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Geological History

Adapted from a leaflet by David McAdam entitled "Geological Guide to the Arthur's Seat Volcano" reproduced with the permission of Edinburgh Geological Society

Holyrood Park is the site of a volcano which was active in early Carboniferous times, about 340 million years ago. The sequence of events which occured is illustrated in the following cartoons:

History Stage A Stage A: Thin layers of calcareous mud and sand (the Cementstone Group sediments) were deposited in shallow tropical seas and lakes fringed by forested coastlines.
History Stage B Stage B: The Castle Volcano erupted to form a cone, and a lava flow spread towards the east.

History Stage C Stage C: The Arthur's Seat Volcano pushed through the sediments and early lavas, erupting first from the Lion's Head Vent, Later from the Lion's Haunch Vent. A cone of basaltic lavas and ash beds built up around the vents, which finally filled with agglomerates and intrusions.
History Stage D Stage D: The extinct volcanoes were flooded and buried under more mudy sediments (the Abbeyhill Shales). 25 million years after the volcano became extinct, more molten rock pushed between the underlying strata, forming horizontal sheet-like intrusions called sills.
History Stage E Stage E: Earth movements folded the rocks around Edinburgh, tilting the Holyrood Park rocks to the east.
History Stage F Stage F: Millions of years of erosion, most recently during the Great Ice Age, wore down the rocks to lay bare the inside of the volcano. Now the two vents form the twin peaks, and Whinny Hill is the remnant of the volcano cone. The cliffs of Salisbury Crags are a thick sill, while the softer sedimentary rocks occupy the lower ground.

Copyright: Edinburgh Geological Society, 1986

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