The North Pennines is mainly made up of layers of limestone, sandstone and shale, which formed in tropical environments during the Carboniferous Period of Earth history.

Tropical world

In Carboniferous times, around 350 to 300 million years ago, the piece of the Earth’s crust that would eventually become Britain lay almost astride the equator. The North Pennines basked in a hot and humid climate – rather like that of the Bahamas today – and was periodically covered by tropical seas, vast river deltas and lush rainforests.

Coral seas

The tropical seas of the Carboniferous Period teemed with life. Sea creatures such as corals, sponges, crinoids and brachiopods flourished in the clear, sunlit waters. When they died their remains accumulated as limy ooze and shelly fragments on the sea floor, eventually hardening to become the hard grey limestone we see today.

Rivers of mud and sand

Large rivers drained into the sea from hills to the north. They washed mud and sand into the sea, forming vast deltas. In time, the deltas built far out into the sea. Layers of mud and sand settled on the sea floor, burying the marine life and eventually hardening into shale and sandstone.

Swamp forest

The deltas built up above sea level into vast swampy areas, similar to the Mississippi delta today. The North Pennines would have been covered in a lush swamp of giant horsetails and clubmosses. We can see relics of this great forest today – in the area’s coal seams and the plant fossils in many local sandstones.

Changing sea levels and cyclothems

Sea levels in the Carboniferous were constantly changing. When sea level was low, the deltas built up and became covered with vegetation. When sea level rose, the forests were drowned and sea life returned. This cycle happened many times, building up repeating layers of limestone, shale, sandstone and coal, known as ‘cyclothems’. Sandstone and limestone are hard, resistant rocks, whereas the softer shale layers wear away easily. This contrast has produced the terraced hillsides of the North Pennines and can be seen in many of the area’s waterfalls.

What’s in a name?

Quarrymen gave the rock layers names based on their thickness, character, location or uses. Examples include the Four Fathom Limestone, the Great Limestone, the Tynebottom Limestone and the Cockleshell Limestone. Sandstone layers known as the Firestone Sill and the Grindstone Sill were named after their use for hearth stones and grindstones.

Diagram showing the formation of cyclothem sequences (from bottom to top). These reflect changing sea levels and the building up of river deltas. The column on the right shows the resulting rock sequence of limestone, shale, sandstone, coal & limestone