Rocks in Roslin Glen: a record of a swampy past

Midlothian is an area of central Scotland that lies to the west of Edinburgh and is an area with strong geological connections due to a history of mining for both coal and oil shale. As a part of the annual Midlothian Science Festival (http://midlothiansciencefestival.com/), the School of GeoSciences at the University of Edinburgh offered a walk to look at some local geology and a talk about climate change research on the Greenland icecap. In addition, a ‘Dino and Rocks Day’ was attended by 380 people, proof (as if it were needed) that dinosaurs continue to fascinate the general public. The Edinburgh Geological Society also contributed with a session about Midlothian Fossils and a local historian talked about the history of coal mining in the area.

The geology walk visited local exposures, in this case Carboniferous sediments including what may be the best exposed fluvial sediments in the area. The walk was advertised as “Rocks in Roslin Glen: a Record of a Swampy Past” and all 25 spaces were quickly booked. The location was Roslin Glen, which may sound familiar if you’ve seen the film, The Da Vinci Code, based on the novel by Dan Brown. We have not misspelled the name of the glen incidentally. For some reason, Rosslyn Chapel lies on the edge of Roslin Glen and the country park of the same spelling. The glen itself is a steep-sided valley of around 20m in depth, which carries the River North Esk roughly west to east towards the North Sea. The glen is wooded and is a popular location for short walks.

Fig. 1. The first stop – a small sandstone quarry. The top half of the ‘exposure’ is actually a stone
wall. (Photo credit: Lee Live http://www.ourdreamphotography.com/.)

The chosen Saturday in October was a cool but dry autumn day and good for the walk. We met in a car park inside the country park, by the river North Esk. Following a short introduction, we crossed the river by a footbridge, arriving almost immediately at the first stop – an exposure of sandstone in a small hollow that looks like it was quarried out at some point in the past (Fig. 1). This seemed like a good place to talk about what the area would have been like when the sandstone was laid down as river sand, in the Early Carboniferous. As the land, which eventually became Scotland, drifted across the equator during this time, it would have been a good deal warmer than a present day October; and it would have looked rather different too. Although there were trees around, they were tree ferns and not the modern angiosperms we have today, which did not evolve until the early Cretaceous. Modern tree ferns are quite modest in size, a few metres tall judging by the modern ones growing in Madeira (a great place to visit for a geology, incidentally). However, fossil tree trunks found during quarrying of Carboniferous sandstones in the Edinburgh area show that some of the tree-ferns where huge. You may have seen one example from Edinburgh outside the Natural History Museum in London, although 150 years of London air haven’t done much for its finer details. This specimen was found in 1854 and then lost for around 15 years, according to the book, Building Stones of Edinburgh by McMillan et al. (1999). How you lose a rock that is around 12m long we are not sure. Other examples of the tree trunks can be found in the Botanic Gardens in Edinburgh. The smaller plants in our modern woods are also largely angiosperms too, so rather different from their Carboniferous equivalents. However, the ferns and occasional horsetails growing below the modern trees look pretty similar to their Carboniferous ancestors.

The Carboniferous landscape would have sounded different too – there were no birds, which didn’t appear until the Jurassic, so no birdsong. Presumably, the huge dragonflies that could reach one metre in wingspan didn’t make much noise, but some of the amphibians might have had calls, as do modern frogs for example. There were also alarmingly large creepy-crawlies, with millipedes a metre long. There are no traces of these to be seen locally, but there is a great example of a millipede track on the Isle of Arran, a few tens of kilometres to the west. We briefly talked about the sandstone and how it was brought here as sand by a river. We didn’t spend time discussing the stratigraphy of the Scottish Carboniferous, although the sediments here are part of the Passage Formation, which is late Namurian in age and marks the transition from relatively limestone rich beds below to the overlying coal measures.


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