Carboniferous Coals and Centipedes

This month we go back in time to an important Geological Period for Edinburgh: and discover what Central Scotland and the UK would have looked like during The Carboniferous. This was a time when Edinburgh was hot, swampy and full of giant insects! The rocks deposited at this time would go on to shape the local human history.

When was the Carboniferous?

The Lower Carboniferous Period spanned between 326 and 359 Ma ago. At this time, Great Britain sat close to the Equator, just north of a large mountain belt named the Variscan Mountains (Figure 2). Being further South than its today, the Central Belt of Scotland was a lot warmer (I know that is not difficult!) The World’s Climate was a very different.

Fig 1: Carboniferous swamp. From National Geographic.

The Carboniferous was an important time in the evolution of life on the planet, as the first reptiles roamed the Earth. These reptiles would go on to become the prevalent dinosaurs in the Permian, Triassic and Jurassic Periods. The Carboniferous was a time when Amphibians and Insects ruled the land (Figure 1).

Lower Carboniferous Edinburgh

Fig. 2: Central Scotland (boxed) during the Lower Carboniferous. From the Millennium Atlas (Evans et al. 2003).

359 Million years ago, the Central Scotland lay at the equator. Since that time, plate tectonics has caused Britain to drift Northward to its current situation. Whilst on the equator, tropical conditions were found, and much of Britain was covered with dense humid swamps. This is known from the rocks that were deposited that this time that show large amounts of plant material including trees (a large tree stump can be seen on the beach at Crail, Fife).

Fig. 3: Shallow Marine rocks on the side of Arthurs Seat show ripples, roots and mud cracks (see insert).

The sea was never far away, and periodic sea level rises, or changes in the delta shoreline led to marine incursions over the region. At times a shallow sea covered all of Edinburgh except for the volcanic peaks of Arthur’s Seat and Castle Rock (see January’s Blog for more info.) Sedimentary Rocks seen at the Camstone Quarry on the East slope of Sailsbury Crags (Holyrood Park) show ripples, roots and mud cracks (Figure 3). The sea must have been a shallow lagoonal sea that at times dried and re-flooded thee area.

Lower Carboniferous Fife

To the North, on the Fife Coast, swamp, river and delta top sediments are found. As a result, we know that in the Lower Carboniferous a large delta was forming sourced by sediments from the Northeast (where the North Sea is today). Large sand beds are separated by mudstones that represent lagoonal or more marine (estuary) conditions. The sands around Anstruther and St Monans show a variety of sedimentary features including cross bedding and climbing ripples (Figure 4).

Fig 4: The Fife Coast Lower Carboniferous delta top and shoreface sands show current features such as climbing ripple. Muddy layers separating the sands represent lagoonal or more marine (estuary) conditions.

Another piece of evidence for the swampy conditions in Fife comes from the fossils and trace fossils found in the rocks there. Crail harbor and beach where a variety of fossils can be found including a large tree stump, many plant and tree fragments, and plant roots. During the Lower Carboniferous, large trees and ferns evolved and with the hot humid climate thick dense forests grew across the Globe. These trees released much oxygen into the atmosphere, and this is thought to be the reason for the evolution of insects that are much larger than today. The centipede tracks found in the west bay of Crail (Figure 5 insert) indicate that this creature reached 2 m in length!

Fig 5: Crail harbor – well worth a visit for some fossil hunting and a paddle.

Oil Shales

Such dense forests shed much organic material into the soil which accumulated in swamps. Where buried, these accumulations of plant debris have formed layers of fine-grained rock with high oil content: oil shales. These oil shales can be seen in South Queensferry, at Port Neuk – identified due to their brown color and oily texture (will not crumble). These oil shales were mined extensively from 1850 until to last mine closed in 1962 (Figure 6). The waste products of the industry are still seen across West Lothian as large red manmade hills called Bings. More information can be found at the excellent Museum of Scottish Shale Oil (Livingston).

Fig 6: Philpstoun Oil Work in 1925, located close to South Queensferry. From the Museum of Scottish Shale Oil.

It is clear that Edinburgh was a very different place in the Lower Carboniferous when compared to today. They mark the start of the evolution towards dinosaurs with the first reptiles roaming the Fife and Lothian swamps with giant insects. These rocks have gone on to play a very important roll in human history with the beginning of the oil industry in the UK.

--- Happy Exploring! ---