Edinburgh Geological Society publication:
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Steep path up Salisbury Crags, Arthur's Seat and Calton Hill. Smooth pavement around Holyrood Park and past Castle Rock
Site of Special Scientific Interest (SSSI)
Text and photographs: Naomi Stevenson
Page updated June 2013
As much part of Scotland’s crown jewels as the historic Honours housed in Edinburgh Castle, Arthur’s Seat and Salisbury Crags are visible across the Firth of Forth or from the train from Berwick long before it reaches Waverley Station. Holyrood Park dominates the geological and human history of Edinburgh as well as its skyline it shares with Calton Hill.
During the early Carboniferous, roughly 350 million years ago, what would later be Edinburgh was a tropical lowland area, at the edge of a continent, with rivers, lakes and lagoons depositing the sandstones, mudstones and dolomitic limestones forming the future Ballagan Formation. The area was being stretched by tectonic forces and, some 60 km deep, the mantle rock decompressed and partially melted, forming magmas which rose to the surface. These rising magmas hit the wet sediments and the type of eruption that resulted was the phreatomagmatic eruption that we have seen in the recent past with Surtsey (Iceland), with the Arthur’s Seat volcano rising to dominate the landscape. Other magmas spread sideways to form sills (horizontal sheets of magma that force the existing apart).
In modern times, Holyrood Park showcases Arthur’s Seat and Salisbury Crags. Entering the park at the Holyrood entrance and walking around anticlockwise to the St Leonard’s entrance, Salisbury Crags dominate the skyline as you walk around Queen’s Drive. The Crags are a prominent dolerite sill which was intruded at shallow depths - analysis shows that it is likely to have been intruded in pulses rather than as one flow. The sill would originally have been lying horizontally – the current tilted profile is the result of later processes. Where the rock is exposed, rough jointing is visible at the top and it has columnar jointing in its mid-section. Towards the southern end are two small but important locations – Hutton’s rock and Hutton’s Section, important as an early example of geoconservation and for demonstrating how the dolerite of the Crags was intruded into the country rock respectively.
Near the St Leonard’s entrance, at the Hawse, Salisbury Crags disappear underground and more geological history is revealed. From this spot, there is an excellent view north across Hunter’s Bog with lava flow one – which is actually from Castle Rock Volcano – to the right. Also from this spot, the peak of Arthur’s Seat rises imposingly. Currently eroded down to almost 250 m, the full height of the volcano was likely to have been some 750 m in its heyday. Most of the structure has been torn away by the ravages of time - what remains is a choked pipe, much agglomerate and a series of lava flows.
The agglomerate is clearly visible in the eroded rock face as you continue to walk along Queen’s Drive. The grey, fallen basalt rubble stands out clearly against the iron-rich, red matrix. Some of the basalt is clearly vesicular (i.e. it has gas bubbles preserved in it).
Further still, as Queen’s Drive curves north, the slopes of Crow Hill, evidencing more recent geological history with their ice-worn and farmed terraces, are a prominent landmark. To the right is Dunsapie, a resistant basalt intrusion. Rounding the layered lavas, tuffs and sediments that make up Whinny Hill (~450 m high) brings you back to the foot of the Long Row lava – the reciprocal view of that from the Hawse. The fault cutting the foot of the Long Row lava is clear and there is an excellent view up to Arthur’s Seat. A further, major, fault is the Calton Fault which edges along Whinny Hill and Salisbury Crags – when it faulted it tilted Salisbury Crags to their current angle.
Castle Rock is an olivine-rich basaltic volcanic conduit – the volcano it fed was probably a secondary cone of the Arthur’s Seat volcano. The cinder cone that would have surrounded the conduit has eroded away. The famous profile of the Castle Rock is from Princes Street, where it is separated from the Gardens by railway lines, but it can be seen at close quarters along Johnson Terrace to its south.
Calton Hill, to the north of Princes Street, is formed by eastward-dipping tuffs and lavas. The hill is bounded by faults and an area of basalt has been polished to a shine by the action of ice. From the top of Calton Hill there is a tremendous view across to Holyrood Park, along Princes Street and to the Castle, and from its northern side across to the Kingdom of Fife and even across the Highland Boundary fault to the Highlands.
Not many cities have such an extravagant visible geology as Edinburgh.
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