SPOTTER’S GUIDE TO BRITISH ROCKS
A simple guide to help you identify the materials that have created the UK landscape
GNEISS
The coarsely grained salt-and-pepper bands of gneiss (pronounced ‘nice’) are found in knobbly ‘knock and lochan’ landscapes of such places as Rannoch Moor and the north west Highlands. These metamorphic rocks formed up to 1,000 million years ago, when mineral grains recrystallised under intense heat and pressure.
SLATE
The smooth blue and green slates of Snowdonia and the Lake District were formed 395-510 million years ago as fine-grained sediments that were altered by intense heat. Their red-hot formation created foliation – or cleavage – that made them easy to split and gave us the slates that once roofed the world.
FLINT
Geologists argue about how mineralised nodules of jet-black or brown flint were formed in chalk and limestone beds during the Cretaceous period 65-140 million years ago. But as early man soon discovered in such places as Norfolk’s Breckland, if broken open, they created an exceedingly sharp edge, as incisive as a surgeon’s scalpel.
GRANITE
Sparkling, silica-rich pink or white coarse-grained granite is an igneous rock formed by the eruption and crystallisation of once-molten material from the core of the earth. Much of Dartmoor, Bodmin Moor and the Cairngorm Mountains are composed of granite, which weathers into the distinctive upstanding towers of rock known as tors.
OLD RED SANDSTONE
The rich red grainy rocks of Old Red Sandstone were laid down under Saharan desert conditions in the Devonian area, between 345-395 million years old ago. They are made up mostly of quartz grains and get their colouring from coatings of iron-rich hermatite. You find them in the Brecon Beacons and parts of the West Midlands.
MILLSTONE GRIT
The abrasive clerical-grey Millstone Grit of the famous Pennine edges so beloved by climbers was laid down 280-345 million years ago in a vast river delta. As its name suggests, it was formerly carved into millstones to grind cereals in corn and windmills, and unused examples can still be seen at places such as Stanage Edge in the Peak.
DOLERITE
Dolerite is a finely grained, dark, dense volcanic intrusive rock that forms dykes or sills often running for miles. It’s in the Great Whin Sill that runs across northern England and on which Hadrian built his wall; at High Force, Cauldron Snout and Lindisfarne and in the volcanic plugs on which Edinburgh and Stirling Castles were built.
MOUNTAIN LIMESTONE
This pearly grey limestone of the White Peak and Yorkshire Dales was formed under a shallow, tropical sea around 350 million years ago. A huge number of sealife fossils are found within it, including the screw-like stems of crinoids, a sea lily-like creature. Weathering creates the clints and grykes of limestone pavements.
SERPENTINE
Found on Cornwall’s Lizard peninsula, this decorative rock is found in shades of dark green, red and grey, and polishes like marble to a deep sheen. It formed during the creation of the oceanic crust over a geologically short time period of about 35 million years, during the Devonian period around 345-395 million years ago.