Thursday 2nd May, 2019

WEGA in the North West Highlands Geopark

Day 5 – Thursday 2nd May 2019

Today was a day for looking at the details. We looked at one of the unusual igneous intrusions of Assynt, the thermal metamorphism of the Durness Limestone, tried to find a trilobite, scratched our heads at the geological complexity on the shores of Loch Assynt and had a look up the glen with the Bone Caves.

Our route can be seen on the map below. A selection of the my photographs are used below. You can see all of them HERE.

Our first stop was the Allt a’Mhuillin Quarry which is the type locality for borolanite, a melanite-biotite-proxene nepheline-syenite. The white spots are aggregates of potassium feldspars, white mica and nepheline and are pseudomorphs after leucite. 

The unusual mineralogy is the result of the lower than normal silica content. There is not enough silica to form feldspars, so feldspathoids (in this case leucite) form instead.

Current interpretations are that the intrusion of these rocks was contemporaneous with the thrusting.

Borolanite with pseudoleucites.

Ledmore Quarry was the next stop but we were not able to enter as the pit was flooded and there was a lot of work going on. But we were able to look at interesting bits near the entrance. Including some nice Fosterite Marble.

Fosterite Marble

I then participated in my fourth unsuccessful trilobite hunt in Assynt in the last fifty years. 

The trilobites of Assynt are important for dating purposes and for illustrating the geological history of the UK. The Assynt trilobites are similar to North American specimens. They are unlike those of Wales and England.

The two sorts were brought together by the closing of a predecessor of the Atlantic Ocean – the same event which led to the Moine Thrust.

It would have been nice to find my own North American trilobite but I will trust the evidence found by other, luckier (or more persistent) geologists.

Looking for trilobites.

Then to the shores of Loch Assynt where we looked at the complexities of the Thrust Zone.

No doubt, with lots of time and relevant experience, we could have seen the truth of the Thrust Zone. As it was we were impressed  with the work of the geologists who mapped and interpreted the zone.

Complex faulting, sills, Durness Limestone. In the thrust zone.

After driving to the “Rock Stop Shop” at Kylesku for lunch. we returned to Skiag Bridge, close to Loch Assynt, to look at the classic spot for Pipe Rock.

The group at Skiag Bridge, looking at Pipe Rock.

The final stop was a pleasant walk up a side valley to look (at a distance) at the Bone Caves.

The burn which comes down the Bone Cave valley
Red deer in the Bone Cave Valley

Eventually we came to a spring which produced most of the water in the burn running down the glen.

Spring producing most of the water in the burn.
Looking up the valley from the spring. The Bone Caves are at the base of the cliffs in the centre of the picture.

Then walk back to the van and home to Ullapool. But we paused to take a last look at Suilven.