Woodbury Common is a heathland area comprising of a number of adjacent commons, including Dalditch, Withycombe, Lympstone, East Budleigh, Bicton, Woodbury, Colaton Raleigh, Hawkerland, Aylesbeare and Harpford. With the exception of Withycombe and Lympstone Commons all the commons are owned by Clinton Devon Estates and mostly managed by the Pebblebed Heaths Conservation Trust. Aylesbeare and Harpford are managed for Clinton Devon Estates by the RSPB. Withycombe and Lympstone are owned by East Devon District Council and Squabmoor Farm Ltd respectively.
Figure 1) Map showing the various different commons that form Woodbury Common
These pebblebed heaths lie on a 20-30m thick bed of conglomerates and gravelly sandstones called the Budleigh Salterton Pebble Beds (BSPB) and it is where the pebblebed heaths get their name. This forms the basis of an important aquifer, meaning a permeable rock that can contain and transfer groundwater, for the area. It provides water for many villages and towns including Budleigh Salterton, East Budleigh, Otterton, Colaton Raleigh and Newton Poppleford.
The BSPB and the overlaying Otter Sandstone forms one of the most important aquifers in south west Enland. The water is transferred through micro and macro fissures within the Otter Sandstone as well as within the largely free draining matrix that makes up the BSPB. This is particularly important in the Otter Valley where much of the groundwater passes through these two geological units to transfer to the River Otter. To supply this transported water a large number of boreholes have been drilled into the aquifers. In 1997 the abstraction from the Otter Sandstone and the BSPB totalled around 25 000 m3 a day. The importance to these boreholes, and the aquifers, in the local water extraction is highlighted by the fact that of the licensed water extractions south of Broadhembury, 80% are groundwater and only 20% are from surface water sources.
The social importance of the BSPB and the characteristic river rounded pebbles is highlighted in the name of nearby Newton Poppleford. The word "popple" is a Westcountry term used to describe a stone worn smooth, and more or less round, by the action of either the sea or a river. The town was built at a ford over the River Otter that contains the pebbles of the BSPB and the name means new town built on a pebbly ford. The pebbles are, and have historically, been used as a local building resource and there are numerous "popplestone" walls and buildings in the surrounding villages and towns.
Figure 2) A pebbelstone cottage from Sidmouth
Figure 3) A pebblestone wall from Sidmouth
The pebblebeds have also been historically important for the area as it has been quarried to supply high quality quartzite sand and gravel for contruction. The only quarrying operation currently occuring on the pebblebed heaths is at Blackhill Quarry, which is a 64 hectare sites adjacent to Bicton Common. The quarry opened in the 1930's by the Blackhill Stone Company Ltd and is now operated by Aggregate Industries UK Limited. Extraction at the site ceased in April 2011 with the exhaustion of the permitted sand and gravel reserves, although there is some process of imported sand and gravel from nearby Venn Ottery and Marshbroadmoor. The plan for the quarry area is to be returned to natural heathland, which is currently underway.
The pebblebed healths have a long history of occupation and therefore have a wealth of archaeological remains, with over 168 features noted in the County Council's Historic Environment Record. They range from the prehistoric remains of a large number of barrows and cairns (burial mounds) to the WWII military installations. The most important sites is the designated Scheduled Monument of the Iron Age hillfort of Woodbury Castle.
Figure 4) Map showing the geology of the area surrounding Woodbury Common
The BSPB form an outcrop as a linear and patchy deposit, never more than four miles across, and underlie the pebblebed heaths, giving them their name. The BSPB are traceable from from Budleigh Salterton on the south coast to Minehead on the north coast. This sequence of conglomerates and pebbly sandstone forms a distinct lithological unit, over 30m deep in places. The rocks forming the pebbles were created 440 million years ago. The pebbles were shaped during the Lower Triassic Age (235 million years ago) and is underlain by Permian Marls. The pebbles were formed and deposited by riverine erosion in a mountainous arid landscape, at a time when the world's continents were joined as the super-continent 'Pangaea'. The beds were partially overlain by sandstones and mudstones, with the former comprising of wind-blown and river deposits, and the later finer-grained material laid down in temporary lakes. The BSPB have been susequently exposed by later erosion of the overlying rocks. As a result of the pebble beds, Woodbury Common is largely freely draining, very acidic, sandy and loamy soils of low fertility that continue to maintain the heath environment seen today.
Approximately 90% of the pebbles, cobbles and boulders in the BSPB consist of well-rounded metaquartzite up to 0.45m in diametre. Metaquartzite is formed when sandstone has undergone intense heat under relatively low pressure and the grains of silica have fused together to create a rock that is more than 90% quartz and has a pale, sugary, fine to medium grain appearance. The rest consists of schorl, vein quartz, porphyry and occasional sandstone pebbles. Schorl is a crystalline aggregate of quartz and tourmaline, which is a crystalline mineral commonly used as a non precious gemstone. Vein quartz is quartz that has formed in 'veins' or sheets within the cracks of other rocks. Porphyry is a term used to describe an igneous rock that consists of large grained crystals such as feldspar or quartz within in fine grained silicate rich matrix. The pebbles occur in a matrix of about 65% course to fine gravel, 20% to 30% sand, and 5% to 12% silt and clay. The BSPB can be seen at many points across the commons, especially in areas with high footfall, such as on the paths like waypoint one.
To log this EarthCache please email me or message through my profile the answer to these questions
Go to the first waypoint and you will see a path with a good exposure of both the pebbles and soil matrix of the Budleigh Salterton Pebble Beds
1) Look at the pebbles and estimate the dimensions of the largest stone you can find.
2) Estimate the average size of the pebbles.
Go to posted coordinates where you will find a stone monument
3) What is the name of this stone monument and is it made out of the local geology?
4) When was this monument donated and for what reason?
5) From the monument if you look in a roughly south westerly direction what human activity can you see?