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Salt Marsh Geology

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What lies beneath the marsh?

Salt marsh geology . . . the geology beneath the salt marsh is surprising and fascinating. In Portishead, we have excellent examples of hard and soft bedrock (solid rock under the sediment). The rocks that form the headland at Portishead are hard, difficult to erode and resist the advance of the sea. It is limestone: a hard, well-cemented rock.

Down on the saltmarsh however, we can’t see any cliffs. In fact, we are down at sea level and there is no evidence of the bedrock at all. Over time, the water of the local rivers and the sea have eroded away the bedrock and thick layers of sediment have been deposited on top. By drilling down beneath the mud, you can get a sample of the rock and identify what it is. This rock is much softer than the limestone on the headland. It is called mudstone and is a very fine grained rock with small, easily eroded particles.

How did the mudstone form? Let’s go on a journey through deep time when the land was a windswept desert like the Sahara of today . . .

Mud and deserts

Many metres of mud have been continuously deposited by incoming tides. As these thick muds are buried and compacted, you could think they solidified to form the underlying mudstone. But that’s not actually the case.

The mudstone beneath Portbury Salt Marsh is called the Mercia Mudstone and was formed a long, long time ago when the land looked very different. It was even in a completely different part of the globe!

The continents on the surface of our planet are constantly on the move. They move so slowly, that we don’t even notice it. On average, they move at the rate our fingernails grow. Over millions of years, the continents move around, assemble in different combinations and split up again. Scientists can trace this dance back in time. They can  determine where on the planet a piece of land was depending on what rock was deposited and what minerals it contains.

So what does the Mercia Mudstone tell us? It contains a salt-rich mineral called halite. This gives us an indication of the conditions in which the mud was deposited. In order for the salt to crystallise in the mud, it must have formed in very high temperatures, like the salt pans in the Kalahari desert in southern Africa, or Death Valley in the US.

Occasionally water floods the salt pans. The water evaporates rapidly under the high heat, leaving behind salt crystals called halite. 

The presence of halite in the Mercia Mudstone indicates that this mudstone was formed in an arid, desert setting. Not at all like the wet, temperate estuary of today!


Palaeogeography is the science of tracing back the location of the continents over time. Using techniques that measure the orientation of minerals in the rock relative to the magnetic field of the Earth, scientists can work out where the Mercia Mudstone was deposited. It formed at a latitude ~15-20° north of the Equator, roughly where the Sahara Desert in Northern Africa is located today.

Minerals in the rocks can be dated using sophisticated techniques and the age of the Mercia Mudstone is estimated at around 220 million years old.

Take a look at this “rock below the salt marsh” clip:

Portishead, a tropical holiday destination?

The rocks on the headland above Portishead are even older. You have to travel back more than 300 million years to discover where they were formed…

One of the rocks that forms the headland at Portishead is the Black Rock Limestone. If you go for a walk on Chesil beach at low tide, you can spot chunks of the grey rock that have fallen onto the beach and been smoothed by the sea. If you look carefully, you may spot fossils in the rock that hint at its exotic origins.

Limestone is created by the precipitation of calcium carbonate (CaCO3) out of sea-water, in the form of lime-mud and the shells of sea creatures. It is usually deposited in tropical seas that are warm and shallow. The mud and shells sink to the sea bed and eventually solidify to form thick layers of rock.

In the Black Rock Limestone, you can see fossils of ancient corals, sponges and sea-ferns called crinoids. We know from the fossils and minerals found in the Black Rock Limestone that it was deposited in a warm, tropical sea when the land was near the Equator, 340 million years ago. Can you imagine it today?

The rocks of the Portishead and Battery Point have a much more varied and complex history than has been covered here. To find out more, check out the Gordano Civic Society Posset Piece, Portishead Rocks!, written by local geologist Mark Howson.

This page is written by geologist Mathilde Braddock. Here is a link to her website Steps in Stone.


Williams, B. P. J. & Hancock, P. L. 1977. The sedimentology structure of the Upper Palaeozoic rocks at Portishead, in Geological Excursions in the Bristol District, Ed. Savage, R. J. G., University of Bristol, ISBN 0 901239 22 4.

Howson, M. and the Gordano Civic Society, 2014. Portishead Rocks!, Posset Pieces No 16, from Posset Pieces: Local History from Portishead and the Gordano Region of North Somerset, ISSN 0924-5839.

British Geological Survey, Geology of Britain viewer,