A county built on two kinds of rock
Much of Cornwall’s bedrock is made of folded sedimentary rocks such as slate and related material, laid down long ago in ancient seas. Pushed up into these rocks is something quite different: a large body of granite. It is not a single block but a chain of linked masses that geologists often describe together, running from the Isles of Scilly through the far west and on to Bodmin Moor, with Carnmenellis and the St Austell uplands in between. The granite is usually dated to roughly 280 million years ago, though figures differ slightly between sources.
This granite began as molten rock deep underground. It cooled slowly, which gave its mineral crystals time to grow large. Typical granite contains pale feldspar, glassy quartz and dark mica, and in parts of Cornwall the feldspar crystals are especially large and obvious. Slow cooling also made the rock hard and tough, which matters for what happened next.
From underground to hilltop
Granite formed far below the surface. It reached daylight only because the rocks above it were gradually worn away over a very long time. As the covering rock eroded, the granite was exposed. Because it resists weathering better than the softer rocks around it, it was left standing as higher ground. That is why Cornwall’s main upland areas, the moors, sit on granite.
How a tor takes shape
A tor is a stack or cluster of rock rising from a hilltop or ridge. The word is widely used in the south-west of England and is often said to come from a Celtic root, though its exact history is a matter for linguists. Tors are not unique to Cornwall, and the best-known examples in Britain are on Dartmoor in neighboring Devon.
The usual explanation of tors involves two stages. First, granite is full of natural cracks called joints, which formed as the rock cooled and as pressure was released when overlying rock was removed. Joints run both vertically and horizontally, dividing the rock into blocks. Second, water and chemical weathering work along those joints. In some conditions the granite below ground weathers into a crumbly sandy material, especially where water can move through it. Where the joints are widely spaced, the rock stays solid and resists. When the loose material is later stripped away, the harder blocks are left exposed.
Many details are still discussed by geologists, including how much of the work was done in warm, wet conditions in the distant past and how much by the freezing and thawing of the later Ice Age periods. Cornwall was mostly beyond the reach of the large ice sheets, though its uplands experienced cold conditions in which frost shattering and the slow downhill movement of loosened material probably played a part. For a simple account, it is enough to say that tors are the survivors of a very long process of weathering.
Shapes you can recognize
Once you know about joints, many tor features make sense:
- Stacked, layered blocks. Horizontal joints split the rock into slabs that look piled on top of one another.
- Rounded edges. Water and chemical attack are strongest at corners, so blocks become rounded over time.
- Hollows and basins. Small pits can form on flat rock surfaces where water collects. Some were once described as man-made, but many are now understood as natural features.
- Scattered boulders. Blocks that have slid or fallen sometimes litter the slopes below a tor.
Well-known examples
Several Cornish tors are familiar landmarks. On Bodmin Moor, Rough Tor and nearby Brown Willy stand among the highest ground in the county. Another famous rock formation on the same moor is the Cheesewring, a balanced stack of rounded slabs that is easy to recognize. Elsewhere, the granite headlands near Land’s End show similar weathering in a coastal setting. Anyone planning a visit should check current local information, since access arrangements and conditions can change.
Granite and human use
Granite was not just scenery. People used it. In the Bronze Age and earlier, communities used moorland stone for monuments, and the standing stones found on high ground are often built from locally gathered granite. In later centuries, granite was cut for churches, bridges, harbor walls and houses, and the practice of granite quarrying became a notable local industry. The sturdy look of many Cornish villages comes from this local stone.
Granite also has a deep connection to mining. Hot, mineral-rich fluids associated with the cooling granite deposited tin and copper in cracks in the rocks around it, and this explains why the richest mining districts sit close to the granite uplands. A separate chemistry story belongs to china clay, which is made when feldspar in the granite breaks down; the pale clay country near St Austell is a result.
Folklore and tors
Unusual rock formations tend to attract stories. In Cornwall, some outcrops are linked in popular tradition with giants who are said to have thrown or stacked the boulders. These tales are folklore rather than history, and they vary from place to place. They show how striking the rocks have appeared to people over the centuries, and how readily the landscape invited explanation.
Visiting tors safely
Rocky outcrops are fun to explore but need care.
- Wet granite is slippery, and moss and lichen make it more so.
- Do not climb or scramble on unstable stacks, and avoid loose or balanced blocks.
- Weather on exposed hills changes quickly, with fog and wind arriving without much warning.
- Keep to paths where possible to limit erosion, and respect any signs about nesting birds or grazing livestock.
- Remember that many tors are also archaeological or protected sites, so avoid moving stones.
If you are unsure about the route or conditions, a local guidebook or visitor information source can help. In general, the best approach is to look closely, take photographs, and leave the rocks as you found them.