Deep Time under our feet on the Northumberland coast

Layers of coals and shale, mudstone and sandstone formed over several million years

The geology of the Northumberland coast tells a good story. It gives us a fairly complete chronology of the Carboniferous era (from roughly 360 to 300 million years ago). At that time our piece of land was not part of a recognisable Great Britain, not next to a North Sea, and a long way further south on the globe. It sat roughly on the equator, gradually becoming part of a super-continent called Pangaea.

Our coast is dominated by sandstones, mudstones, shales and limestones, and of course, in the southern half, by coal measures. You will also see the hard dark whin sill rock which can be seen under Lindisfarne, Bamburgh and Dunstanburgh castles and which makes up the Farne islands, too. This volcanic rock travelled underground just after that Carboniferous period so is also of a similar age. Recently (20,000 – 12,000 years ago) and in our current location we had the last of the ice flows, which removed the rock that had formed after the Carboniferous, and brought rock boulders and smaller cobbles from places further north such as the Cairngorms. But the majority of our bedrock which we see along the beaches or rising out of the sea at low tide is sedimentary, which means it was deposited, in visible layers, over time, by the action of water in rivers or the sea. That process continues and its reverse, the erosion of those layers by wind, water or friction is also continually shaping our surroundings. Where the land was actually covered by a shallow sea, to the north, we have some limestones, rich in fossils. Where it was part of a river delta system we have the sequence of rocks which involves sandstones, shales, seat-earth (a rock formed of clay) and, often, coal.

In places we can see the cycle that was formed in the Carboniferous  – everything else that was added later has been removed by erosion and ice so we have a 300 million-year gap when we look at a section of coast. A site such as Druridge Bay has in parts sandstone bedrock daring from that distant period, boulder clay (left by the ice), peat and the remains of trees from 12,000 - 7,000 years ago, and the much more recent dunes which are ever-changing in winds and tides.

Philip Hood