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Salt Marsh Geology

geology header

Work in progress but it will be worth coming back later as we plan to add some really interesting stuff. The salt marsh geology is surprising as what lies beneath is fascinating.

In Portishead, we have excellent examples of hard and soft bedrock (solid rock under the sediment). The rocks that form the headland at Battery Point are hard, difficult to erode. See how they form cliffs that resist the advance of the sea?  It’s limestone. A hard, well-compacted and 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 rock. Thick layers of sediment have been deposited on top. By drilling down, 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, 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 the mud is buried and compacted, it could have formed the underlying mudstone. But that’s not the case.

It is called Mercia Mudstone and was formed a long time ago when the land looked very different. It was even in a completely different part of the globe!

The landmasses (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 millennia 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 that takes us back to the Mercia Mudstone. The mudstone that forms the bedrock beneath the saltmarsh has a very high concentration of salt. So high that it can only form in hyper-saline salt pans with very high temperatures, like the salt pans in the Kalahari desert.  Occasionally water floods the salt pans. The water evaporates so quickly under the high heat leaving behind salt rock called halite. The presence of halite in the Mercia Mudstone indicates that this mudstone was formed in an arid, high temperature, in-land desert setting. Not at all like the wet, temperate estuary of today!


Palaeogeography is the science of tracing back the location of landmasses over time. So  scientists can work out where the Mercia Mudstone was deposited. It turns out that it came from at a latitude ~15-20° north of the Equator, roughly where the Sahara Desert in Northern Africa is located today.

Minerals in the rocks are 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: