The geology of
The Common is underlaid by the Triassic Bunter pebble beds around 80 feet thick. These can be seen exposed in the Blackhill quarry and in the cliffs at Budleigh Salterton. The beds extend north into Somerset.
The pebbles are composed of quartzite grit and quartz, usually bedded in a dark red matrix of sand. Many of the paths on the Common are formed from the pebble beds, with the sand matrix being washed down valleys. The Common is mostly covered by a thin layer of soil over this, resulting in a shortage of rich vegetation and an unsuitability for farming.
Some of the quartzite pebbles contain Ordovician fossils, which indicates the origin of the pebble beds. Opinion is divided as to the location of the mountain range, from which these derive.
The rainfall of the area largely filters down into the pebble beds and follows the slope of these off towards the East. At Dotton there are a number of boreholes where this water is pumped up to supply much of East Devon.
Below the pebble beds are Permian marls which, when exposed, result in richer soils. This is readily apparent at the base of the escarpment to the West of Woodbury Castle, where farming restarts with several fields used for grazing.
The Budleigh Salterton Pebble Beds (BSPB) outcrop as a linear and patchy deposit, never more than four miles across, but traceable from Budleigh Salterton on the south coast to Minehead on the north coast. The BSPB forms a distinct lithological unit consisting of a 20-30 metres thick sequence of conglomerates and pebbly sandstones. They are of Lower Triassic age (235 million years) and are underlain by Permian Marls.
The deposit changes in character northwards from Uffculme. To the south metaquartzite cobbles and boulders are dominant. North of Uffculme the pebble beds are composed mainly of locally derived material, such as Devonian limestone and sandstone pebbles.
Blackhill Quarry is seventeen miles due south of Uffculme. Here the BSPB form a sequence, up to 31 metres thick, of well-rounded metaquartzite cobbles and boulders set in a coarse to fine gravel and silty sand matrix. Also present are cobbles and pebbles of schorl, vein quartz, porphyries and occasionally sandstone. Discrete lenses and sheets of sandstone also occur in the sequence. The cobbles and pebbles account for 80% of thre formation; the remaining 20% being the fine gravel, silty sand and sandstone.
The BSPB at Blackhill Quarry were deposited by fairly straight, fast-flowing, relatively deep, braided gravel bed streams. Evidence for this can be found in the extreme coarseness of the deposit and the absence of separate silty sand and mudstone beds is accasionally present and is characteristic of braid plain bars. These are formed when the amount of material being transported exceeds the river's carrying capacity and is deposited. Material is deposited on the dowstream face of the bar forming the cross-stratification.
The well-rounded nature of the cobbles and pebbles indicates prolonged abrasion which could not have occurred during the deposition of this formation. Therefore, the BSPB must be the product of frequently reworked coarse conglomerates, the BSPB being the final phase of erosion and deposition.
The predominance of metaquartzite pebbles, paleocurrent evidence and the isotopic compostion of detailed micas strongly indicate a southerly source for the material. Almost certainly the Brittany-English Channel (Armorican) area.