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Geology Archive

MacKinnon’s Cave

Mackinnon’s Cave is situated on the Isle of Mull’s west coast near Gribun.  The area is dominated by sheer cliffs and very broken country that afford a great view of several important geological time periods.  MacKinnon’s Cave is said to be the longest sea cave in the Hebrides at around 500 feet in length.  A torch is therefore essential to explore the cave, and as the mouth of the cave is tidal you must consult the tide times before setting off and plan your visit on a low tide.

MacKinnon's Cave shore at low tide

Walking from the parking area at Balmeanach Farm you follow the fence line before reaching the coast, where you get stunning views west along Ardmeanach.

Walking to MacKinnon's Cave

On the shore and in the vicinity of MacKinnon’s Cave, the main rock type is called psammite and it belongs to the “Moine Supergroup” which are around 1000 million years old.  This rock is mainly comprised of  sandstones that have been subjected to heat and pressure which causes the rock to change “metamorphose” into a much harder, crystalline rock, the psammite.  The rocks dip at an angle of about 40 degrees which makes for difficult walking.  Great care should be taken while scrambling toward the entrance to the cave.  The psammites continue all the way round the coast and provide the bedrock for some of the most rugged coastal scenery that Scotland has to offer, a wild landscape of sea stacks, caves, fissures and natural arches.

MacKinnon’s Cave consist of a large main chamber with a sand filled floor.  Some light enters this part of the cave and plants thrive near the entrance.  A narrower tunnel leads deeper into the dark of the cave before arriving at a second chamber where rockfall blocks the way.  Deep caves usually have a good geological reason for their formation and Mackinnon’s Cave is no exception, having been eroded out along the line of a fault by the relentless pounding of the waves.

MacKinnons Cave on Mull

Above the psammites, there are rocks that are much younger although still very old. These are sediments of Triassic age (200 to 250 million years old). The shore around MacKinnon’s cave is strewn with large boulders that have fallen from the cliffs above of these Triassic conglomerates, which resemble mixed concrete.  Most of the lumps, technically called “clasts” in the conglomerate are of the Moine psammites from which they were derived. Heading further west along Ardmeanach the way is largely pathless, skirting between the cliffs both above you and below to the coast.  The views over the sea to Iona, Staffa and Treshnish Isles are mesmerising!

Deep inside MacKinnon's cave

Above the Triassic conglomerates can be found on the obvious “terrace” of cliffs that were formed by great flows of lava dating to the Palaeogene period ca. 60 million years ago, when the Isle of Mull was volcanically very active and great flows of lava poured over the landscape.

Geology of Mackinnons cave and Ardmeanch Mull

In this part of Mull the stepped terracing in the lavas is very clear, each layer being a distinct separate lava flow, a topography known as “trap”.  The westernmost area of Ardmeanach is known as ‘The Wilderness’ and it certainly feels like an apt name as you venture below the towering crags.  The fossil tree is close by too, but most of people will walk to it from Burg – another adventure we will cover at some point!

Top 5 Locations for Columnar Basalt!

The Isle of Mull and its neighbouring islands are rightly famous for their geology.  These islands have a richness and complexity in their geological make up that is quite remarkable. When the Survey officers were carrying out the geological survey of Mull and surrounding islands, in the early part of the 20th century, they quickly realised how complex an area that they were studying.  The rocks of Mull have played an enormous part in the advancement of the science of Igneous Petrology and Earth Scientists from all over the world come to study the amazing geology of this place, however, you do not need to be a scientist to appreciate one of the truly remarkable geological wonders – the rock known as Columnar Basalt, which is on display in several places in these islands.

Most people will be familiar with pictures of Fingals Cave on Staffa or the Giant’s Causeway in Northern Ireland.  These are both excellent examples of this strange rock formation which almost does not look ‘natural’.  The mainly six sided columns have a symmetry which is fascinating.  So what is it and where can it be seen on Mull?

Basalt is a type of lava that is very common throughout the world. Look at pictures of Hawaii and you will see lava pouring into the sea and forming great clouds of steam. That is basalt.  The volcanoes in Iceland that caused such disruption to air traffic a few years ago were mainly basaltic.  In fact Iceland today is very like how Mull would have looked 60 million years ago!  Not all basalt lava forms columns however – much of it is massive and uniform, lacking the pillar-like structure.  There is a  lot of variation in the basalt lava seen in Mull and the other islands – some of it is very crumbly, a lot of it contains white crystals of a group of minerals called zeolites, and sometimes it can appear reddish in colour.  But the columnar form, which is easily the most visually spectacular, can be seen in lots of locations on Mull, so here are five areas for you to see some impressive examples of it during a visit.

1. Staffa:
The Island of Staffa is the prime location for seeing basalt columns. No other location really matches it for grandeur. Whether seen from the boat or from the shore, it looks spectacular. The name Staffa comes from the Norse and means “Pillar Island”. Very well named!  Staffa is easy to access on foot but requires a boat trip to get to it.

Staffa Basalt

Basalt columns on Staffa

2. Ulva:
Ulva is an island just to the west of Mull and is accessible from Ulva Ferry by a small regular boat service. The columns are on the south coast of the island, approximately 45 minutes from the slipway. The walking is easy and the columns are well sign-posted.

Ulva Basalt

Basalt on Ulva

3. Macculloch’s Tree (The Fossil Tree):
The famous Fossil Tree lies at the very western extremity of the Ardmeanach peninsula. It involves a long walk over rough terrain and the descent of a ladder to reach the shore. The scenery is spectacular and basalt columns can be seen near the tree and on the shore leading up to it. This is wild country where the rewards for the effort are great scenery and spectacular coastal scenery. Beyond the headland of Rubha na h-Uamha (Point of the Cave, and well-named) there is more columnar basalt to be seen but great care is needed as it can only be easily accessed when the tide is out.

Fossil Tree

The Fossil Tree

4. Carsaig Arches:
This is another difficult to reach location, with a lot of rough walking and a need for a steady gait and a good head for heights. The Arches are at Malcolm’s Point, west of Carsaig itself. One of them forms a sea-stack. The other is a cave. Both are spectacular.

arches1

Carsaig Arches

5. Ardtun:
Ardtun, near Bunessan is famous for another geological find – fossil leaves, dating back 60 million years. The leaves are found in between lava flows, some of which are beautifully columnar. This is dramatic coastal scenery with ravines, a sea stack, caves and other delights. Many of the columns are curved or even horizontal. The approach is over extremely boggy ground, straightforward to walk but very wet underfoot. Care is required at the gorge of Slochd nan Uruisg (the defile of the goblin) where the leaf beds and the basalt columns are best seen.

Ardtun

Ardtun

There are lots of further reading available on the subject, should you wish to find out more about Mull’s geology.  Mull in the Making by Ros Jones being  a great introduction.

Author: James Westland

All images copyright James Westland, 2016