on top of an eroded surface of the Chalk.
The earliest Reading Beds and London Clay Formations have been interpreted to have deposited during a marine transgression, inundating river-flat and near-shore environments. The best quality sandstone consists of rounded grains of quartz sand that suggest beach conditions, and there are several pebble beds with rounded pebbles similar to those on Chesil Beach. Large boulders are found at the base of the sand in the West Park Farm Member, often with pebbles and gastropod fossils on the top surface. The Lytchett Matravers Sand occurs higher in the London Clay. This is a very fine to medium grained (‹0.5 mm) ferruginously cemented sandstone, well sorted and rounded, which when used as ashlar can be seen to be current bedded.
There are pebble beds at the base of the Reading Beds, and at the base of the London Clay. Outliers of these rounded pebbles in a matrix of loose sand are scattered over the Chalk Downland. The loose pebbles, reminiscent of the pebbles on Chesil Beach and between 5 and 10 cm on the long axis, have been used for roadstone. On the Edmondsham estate the height of Castle Hill has a deep pit in the pebble bed that contains patches of iron-cemented sand creating a conglomerate with the pebbles. Some of these conglomerate blocks are used as boundary markers, or in old rubble walls.
There are two iron-rich sandstones in the Barton Beds, the Hengistbury ironstone nodules and the smaller nodules from Bed G of the Barton Beds at Barton cliff. Both of these have very fine-grained iron rich sand. The Hengistbury nodules are large boulders formed as if in aggregating layers. The Bed G at Barton contains many small white gastropod or bivalve fossils.
The overlying Poole Formation is a series of sands and clays that formed in river systems or estuaries. The sandstone from this Formation tends to be iron-cemented ‘sharp sand’ (angular grains). I have not identified any buildings using the Branksome Sand, though there is a question about some ‘sharp sand’ sandstone in Christchurch, which could just as likely be Poole Formation. In a building they are indistinguishable.
All the heathstones are found in lenses, or small patches, rather than in beds which can be followed across country. For this reason, there are few excavations or quarries specifically for heathstone. This stone was collected for building from isolated boulders, from exposures in sand or clay pits, or occasionally from small ‘delves’ (delph or delve = a small excavation). This is in sharp contrast to the building sandstones in other parts of England, where continuous beds of well-cemented sandstone have been extensively quarried from thick deposits. JT Feb 2017
See Thomas, J. 2016: Note on the use of Tertiary Heathstone in buildings over the area of outcrop of the Palaeogene in the Isle of Purbeck, South-central and East Dorset. Proceedings of the Dorset Natural History & Archaeological Society. Vol. 137.
Barton Bed G
The images below show two different views of stone bed G from the Barton Beds
Poole Formation Heathstone
The Poole Formation sandstones are variable, these are examples from Studland.