By Dawn Collis
As a small isle Bute has a relatively large proportion of its area that is coastline. That is a simple and self-evident fact.
What may not be so obvious to most people is what a wealth of geological features can be found on our coastline.
While many of the interesting bits of geology are easy and obvious to see, some are a little more challenging either to spot or to access. However in this quick tour of some of the features that are of particular interest many of the items, if you can’t get along to see them, are illustrated by exhibits in the museum.
Most people will be aware that the Highland Boundary Fault crosses the island and is responsible for the central lochs. From Rothesay Bay through Loch Fad and Loch Quien to Scalpsie Bay the chain is very visible from aerial photographs. To the north of the fault line on Bute is highland terrain with highland rocks, while south of the line is lowland Scotland. Here the soil is less acidic, the terrain is more gentle and the geology has other geological features.
Rocks on the shoreline near Rhubodach show evidence of the scratches scored into even very hard rock when subjected to the flow of glaciers during the last ice age. Here too the effects of changing sea levels can be seen with the quite steep steps of the land from the current sea level to the much higher, raised beaches where the sea level was in the immediate aftermath of the last ice age.
The coastline south of Rothesay is peppered with long ridges of very hard stone. These are old volcanic lava flows, not, as might be thought, from nearby Arran but from the mighty volcanoes that once dominated Mull. They have produced one very rare geological feature found on Bute and in only a small number of other locations worldwide - columnar sandstone. Most people are aware that basalt, a rock produced by volcanoes, produces very distinctive hexagonal columns as it cools, giving us such spectacular features as the Giant’s Causeway and Fingal’s Cave. What is less well known is that in a small number of places in the world sandstone, a rock created from sandy sediments, also shows this feature. When the lava flows from Mull crossed the south of Bute they forced their way through cracks in the sandstone, the intense heat from the lava heated up the sandstone to such an extent that is started to change form - it was being ‘cooked’. As the sandstone cooled it started to form a more crystalline structure and this caused the hexagonal columns to form. The best place to see this strange and extremely rare feature is at the far end of Kilchattan Bay, close to the bus turning circle. There is now an interpretation board to help visitors to spot the formations. A piece of the sandstone is on display at the museum for those who cannot get down to Kilchattan.
There are other signs of the turbulent past of Bute. Volcanic ‘bombs’ are created when a piece of stone thrown up during volcanic eruptions lands in volcanic ash and then the whole is subsequently compressed to form rock. One such bomb can be spotted at Glencallum Bay. There are also volcanic vents on the island. Dunagoil and Dunstrone, utilised by Bute’s early settlers as defensive positions, are both a result of lava solidifying as it bubbled and flowed out during the volcanic past of this area.
If this short article on some of the geological features of Bute has whet your appetite there is a short book on the Geology of Bute available from the museum.