Mendip Hills, Somerset

The landscape

The Mendip Hills, situated about 15 kilometres south of Bristol, is a ridge that rises abruptly from the flat Somerset Levels, to a bare plateau that reaches over 300 m above sea level in some places (e.g. around Priddy and Charterhouse). The landscape contains a variety of natural habitats including limestone pastures, ancient woodland and gorge cliffs where rare plants survive because of their inaccessibility to grazing animals. This is a landscape that has been long colonised by people and evidence of Bronze Age, Iron Age, Roman and medieval occupation have been found on the hills.

The Mendip Hills is made of massive Carboniferous limestones with Devonian sandstone and shales on its flanks. A number of karst features such as sinkholes and dolines are scattered over the plateau; and dry valleys, gorges and combes have been cut into its sides. Subterranean solution of the limestone has produced spectacular caves, such as Gough Cave and Wookey Hole, which are full of precipitation features including stalactites and stalagmites.

Read's cave folded rock

Late in Carboniferous times (about 290 million years ago), the rocks were folded, faulted and uplifted by earth movements, which eventually formed a range of mountains about 1500 meters high. These mountains have long since been eroded away so that only the sandstone and limestone core remains. But the earth movements mean that the limestones dip down steeply, in contrast to those in Yorkshire, and this is important in terms of the way in which the water moves through the limestone.

Pressures on the landscape

We have already seen that there are environmental pressures in the Yorkshire Dales National Park and here we see that there are similar pressures in the Mendip Hills:

Agriculture

Loxton, Mendip Hills. Looking WSW.

Modern Agriculture has changed the traditional landscape of the Mendip Hills. Much of the early wealth of the region was the result of sheep farming and the wool trade. However, these traditional activities have been replaced by dairying and horticulture as the main types of farming. Many of the dry stone walls, that were made from the local limestone several hundred years ago, have fallen into disrepair or have been lost. Hedgerows have been replaced by more easily maintained wire fences. Sinkholes that were scattered over the surface of the limestone have been filled and old mine workings have been removed. Modern farming has also resulted in the "improvement" of limestone grassland, replacing the natural grasses with modern seeded varieties, and fertilised so that the wild plants are unable to compete. Forestry plantations comprising tree species that are not native to the area, have also been planted, obscuring the traditional landscape of the Mendips Hills.

Mining and quarrying

Odin Vein, near Castleton. Old open workings for lead.
Ecton, Manifold Valley. Looking SE. Adit to Birches' Level.

Millions of years ago hot mineralizing fluids moved through the rock and cooled so that metals crystallised out in veins now exposed in the hillsides. As in the Yorkshire Dales, these veins were extensively mined during the 18th and 19th centuries and the old mine workings are found in many placed in the Mendip Hills. Metals mined here, including lead (mining was at its peak in the 17th century), zinc (from the mineral Smithsonite mined until the 18th century), iron (especially ochre used for pigments and still mined in the 1970's), manganese (mined for glass making and pottery until the late 19th century), barytes (mined in the 19th century for paint and paper making) and silver (mined in the middle of the 19th century). This industrial activity placed an environmental pressure on the region.

No metals are mined today, although limestone quarries are in operation exporting Carboniferous Limestone aggregates. Callow Rock and Battscombe quarries are the two major quarries in the Mendip Hills and others may open as reserves in the working quarries are depleted. The quarry owners are fully aware of their responsibilities to the environment and to the local population and discuss their activities with the local authorities. However, inevitably, quarrying results in noise, dust pollution and increased road congestion, so that careful management of the quarrying is important.

Tourism

Tourism is important to the local economy and hotels, restaurants, tourist sites, etc, employ a large labour force. Tourists come from miles around by car or on coach excursions to visit such 'honey pot' sites as Cheddar Gorge, Burrington Combe and Wookey Hole. Other tourists pursue there hobbies including walking, caving and riding. However, tourism places considerable environmental pressures at popular 'honey pot' sites, in terms of road and parking congestion, the erosion of footpaths, damage to wild plants and disruption of wild animals.

Karst features in the Mendip Hills

Karst features are also seen in the Mendip Hills. Although glaciers and ice fields were strong influences on the erosion of the limestone areas of Yorkshire, they did not extend into southern Britain. So the 'glaciokarst features' of Yorkshire, such as limestone pavements and scars are not found in the Mendip Hills. Here the erosion that has taken place is related to rivers, especially during the periglacial conditions of the last Ice Age. This type of landscape is known as 'fluviokarst'.

Cheddar Gorge

Cheddar Gorge, looking northwards

In the Mendip Hills, extensive erosion of the Carboniferous limestones took place during the last Ice Age with the release of water from the glaciers and ice fields to the north. One of the most famous examples of an erosion feature is the Cheddar Gorge. Here long dry valleys are all that remains of the river system that fed water into the Cheddar Gorge during the last Ice Age. The steep sides of those dendritic river system is evidence of the considerable amount of water that once flowed over the landscape.

Water was concentrated into Cheddar Gorge and a considerable amount of erosion took place. It has been calculated that the water eroded down 0.25 m every 1000 years and that that Cheddar Gorge took about one million years form. Some of the cliffs on the side of the gorge are 120 m high. The melt water cut vertically downwards through cave systems, which are left as open caves in the gorge sides. Within the cave systems (e.g. Gough's Cave) speleothems are up to 375,000 years old.

Burrington Combe

Great Scarp, Burrington Combe. Looking W.

Although Cheddar Gorge is probably the most famous of the Gorges of the Mendip Hills, another worth mentioning is Burrington Combe. This gorge, which is in the north western end of the Mendip Hills, is only about 1.5 km long, but is an excellent example of a fluviokarst gorge, cut by melt water during periglacial conditions. The Gorge extends in an east-west direction for about 1km then bends sharply to the north. It is quite steep, dropping more than 120m over its total length. It is now dry, all drainage being underground. Where streams flow off the sandstone and shale to the south of Burrington Combe, sinkholes have developed (e.g. East Twin and Bath holes). Cave systems in this area, like Goatchurch Cavern, are very old, abandoned phreatic systems, that prove the long history of solution in the area. Aveline's Hole, was a resurgence cave that has been abandoned by the water; resurgence is now at Rickford Rising and Langford Rising.

Formation of Gorges

The gorges of the Mendip Hills are now dry (except in years of flood) and all the water is subterranean. It was once thought that they were originally long caverns that had collapsed to form the gorges. This is now known to be wrong. The gorges have a V-shaped cross-section, a steep long profile, dendritic river systems (now dry valleys) and lack any evidence of material left behind from a collapsed roof.

  1. The water table is high and rivers flow over the landscape eroding a valley. Underground caves are slowly forming in the phreatic zone.
  2. With the arrival of glacial conditions cave development and underground drainage was halted and the Mendip Hills came under the influence of periglacial conditions. A large volume of water from melting ice was concentrated into the Mendip hills and rapid erosion took place, especially around the steeper margins. Cave systems were cut through by the eroding rivers.
  3. During Interglacial phases when the ice had retreated and the periglacial condition thawed, underground drainage returned and the limestone surface was dry.
  4. In the early Pleistocene there were several periods of periglacial conditions but later there were three phases of major glacial advance (Anglian, 250-350 thousand years ago; Wolstonian, 125–200 thousand years ago; and Devensian 12 to 72 thousand years ago).
  5. The water table gradually fell during the Pleistocene, resurgence level was lowered and many caves were abandoned in the vadose zone. Water in the cave systems began to open bedding and joints to find a new route to the resurgence.
  6. The gorges reached present day levels about 10 000 years ago, but the water table continued to fall so that the river systems moved underground and the gorges are dry.

Wookey Hole, Somerset

Wookey Hole

In Topic 4 we saw that geology controls the shape of the cave system and the way water moves through it. In Yorkshire the limestone is almost horizontal, but in the Mendips, earth movements in the geological past have resulted in steeply dipping rocks.

Much of the water in the Wookey Hole cave system enters at North Hill and Pen Hill and joins the underground river. The steeply dipping rocks means that the river descends rapidly down a bedding plain to the water table. But the high hydrostatic pressure means that the river continues to flow below the water table as well as above it. It descends a bedding plain and is forced up joints, before it flows down another bedding plane. The river eventually resurges at the show cave at Wookey Hole.

Dry Valleys

Top of the Ebbor Gorge.

Dry valleys are characteristic features of the Mendip Hills. Their dendritic (tree-like) patterns show that they were eroded by ordinary streams at a time in the past when water was able to flow over the surface. This apparently took place at a time when the water table was much higher than today, either during the last Ice Ages or possibly earlier. One of the best examples of a dendritic system of dry valleys is on the Mendip Plateau to the east of Cheddar and near Burrington Combe.

Although glaciation is not a feature of the landscape of the Mendip Hills, the ground was permanently frozen (permafrost) similar to the Tundra in northern Russia and Canada of today. During much of the year, the rivers were frozen, but during the summer months, some of the ice and snow melted and torrential streams eroded the frozen ground. Where the hills were steepest, the water would have flowed more rapidly, erosion was greater, and gorges formed. During interglacial times, when the ice retreated, rivers took an underground routes, as they do today.

Caves

There are a large number of cave systems below the Mendip Hills that can only be entered by expert cavers with specialist equipment. Few of the caves have been explored in full, because of their complex routes in the phreatic zone and the quantity of sediment that partially blocks many cave systems. Two show caves in the Cheddar Gorge are well worth a visit because many of the subterranean karst features of the Mendip Hills can be observed in safety: Gough's Cave and Cox's Cave.

A major resurgence occurs at Cheddar Rising. This is where all the water from the active caves in the Cheddar Gorge come to the surface. Water comes from long distance to this resurgence, including streams that disappear down sinkholes in the Charterhouse area to the north and the North Hill area 11 km to the south east. The largest Caves in the Cheddar area are those nearest to the resurgence and there are a large number of passageways that link up at Gough's Cave (the explored part extends over 2 km into the hill side). The lower part of Gough's Cave has a river flowing through it and the phreatic tubes are as much as 3m high and 7m wide. The deeply dipping limestone takes the water down the bedding planes to great depths (more than 30m below sea level) before the hydrostatic pressure forces it back up joints to the resurgence. The show cave follows about 200m of passages and caverns, in which speleothems (stalactites and stalagmites) are abundant (some of them dating to over 120, 000 years ago). Deeper in the hillside, Great Oones Hole and Long Hole have caverns with names such as Boulder Chamber, St Paul's Chamber and Bishop's Palace, and speleothems between 230, 000 and 380, 000 years old.

Dolines

Read's Cave, Burrington. The eastern swallet.

The limestone plateau of the Mendip Hills is covered by basin-shaped depressions, varying in size from a few metres to several hundred metres in circumference and up to 10 m deep. These are almost entirely solution dolines, although a very few are subsidence dolines. During periglacial times, these depressions were lakes during summer months.

In a very few cases a stream enters a doline, but as there are few areas on impermeable high ground (as there is in the Yorkshire Dales), swallow do not form. However, near Burrington Combe streams flow over the older sandstones and shales, sinking into the ground when they reach the Carboniferous limestone at, for example, Read's Sinkhole and Bath Sinkhole.

The two best formed subsidence dolines are the Devil's Punch Bowl (50m in diameter and 20m deep) and Wurt Pit (100m in diameter and 15m deep), formed by solution below ground.