What is the stone that Stirling Castle is built out of?

Question

My 6 year old son in fascinated by geology and would love the answer to this question. Also, we have looked up the general geology of the Stirling area, but all explanations are quite technical - for both of us! Can anyone explain in relatively straightforward language what the local rocks are and how they were formed. We know the Castle Rock is a volcanic plug - what rocks will this consist of? And what lies beneath?

Many thanks.

Answer 1

Hi, I will try and give a simplified answer, with a few technical terms which I will try and explain.

Some time, about 300-290 million years ago hot, molten lava rose from deep within the earth. This was a time quite a while before the Dinosaurs ( about 100 milion years before they started to become numerous, and nearly 240 million years before the dinosaurs died out.

The molten lava reached rocks laid down in flat layers, sandstones (made of bits of sand held together by a natural cement) and layers of coal, that were the squashed remains of plants and vegitation from ancient swamps.

The molten lava pushed apart some of these layers and spread out in a big, flat layer of its own. The lave cooled down and became solid. The layer of cooled lava was harder than the sandstones and coals, when it was uncovered by the rocks on top wearing away (what we call "erosion") it formed a step in the landscape. This is what geologists (scientists who study rocks and fossils) call a "Sill"

Some people call the rock a "Basalt" which is a dark rock made of very fine crystals (you might just see them with a good magnifying glass) . Basalt is a rock made from lava that has come from very deep in the earth. Other people call the rock a Dolerite ( I think this is more correct) Dolerite is very similar to Basalt, but has slightly bigger crystals.

The castle itself is not made out of Dolerite, as it is a very hard rock and difficult to make into blocks for building. I have not found any informatio on the original building stones, but repair work was done using a sandstone called Catcastle Grey Sandstone http://www.dunhouse.co.uk/proj18.htm

this is a sandstone from County Durham, of about the same age as the sandstones the lava pushed itself into (From a time geologists call the "Carboniferous" which is about 350-300 million years old) I think the original castle will have been built from similar, but local sandstones.

The flat stones (flagstones) used for the floor of a lot of the castle are a famous stone called "Caithness stone" that comes from Caithness in Scotland. This is a sandstone but made of very small sand grains. Because it can be split easily into flat sheets but is very hardwearing it is perfect for making stone floors