The caves of Cheddar Gorge are located in the village of Cheddar, Somerset, within the Mendips Area of Outstanding Natural Beauty (AONB). This geological postcard focuses on Gough’s and Cox’s caves within Cheddar Gorge.
Geological overview
Chedddar Gorge is part of an anticline–syncline structure that runs through much of Somerset. The Somerset Plains are part of the syncline and the hills that make up much of the Mendips AONB are part of the anticline.
Synclines and anticlines form when rocks are folded as they are compressed along an axis. In the geological map of this part of Somerset (Figure 1), the anticlines are the light blue rocks (358 to 337 million years old) that disappear and reappear from Stoke St Michael to just west of Bristol. These are limestones of the Pembroke Limestone Group and were deposited at the bottom of a shallow sea, which closed about 350 million years ago. The landmasses between which the sea was located were compressed into one another and the rocks folded to form the hills found here today.
Figure 1 Geological map of the Mendip region in Somerset (adapted from BGS Geology Viewer). BGS © UKRI.
The light red rocks that overlay them are mudstones of Triassic age and are between 252 to 201 million years old. These are part of the Mercia Mudstone Group. These rocks lie unconformably over the Cheddar limestones; they are flat compared to the limestones, suggesting that the limestones were folded before the mudstones were deposited. Another clue to the unconformity is that there are no rocks between the ages of the limestones and the mudstones, with a gap of about 85 million years between them. This could be because the rocks previously there were eroded before the mudstones were deposited, or because there was no deposition in those 85 million years.
Inside the caves
The gorge itself was cut by glacial meltwater. The water found weaknesses in the rock and eroded much of it away to create the route that the main road that passes through the gorge follows today. This is also how the caves formed, but they would have been much smaller before the excavations in the late 19th century. We know that people have had access to some of the caves long before these excavations as the famous Mesolithic-age ‘Cheddar Man’ was found close to the entrance to Gough’s Cave.
Figure 2 Flowstone speleothem in Gough’s Cave. See if you can spot the straw (icicle-like) speleothems on the left-hand side of the photo. © Kotryna Savickaite.
It can be difficult to identify different types of limestone inside the cave because it is so dark, but one of the main attractions of the caves are the formations called speleothems, more commonly known as stalagmites and stalactites. When rain falls onto soil above the caves, it picks up large amounts of carbon dioxide (CO2). This water becomes saturated with CO2 and this dissolves the calcium carbonate (CaCO3) in the limestone rock as it travels through it. When this water reaches the cave, the conditions become right for the CaCO3 to precipitate and grow into various speleothems. If there is a lot of water with the right concentration of CaCO3, the speleothem will be larger; if there isn’t a lot of water (or too much!), it will be smaller. There is a good example of this in Figure 2.
Figure 3 Flowstone speleothems in Gough’s Cave. © Kotryna Savickaite.
The rock that makes up the caves is all limestone and the speleothems are all made from the same material. However, they can be of a different colour from one another. There is some variation in the rocks because there are actually a few different types of limestone that make up the cave as well as different levels of iron oxide, or rust. The speleothems, on the other hand, tend to be white or off-white in colour, but many of them in the cave are pink, yellow or orange (Figure 3) because of the discoloration from the water running through the rocks and rust.
Figure 4a Various types of speleothems. Can you name them all? Also, the grey rocks at the back of the photo show the bedding planes in the limestones. © Kotryna Savickaite.
Figure 4b Quartz veins in the limestone. © Kotryna Savickaite.
Figure 4c Rusted rock and growing lichen (green patches). The lichen can only grow here because of the artificial lights; natural light doesn’t reach far into the caves. © Kotryna Savickaite.
Figure 4d Column speleothems in Cox’s Cave. © Kotryna Savickaite.
Other features
Some other features inside the cave include pools of water that form from the water that percolates into the cave. Some of these pools are on a platform maybe 30 cm thick, under which you can walk. The water is also very clear and undisturbed, making for some great mirror-like effects (Figure 5).
Figure 5 Pool of water in Gough’s Cave. © Kotryna Savickaite.
Cheddar Gorge walks
If you want to take a longer hike in the area, there are many walks to choose. You can choose to walk up the steps of Jacob’s Ladder up the steep side of the gorge. Once at the top, you can take a near-circular stroll around the top of the gorge and admire the views. You can also park your car higher up the gorge in a place called Black Rock and use the Cliff Top Gorge Walk path to get to Cheddar.
Access
The caves are located on the B3135, just to the north-east of Cheddar village. The road follows the gorge, so it is steep and narrow, which means that walking along the road can be dangerous as there is no pavement.
Wear shoes with good grip as the paths inside the caves can be wet and slippery.
Further information
About the author
Kotryna Savickaite
Geochemistry Technician
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