Sleuthing the Silurian Volcano

Graham Worton, NCC trench

Graham Worton, Keeper of Geology at Dudley Museum and Art Gallery, outlines some of the evidence at Wren's Nest

Often the work of a geologist is like that of a detective – you need to work out the full story using only a few bits of available evidence. However, for geologists, the evidence is often millions of years old!

So it is with these layers of rock at Wren’s Nest, near Dudley. On close examination the layers of limestone and shales are occasionally broken by layers of yellow clay, like the one shown in the picture below.

Bentonite clay layer

Bentonite clay layer, running diagonally from top left of picture to bottom right.

Geologists know these clays as bentonite clays, and know that they are formed from decomposed volcanic ash. So the question for geologists studying Wren’s Nest was “Where was the volcano?”

Applying a broad principle known as uniformitarianism (originally formulated by geologist James Hutton), which states that natural laws operated in the past much as they do today, it was possible to suggest that the ash layer might have come from a volcano around 50-60 kilometres away.

This at least gave an area in which to search for the volcano. Further evidence in sedimentation patterns suggested that the wind direction was predominantly from the south, so narrowing the search further.

The final piece of evidence came from a British Geological Survey drill core which had been drilled to examine the geology of water supplies around Cheltenham. Beneath the much younger Jurassic limestones the drill found 800m of coarse volcanic material, a pile large enough to suggest that the volcano had been found at last!

There is one more piece of information which has been locked away in these clays. By extracting a sample of zircon crystals from them it is possible to very accurately determine their age. By dating several layers, it is possible to measure the amount of time that the intervening rock layers too to form. We’ll look into that in more detail in a later post.

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