How many of you have wondered about the boulder outside the Hack and Spade, by the village notice board? Did you know that the rock of which it is made came from the Shap area, in eastern Cumbria? How did it form and how did it get here?
The boulder is made of Shap granite, which is very distinctive with its pinky red oblong crystals of a mineral called orthoclase feldspar, (potassium aluminium silicate). It is often used in memorials and facing for public buildings as it is so decorative and is still being quarried today at Shap. You can see the quarries from the M6. It is a type of igneous rock formed from the very slow cooling and crystalisation of molten rock, many kilometres below the Earth’s surface, within the Earth’s crust. Gradually, the overlying layers of rock are worn away by erosion to expose the granite at the surface. Granite is usually divided by many rectangular joints or cracks which form as it cools and also with the reduction of pressure as the overlying rocks are worn away. This takes hundreds of millions of years. Shap Granite formed 393 million years ago during a period of geological time called the Devonian, within the rocks making the Lake District. We know this from the dating of radioactive elements within the minerals making up the granite.
But how did it get to Whashton? You will notice that the boulder is very rounded in shape and this is because it has been transported here by glaciers during the latest ice age, at some point during the last two million years. It tells us that the ice was moving out from the eastern Lake District/Cumbria area and spilling over the Pennines eastwards, eroding and transporting rock material as it flowed. The glaciers broke away and picked up pieces of Shap Granite from the Shap area, as they went along. This rock material was scraped over the land surface and rounded as it was transported. Eventually, the ice melted about ten thousand years ago and the rock material was dumped and left on the land surface. The larger pieces of rock like our boulder are called glacial erratics, because their rock type doesn’t match the geology of the underlying rocks where they finally come to rest. Whashton is underlain by rocks of Carboniferous age ie limestones and sandstones about 350 million years old. In this way, erratics can tell us about the direction of movement of glaciers.
This boulder has obviously been rolled or transported by a tractor a short distance to its current position from somewhere else in the locality. Similar boulders of Shap Granite can also be seen along the edges of fields just to the west of Barnard Castle. There is another, smaller boulder of Shap Granite to be seen in Whashton but do you know where? (Answer next month…)