The rock a crag is made of shapes almost everything about its climbing: the friction, the shape of the holds, how a route is protected and even which styles of ascent are considered acceptable. Below is a short description of each rock type used on crag pages, grouped by how the rock formed. Each description is summarised from the sources listed beneath it. See also the main glossary.
Rocks that formed as molten magma or lava cooled and crystallised, either slowly deep underground or quickly at the surface.
A grey, fine-grained volcanic rock, between basalt and rhyolite in its silica content, made mostly of plagioclase feldspar and typical of volcanoes above subduction zones. It is less well known as a climbing rock than its neighbours, but much of the climbing in Glen Coe is on andesite and rhyolite, as are the crags of Zámutovské skaly in eastern Slovakia and the sport climbing at Long Beach near Dunedin.
A dark, fine-grained volcanic rock formed when runny, iron- and magnesium-rich lava cools quickly at the surface. Thick flows shrink as they cool and split into polygonal columns, and the joints between them give long, parallel cracks and stemming corners that take plenty of cams. It can be rough on the hands but slippery when wet. Trout Creek in Oregon and the Gorge at Smith Rock are well-known basalt crack venues.
Known as diabase in North America, dolerite has the same chemistry as basalt but is medium-grained, having crystallised in shallow intrusions such as dykes and sills. It is usually rough with good friction and often splits into columns, giving long crack and corner systems that take nuts and cams readily. Fair Head in Northern Ireland, a columnar sill up to 100 m high, is the outstanding example in the British Isles; Tremadog, Dinas Mot and Tasmania's Totem Pole are also dolerite.
The coarse-grained, slowly cooled equivalent of basalt, formed deep underground or in the lower oceanic crust. Its large, interlocking crystals give a famously rough surface with outstanding friction that dries quickly, though it is hard on skin, while the thin basalt dykes that cut through it are much more slippery. The Black Cuillin of Skye is Britain's great gabbro range, home to the Inaccessible Pinnacle and many mountain rock routes.
A light-coloured, coarse-grained rock that crystallises slowly from silica-rich magma deep in the continental crust, made mainly of quartz and feldspar with some mica. It is prized for its soundness, friction and joint-controlled crack systems, which take natural protection well, and it gives everything from friction slabs and rounded breaks to splitter cracks and big walls. Yosemite Valley is the archetypal granite big-wall area; in Britain the Cairngorms, Arran and the Cornish sea cliffs around Land's End are the main venues.
A silica-rich volcanic rock, the fast-cooled surface equivalent of granite, fine-grained or glassy and often flow-banded. Much of Britain's classic mountain rock is rhyolite, particularly in Snowdonia, where Clogwyn Du'r Arddu and Dinas Cromlech, home of Cenotaph Corner, are both rhyolite cliffs known for steep, high-quality rock.
A coarse-grained intrusive rock that looks like granite but contains little or no quartz, being made mostly of alkali feldspar with hornblende or pyroxene. Climbers know it mainly from Hueco Tanks in Texas, widely regarded as one of the best bouldering areas in the world, where weathering has hollowed the rock into the pockets and basins ("huecos") that give the area its name. Access there is tightly controlled.
Rock made of volcanic ash that settled from eruption clouds or pyroclastic flows and then hardened; where the ash was hot enough to fuse it forms a tougher welded tuff. Quality varies widely: much tuff is soft and crumbly, but the best walls are compact and give technical, often bolted, face climbing. Smith Rock in Oregon, regarded as the birthplace of American sport climbing, is welded tuff, and the Idwal Slabs in Snowdonia, first climbed in 1897, are ash-flow tuff.
Rocks that have been transformed by heat and pressure, recrystallising into new minerals and textures without melting.
A high-grade metamorphic rock formed when igneous or sedimentary rocks are subjected to intense heat and pressure deep in the crust, recognisable by its banding of light quartz-feldspar and dark mica or amphibole layers. It is typically rough, often cut by quartz veins and swirling bands that create edges and incuts. The Lewisian gneiss of north-west Scotland and the Outer Hebrides, among the oldest rock in Europe, gives superb sea-cliff climbing on Lewis and Harris.
A field name rather than a precise rock type, covering dark basic igneous rocks such as basalt, dolerite and gabbro that have been altered by low-grade metamorphism, growing green minerals such as chlorite and actinolite. In Cornwall these tough rocks resist the sea better than the slates around them and form headlands. The rock is generally solid with good friction when dry but can stay greasy in damp weather; Gurnard's Head, home of the classic Right Angle, is the best-known greenstone crag in Britain.
A very hard rock made almost entirely of interlocking quartz grains, usually formed when sandstone is recrystallised by heat and pressure. It resists weathering and tends to form steep, blocky cliffs with square-cut edges, roofs and cracks; the surface can be glassy and some cliffs are loose in places. Gogarth on Anglesey is Britain's great quartzite sea cliff, and Seneca Rocks in West Virginia and Devil's Lake in Wisconsin are well-known American examples.
