Holyrood Park Geology

How lucky we are to have, in the centre of Edinburgh, a place which encapsulates so much of geological interest in a such small area. We have the evidence of ancient volcanoes, remains of prehistoric fish, and lava flows. There are limestones, coal, plants from bygone forests, corals from former seas, and fossilised beaches, together with the signs of rather more recent glaciers.

That area is Holyrood Park.

Luckily, the Edinburgh volcanoes have been extinct for a very long time and will not erupt again.

The best preserved of these ancient volcanoes is Arthur's Seat. However, erosion over millions of years has dug deep into the core of the volcano and exposed the vents, lava flows and ash falls, which are all today clearly visible. The vents, which carried the lava to the surface, still reach deep into the earth, although today are cold, solidified and completely blocked.

This map shows the Geology of Holyrood Park today. The evidence for the ancient environment we have just described in clearly visible, in what must have been one of the most closely studied areas in terms of Geology in the world.

The combination of the diagram and photograph above show how today's landscape related to the ancient volcano. The volcano has been tilted approximately 20 degrees to the East due to the immense forces which have acted on its in the millions of years since it was active. The volcanic pipe, which was the conduit through which the lava was erupted, forms the bulk of Arthur's Seat itself, and the lava flows which form the flanks of the volcano are exposed on the eastern side of the park (left hand side of the ilustration above, which is looking South).

The volcano sits on top of sedimentary rocks which were deposited before the volcano existed within a shallow sea. These sedimentary rocks contains some fossils, but more fossils are to be found in thin layers of younger sedimentary rocks which are found between the lava flows. This shows that the vaolcano was erupting into a sea, and between eruptions, these sediments were depositing. It is these rocks which include the plant fragments and fish remains.

This map shows Arthur's Seat and Salisbury Crags in the context of the Geology of Edinburgh and the Lothians.

From ancient times the earth had been regarded as having a short and finite life, closely related to the history of humankind. This thinking was developed in medieval times, with scholars placing an age of 6000 years on the Earth, based on literal interpretation of the bible. They were equally convinced that a catastrophic end was not far in the future.

This view persisted until late in the 17th Century, when Hutton, through careful observation and scientific inquiry came to the conclusion that the Earth had 'no vestige of a beginning, no concept of an end'. Although we know today that the Earth did have a 'beginning' and indeed will ultimately have an end, both of these events are so far away from the present time that Hutton's view can be regarded as for the most part accurate.

Hutton was a remarkably perceptive observer, and lived at the time of the Scottish Enlightenment, in an environment which challenged preceding doctrines. He not only questioned the creationist view whereby the earth has always looked exactly as it is today, but also argued against the prevailing view that the landscape had been carved by a biblical flood. This Neptunist philosophy propounded by Abraham Werner of Freiburg (Saxony) suggested that all rocks had been deposited from the primaeval ocean which existed after the flood. Although volcanoes had been studied by this time, the known world was very much centred on Europe, and thus the only Vesuvius, Etna and a few extinct volcanoes in France were really considered, and were therefore dismissed as a 'curiosity'.

Through careful scientific observation, Hutton realised that the Neptunist view could not be true. He collected evidence to prove his point, and among this evidence was 'Hutton's Section' at the base of Salisbury Crags. The rocks in that area clearly showed that molten (igneous) rock had been pushed in between existing sedimentary rocks, breaking them apart. Molten rocks injected between existing sediments had no place in Werner's simplistic scheme, and thus it was discredited.

Hutton's concept of uniformitarianism, where the processes of the past are the processes of the present and the processes of the future, applied across the immensity of time, won through. However, acceptance amongst even the scientific community took more than 50 years. Hutton, for the first time, had realised that both the surface and interior of the Earth were constantly changing, through processes of creation and decay.

The igneous rock which forms Salisbury Crags is called teschenite by geologists, but is known as whin to quarrymen, who used this hard rock primarily as cobbles to cover the streets of the City. Quarrying was ongoing in the 18th Century, but Hutton asked the quarrymen to preserve certain features of geological interest, for example Hutton's Rock which shows a fine example of an iron ore vein cutting across the intruded rock (or sill) which forms Salisbury Crags. Thus in addition to being the Father of modern geology, a scientist and philosopher, Hutton was also an early conservationist.

We can see evidence of a number of different events around Holyrood Park, which must have occurred at different times. For example, Cat's Nick is a notable cleft in Salisbury Crags, at almost its highest point. This represent a further intrusive igneous body of rock, which has cut across the sill which forms the crag. This body is almost vertical, and is called a dyke. Its significance is that the sill must have cooled before this dyke was intruded, and therefore it must be a much later feature.

These pages are provided by the Department of Geography, University of Edinburgh as part of Scottish Geology Week. Scottish Geology Week is coordinated by Scottish Natural Heritage.