Wurt Pit and Devil's Punchbowl
Encyclopedia
Wurt Pit and Devil's Punchbowl is a 0.2 hectare
(0.5 acre) geological Site of Special Scientific Interest between East Harptree
and the Priddy Circles
in the Mendip Hills
, Somerset
, notified in 1987.
This site consists of the two largest subsidence depressions in the Mendips, formed by the underlying limestones having been dissolved by subterranean waters causing the surface rocks to collapse into the void. Clear evidence of their being collapsed structures rather than erosional or solutional features, is gained from the fact that the surface rocks at both localities are insoluble, being marl
s at Devil's Punchbowl and a series of limestones and clays which have been impregnated by silica at Wurt Pit. The silica-enrichment of the limestones and clays at Wurt Pit (known as the "Harptree Beds", of early Jurassic
age) is also of considerable mineralogical importance since it took place as part of the main phase of mineralisation which emplaced the principal Mendip orefields during Jurassic times. The Harptree Beds show varying degrees of silica-enrichment, and also contain traces of other minerals, such as limonite
and yellow ochre (hydrous ferric oxides), barite
(barium sulphate), sphalerite
(zinc sulphide) and galena
(lead sulphide).
Wade and Wade, in their 1929 book Somerset, described the Devil's Punch Bowl as one of the most notable Swallet Holes on the Mendips
Hectare
The hectare is a metric unit of area defined as 10,000 square metres , and primarily used in the measurement of land. In 1795, when the metric system was introduced, the are was defined as being 100 square metres and the hectare was thus 100 ares or 1/100 km2...
(0.5 acre) geological Site of Special Scientific Interest between East Harptree
East Harptree
East Harptree is a village and civil parish in Somerset, England. It is situated north of Wells and south of Bristol, on the northern slope of the Mendip Hills overlooking the Chew Valley. The parish has a population of 680...
and the Priddy Circles
Priddy Circles
Priddy Circles are a linear arrangement of four circular earthwork enclosures near the village of Priddy on the Mendip Hills in Somerset, England. The circles have been listed as Scheduled Ancient Monuments, and described as 'probable Neolithic ritual or ceremonial monuments similar to a...
in the Mendip Hills
Mendip Hills
The Mendip Hills is a range of limestone hills to the south of Bristol and Bath in Somerset, England. Running east to west between Weston-super-Mare and Frome, the hills overlook the Somerset Levels to the south and the Avon Valley to the north...
, Somerset
Somerset
The ceremonial and non-metropolitan county of Somerset in South West England borders Bristol and Gloucestershire to the north, Wiltshire to the east, Dorset to the south-east, and Devon to the south-west. It is partly bounded to the north and west by the Bristol Channel and the estuary of the...
, notified in 1987.
This site consists of the two largest subsidence depressions in the Mendips, formed by the underlying limestones having been dissolved by subterranean waters causing the surface rocks to collapse into the void. Clear evidence of their being collapsed structures rather than erosional or solutional features, is gained from the fact that the surface rocks at both localities are insoluble, being marl
Marl
Marl or marlstone is a calcium carbonate or lime-rich mud or mudstone which contains variable amounts of clays and aragonite. Marl was originally an old term loosely applied to a variety of materials, most of which occur as loose, earthy deposits consisting chiefly of an intimate mixture of clay...
s at Devil's Punchbowl and a series of limestones and clays which have been impregnated by silica at Wurt Pit. The silica-enrichment of the limestones and clays at Wurt Pit (known as the "Harptree Beds", of early Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
age) is also of considerable mineralogical importance since it took place as part of the main phase of mineralisation which emplaced the principal Mendip orefields during Jurassic times. The Harptree Beds show varying degrees of silica-enrichment, and also contain traces of other minerals, such as limonite
Limonite
Limonite is an ore consisting in a mixture of hydrated iron oxide-hydroxide of varying composition. The generic formula is frequently written as FeO·nH2O, although this is not entirely accurate as limonite often contains a varying amount of oxide compared to hydroxide.Together with hematite, it has...
and yellow ochre (hydrous ferric oxides), barite
Barite
Baryte, or barite, is a mineral consisting of barium sulfate. The baryte group consists of baryte, celestine, anglesite and anhydrite. Baryte itself is generally white or colorless, and is the main source of barium...
(barium sulphate), sphalerite
Sphalerite
Sphalerite is a mineral that is the chief ore of zinc. It consists largely of zinc sulfide in crystalline form but almost always contains variable iron. When iron content is high it is an opaque black variety, marmatite. It is usually found in association with galena, pyrite, and other sulfides...
(zinc sulphide) and galena
Galena
Galena is the natural mineral form of lead sulfide. It is the most important lead ore mineral.Galena is one of the most abundant and widely distributed sulfide minerals. It crystallizes in the cubic crystal system often showing octahedral forms...
(lead sulphide).
Wade and Wade, in their 1929 book Somerset, described the Devil's Punch Bowl as one of the most notable Swallet Holes on the Mendips