A medium-grained metamorphic rock, most often formed from mudstone or shale during mountain building, whose mica grows in parallel layers so the rock splits easily along one plane. Glacier-polished slabs give delicate friction climbing while steeper walls offer flat holds and cracks, and protection can be sparse. Glen Nevis and Creag Dubh are the main Scottish schist venues, and Wanaka in New Zealand has more than a thousand schist sport routes.
Formed when olivine-rich rock from the Earth's mantle is altered by seawater, usually beneath the ocean floor. It is soft, often green and red, and has a smooth, slippery feel, especially when wet, making it an unusual climbing surface. In Britain it is found mainly on the Lizard peninsula in Cornwall, a slice of oceanic crust and mantle pushed onto the continent, where it sits alongside schist on the sea cliffs.
A fine-grained metamorphic rock formed when mudstone or shale is compressed at relatively low temperature, growing aligned minerals that let it split into flat sheets. Natural and quarried slate is usually smooth, so climbing relies on small, sharp edges, delicate footwork and thin cracks, with protection often sparse, though more routes are now bolted. The disused Dinorwig quarries above Llanberis, developed by climbers from the early 1980s, are the classic venue, with Rainbow Slab among the best-known walls.
Rocks formed from layers of sediment, such as sand, mud, pebbles or shells, laid down and then compacted and cemented over time.
A soft, white, porous limestone made almost entirely of the microscopic shells of plankton, laid down on a shallow sea floor in the Late Cretaceous and often banded with flint. It is one of the least reliable climbing rocks: the soft chalk at Dover is climbed like ice, with axes, crampons and hammered-in warthogs for protection, and routes there can change or collapse.
A hard, fine-grained rock made of microcrystalline quartz, formed either from the silica skeletons of sea creatures or as nodules and bands replacing the carbonate in limestone and chalk (where it is called flint). It breaks with a glassy, shell-like fracture, so climbers mostly meet it as sharp nodules and bands within softer limestone. Crags made mainly of chert are uncommon; the Chert Locker at Mount Charleston in Nevada is one example.
Made of rounded pebbles and cobbles set in a finer matrix and bound by a natural cement, formed where fast-flowing water moved coarse gravel, such as in rivers and alluvial fans. The holds are the embedded stones themselves, from tiny knobs to large blocks, and with few cracks the routes tend to be steep, bolted face climbs. The towers of Riglos in Aragon and the mountain of Montserrat near Barcelona are the classic examples.
The Culm Measures are a thick sequence of Carboniferous shales and thin sandstones in Devon and Cornwall, intensely folded during the Variscan mountain building. On the coast between Clovelly and Bude, erosion has left the harder sandstone beds as steep slabs, walls and fins up to about 50 m high. The rock varies from very loose to solid and compact, slab routes typically need lots of small wires, and Lower Sharpnose Point is the best-known crag.
The climbers' name for the coarse, hard sandstone of the Millstone Grit, laid down by a large river delta in the Carboniferous and once quarried for millstones. Its rough texture gives outstanding friction, so rounded breaks, slopers and tiny edges can all be used, alongside steep cracks and arêtes. Routes are short and protected with traditional gear, with a long-standing consensus that gritstone stays bolt-free. Stanage and Froggatt Edge in the Peak District are the best-known crags.
Made mainly of calcium carbonate, much of it from the shells and skeletons of marine organisms such as corals. Because it slowly dissolves in rainwater it develops pockets, caves and other karst features, and redeposited carbonate can form tufas, the column-like features prized on steep sport routes. Climbing ranges from pocketed walls and sharp crimps to overhanging caves, and popular routes polish easily. Cracks are less common than on granite, so many limestone crags are bolted. Malham Cove, Céüse and Kalymnos are well-known examples.
Made of sand-sized grains, mostly quartz, cemented together by silica, calcite or iron oxides, and ranging from very hard to soft and friable depending on the cement. It usually gives good friction with breaks, slopers, pockets and cracks, but softer sandstone is easily damaged, especially when wet, so several areas have strict local rules. In Saxon Switzerland chalk and metal protection are forbidden; on southern English sandstone, such as Harrison's Rocks, climbing is limited to top-roping, bouldering and soloing without metal gear; and at Fontainebleau climbers are asked to use little chalk and never climb on wet rock.
A fine-grained rock formed from mud deposited in quiet water, whose clay particles lie in parallel sheets so it splits easily into thin layers and weathers quickly; with heat and pressure it becomes slate. It is usually poor, friable climbing rock, and in sea cliffs it tends to erode back, leaving harder beds standing proud. Climbing on shale itself is mostly limited to small venues such as the tidal bouldering at Downderry in Cornwall.
Values used where a crag isn't natural rock, or where its rock type hasn't been recorded.
Man-made structures climbed outdoors, such as buildings and bridges, for example the Derek Dooley Bridge in Sheffield.
Climbing walls and standardised training boards, such as the Moon Board and Tension Board, with artificial holds.
The rock type hasn't been recorded or couldn't be confirmed.