Welcome to my blog about all things geology in the British Isles. What better place to start in the birthplace of modern geology: Edinburgh. My home town and true love, this city provides geological wonder wherever you look. I begin with one of the most prominent features of the city and the largest of its seven peaks: Arthur’s Seat.
Set in Holyrood Park, Arthur’s Seat rises 251m high and gives beautiful views across the city. The steep trek to the top is worth the effort, and the surrounding park gives some fascinating geological outcrops of historical significance. Here, some of the key features are outlined and can easily be visited on a walk round the park (see map for locations).
Arthur’s Seat Volcano
Arthurs Seat is the remnants of a large volcano (see figures) that was active in the Carboniferous, some 350 million years ago. The summit of Arthur’s Seat, Lions Head, represents an ancient plugged volcanic vent. However, within Edinburgh there are a further four vents that at various times during this volcano’s formation. The main vent, forming the Lions Haunch, lies to the South East of the main summit of the hill. Two further smaller vents occur elsewhere in Holyrood Park (see map) and another major vent now forms Castle Rock on which Edinburgh Castle is now perched. It was therefore a large and complex volcano, with multiple episodes of activity.
Evidence from the rocks in the area suggest that the volcano formed in a shallow sea, with the peaks of the volcanic vents forming islands with lava descending into the sea after each eruption and ash settling through the water column. Some of the sediments deposited on the sea floor can be seen in the small Camstone Quarry at Locality (1) where Carboniferous Cementstone Group sands and shales can be seen. Here, burrows and ripples can be seen in the stone as well as some mud cracks that indicate that this sea was very shallow, and at times the sediment was dried above the water level.
Over much of Holyrood Park and the surrounding area, lavas from these five vents are seen and make some impressive features. Nearby Calton Hill is largely made up from lavas and ashes from the various vents outlined above. Lavas are a type of igneous rock defined as extrusive molten rocks. That is to say that they are laid down on the surface and cool quickly in the air.
Under the earth’s surface, molten rock is known as magma, or intrusive igneous rocks. The Arthur’s Seat volcano was likely fed by a large magma chamber deep under the Earth’s surface. The magma made its way to the surface through dykes (vertical routes) and sills (horizontal routes). One spectacular intrusive feature can be seen at Locality (2): Samson’s Ribs. Here, columnar jointing seen from the Duddingston Low Road. The columns of basalt rock are formed when the magma cools and contracts. Similar features are seen at Giant’s Causeway in Northern Ireland.
Salisbury Crags
Another intrusive igneous feature dominates the Edinburgh skyline: The impressive Salisbury Crags. Although younger than the main Arthur Seat Volcano, the cliffs of the Sailsbury Crags are formed by an intrusive sill made from a rock called teschenite. It squeezed its way through the sands and silts laid down around the Arthur’s Seat Volcano around 320 million years ago. This can be clearly seen at the famous ‘Hutton’s Section’ outcrop at Locality (3).
Hutton’s Section marks the outcrop that, in the 1700s, James Hutton first identified that Salisbury Crags had been formed from molten rock that squeezed between rock layers, deforming the surrounding sediment. This observation, along with other key observations that Hutton made around Scotland fed into his seminal work on the fundamental principles of geology as we understand it today. At the base of the igneous sill, clearly layered sedimentary rocks are seen, and in two places the igneous rock is seen to have protruded in between the layers and deformed the sediments.
There is another key geological learning to come from visiting this outcrop. The site I now proudly advertised with a descriptive plaque with an informative diagram. However, upon closer inspection the geologists drawn on the diagram would in reality be approximately 10 cm high! It is always important to have an accurate scale in any geological picture or diagram – see if you can find my keys in the outcrop photograph for scale!
Continuing a short way up the path at the base of the Salisbury Crags there is another outcrop of historical significance. At Locality (4) Hutton’s Rock stands alone within an old disused quarry. The rock is a giant vein of hematite cutting through the Salisbury Sill. It takes its name from James Hutton who, on visiting the quarry during operations, persuaded the quarrymen to preserve it due to its geological significance.
‘Après Geology’
There are many other nooks and crannies to explore around Holyrood Park. Too many for this blog entry. However, a few key nearby delights are:
The Innocent Railway Tunnel: A little-known ‘secret’ of Edinburgh is the Innocent Railway Line Cycle Path Tunnel. One entrance is directly below Samson’s Ribs (Locality 2) and it extends over half a kilometer below the park.
Our Dynamic Earth: A whole museum dedicated to geology and the natural world. Well worth a visit to the museum, and as an added bonus there is a great gift shop and café!
The Sheep's Heid: Scotland’s Oldest Pub. And a cozy place to have a post-walk pint and some great grub by the fire. Established in 1360, I wonder if James Hutton was a regular!?
Happy Exploring!
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