It is still cold, and because all the May colour has been wiped out by frost, I don’t have any pretty photographs of flowers. So this is a post about rocks. Do gardeners need to know geology? Probably not, although being curious about your soil – where it comes from, what lies underneath it, what minerals it holds, does help when it comes to avoiding expensive mistakes.This geological map of East Central Scotland (source here – an interesting booklet) shows that I live in an interesting area as far as rocks are concerned. The red bits are volcanic, the pink is Devonian red sandstone, and the grey, dark green and light green are carboniferous rocks containing coal seams, oil shales, and not a lot, respectively.
But as a gardener, what affects me most is the last ice age – about 15,000 years ago. The blue arrows on the map show the direction the glaciers travelled in, but also the course of the water that poured over the land when the ice started to melt. Now, here’s a look at my humble burn, a little upstream of my gardenYou can see how the valley was carved out by a much bigger river than the little burn that runs through it today. And that river of meltwater created the banks and slopes in what is now my garden. Maybe it looked something like this Icelandic river (although there would have been no humans with outstretched arms)The river, and the glaciers before the river, left behind the yellow clay in which I try to grow things. But underneath the clay – sometimes exposed where the ancient river has washed most of the clay away, are older rocks – sandstones laid down when central Scotland was a shallow sea.At the back of the pond, where that ancient river carved a bend, I’ve dug out the covering of clay. I’ve always wanted a cliff face – and this is the nearest I’m going to get. But the layers of sandstone aren’t consistent. Conditions in that ancient sea must have changed quite often, and between the beds of sandstone, there are layers of mudstone – little more than compressed clay. This wears away easily, leaving the sandstone blocks unstable:Of course, once humans arrived on the scene and needed to build mills or dams, these loose sandstone blocks were a godsend:And in digging them out to build the mill, they created my pond:Now and again I find fragmentary fossils in the clay – ripped from carboniferous rock by the glaciers and dumped in my garden. They would appear to be stigmaria – roots of a club-moss which flourished in central Scotland about 350 million years ago. Its modern descendant, the horse-tail (equisetum) infests the garden today – there’s continuity for you!