GEOLOGY

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1. Please start by looking out to sea from the campfire (map D3).  On your right, past Castle Duart, you will see the Stevenson lighthouse at the South end of the island of Lismore, and on your left the peninsula of Morvern.  Under the sea between them is the Great Glen Fault running straight through the Great Glen (Glen Albyn) and Loch Ness to Inverness and beyond.  This fault cut Scotland into two blocks of metamorphic rock about 400 million years ago.  Much later, Mull was thrown up right across the fault by volcanoes round Loch Ba.

 

This rock looks like a Pictish sculpture but is part of a “cone sheet” – see para 4 below -running under the campfire  (map D3) and re-appearing in front of the house.  The molehills are more recent.

2. Just to the right of the fault, 30 miles away, is Ben Nevis, with flat top and shoulder this side.  To the North the cliffs are awesome.  To the right of Ben Nevis you will see more Munros (hills over 3000 ft) than anywhere else in Scotland.  They are the stumps of the Caledonian Mountains, thrown up about the same time as the Fault.

 

The craig (map F5).  The force of the volcanic intrusion stood the sedimentary limestone on its end.  If you start up the woodland track you will see the limestone clearly on your left (map G4).

 

3. About 60 million years ago there was a spectacular eruption of volcanoes along Scotland's western edge, in St. Kilda, Skye, Rum, Ardnmurchan, Mull and Arran.   Look across to Morvern to see some of the dramatic effects.  A big fault  cuts the hillside immediately opposite.  To the left is a plateau of volcanic basalt lava, where in a SW wind, the waterfalls blow straight up in plumes of spray.  At the centre and rear is very old metamorphic rock, changed from other rocks by heat and pressure.  The right, parallel to the coast, is all granite, igneous rock cooled beneath the surface, called “plutonic” after the Roman God of the Underworld.  Huge bulk carriers come worldwide for this quality granite from the quarry at Glensanda further up the coast.

 

The abseil (map F5).  The sea broke here at the end of the Ice Age.

 

4. The volcanoes were in the centre of Mull, running SE from Loch Ba.  Ben More and other hills like Beinn Fhada and Beinn Talaidh are all that remains of the caldera or cauldron from which the basalt erupted.  As the surrounding land fell back toward the centre of the empty caldera, concentric cracks opened and were filled with “cone sheets” of igneous rock like granophyre.  Intrusive “dykes” appeared, typically in a  NW direction parallel with the Sound of Mull.  Later, more granophyre intruded into the basaltic caldera, causing peripheral folding in the original “country” rocks, rather like the ripples when a stone is thrown in water.

A dyke of columnar basalt, looking rather like Staffa on its side (map G4)

 

5. You are actually standing on one of these folds or “anticlines”.  The granophyre round here is so distinctive it gets the special name of “Craignurite”.   It has intruded and shattered the limestone, a sedimentary rock of calcium carbonate formed from shells millions of years earlier.  We hope you agree that the grass in the field has benefited from the fertility and drainage of the limestone.

 Limestone and greensand beds with shell fossils (map G4)

 

6. During the Ice Age a major glacier came down the Great Glen Fault to meet minor glaciers from Mull itself.  At the end of the Ice Age, about 10,000 years ago, the sea came to the base of the craig on which you are standing.  As the weight of the ice was removed, the land rose gradually to its present level leaving a wave cut platform behind.