The Cumbrian invasion of Devon

Devon was invaded by the Dutch in the 17th century at Torbay. The army headed north to Exeter and then on to London to stage a (relatively) bloodless coup and depose the Scottish King James II. William of Orange was welcomed as the new King of England coregent. Most people don’t know that Devon was also invaded in the north. Sort of.

Many times during the Ice Ages of the last 2.6 million years (the Quaternary Period), ice accumulated over Scotland into large glaciers as part of an ice sheet. As it got thicker, a glacier flowed down the Irish Sea. We know for certain that at least once the ice was so thick that it pushed into the Bristol Channel and flowed up onto the north coast of Devon. The evidence of this is found in the form of erratics -rocks that have been moved from their point of origin and now sit on rock of a different kind. At least 20 erratics have been found, mostly volcanic and metamorphic rocks, often well above sea level. The erratics sit on the Pilton Shales, a drab grey fine-grained sedimentary rock. This rock is about 365 million years old or Devonian in age (named after this county where rocks of this age were first defined) and the sediment was deposited on the floor of a deep sea as a series of thousands of submarine landslides.

The best known erratic is the most obvious and the first to have been documented by Williams in 1839. It is a 2.4 m long pink granite (above) sitting directly on the rock platform. Any till it was deposited with has long since been washed away. The shales themselves have been sculpted by the action of waves into a platform that sits above modern sea level. The period that the waves did this cutting must have had higher sea level than present, meaning that either Greenland or West Antarctica had substantially melted, much more so than today. Over the top of the shore platform is a series of wave washed beach sands (a raised beach) and then beach dunes over the top of that. Topping everything off is a thick layer of a mixture of frost shattered shales mixed with mud. This is a typical deposit that covers Devon and signifies that it was a tundra environment with seasonal freezing of the ground surface -an Ice Age.

This spectacular sequence of sediment and rock is a classic site to teach students at, because it shows the dramatic swings between cold glacial and warm interglacial (between glacial) climates through time. Working out exactly when everything happened here is somewhat more difficult. We know that the glacial period was the most extreme on record because there aren’t any other glacial deposits in the stratigraphy. This was followed by at least one interglacial period. The non-glacial deposits over the top mean that the ice sheet didn’t reach here in the last Ice Age. The likely candidate for depositing the erratic is the Anglian Glaciation half a million years ago, which was the time ice reached its furthest south in Britain, just north of London. An interglacial followed, probably the last one, when the shore platform was cut and beach sands were laid down. The exposure dating technique could be conclusive, but this is a Site of Special Scientific Interest and exposure dating is a destructive technique.

Where did the boulder come from? Someone suggested a similar rock in Gruinard Bay, Wester Ross in western Scotland. Others have suggested that some of the erratics derived from Ireland and others from Wales. At first these suggestions make sense because ice originated from all these places. However, glaciologist Alun Hubbard and colleagues ran a glacier computer model for the British Isles by simulating much colder temperatures than present. A physical ice flow model predicts where ice will flow. Looking at his map of flowlines, we can narrow the options. As expected, ice grows from centres on the highlands of the British Isles. However, once the glaciers coalesce, ice in western Scotland flows west (moraines and striations confirm this) and ice in Ireland is also pushed west (again confirmed by drumlin fields and erratics). The flowline of the icesheet that reaches north Devon actually originates in northern England at the Lake District. Ice flowed down the Irish Sea and then around and across Wales. This narrows the search area considerably. A prime candidate is the Shap Adamellite (monzogranite), a well known granite from Shap Fell. There are other possibilities and direct analysis of the rock would be conclusive.


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