Nature Notes from Argyll
(and occasionally other places)
Fri 28 May 2010 Kerrera (part 1)
A geological walk on Kerrera led by Dr John Howe of SAMS and Dr Zoe Sayer, as part of the Festival of the Sea.
The oldest layer of rock on the island is Easdale Slate, seen here outcropping
on the beach. It was formed from deep ocean sediments about 600 million
years ago. The white lines in it are quartz which is a bit younger.
Very close to the slate on the beach are these much younger igneous rocks which originated from the Mull volcano about 53 million years ago. Dykes like this radiate in all directions from the site of the Mull volcano, and can be found as far away as Yorkshire.
The RH pic shows the conglomerate rock that forms the cliff
behind the beach here. It is about 400 million years old and has a matrix
of Devonian Old Red Sandstone. It forms an unconformity with the layer of
slate below it, which means it must have been laid down on ground where layers younger than the slate had been eroded away.
The Old Red Sandstone is in turn overlaid by Lorn Lavas, which are also from the Devonian
age. In the lower half of the LH pic the lava is rubbly, suggesting that
the it settled on wet ground here. Smooth areas represent the interior of
the lava flow. The red bit in the middle of the RH pic is formed from soil
that was on top of the lava flow.
These holes were made by gas bubbles and prove that the material is lava.
There was more Lorn lava visible on the mainland opposite. Once it was
continous all the way across, until ice carved out the Sound of Kerrera
Moving along to Little Horseshoe Bay, we were shown this erratic boulder of
Cruachan monzogranite on the shore. This was transported by glaciers that
travelled down Loch Etive; it was dumped here when they melted. On close
inspection the granite is made up of white (probably alkali feldspar), pink
(probably plagioclase, a kind of feldspar), grey (quartz) and black (biotite)
Here the Easdale slate is higher than its usual level on Kerrera, due to
faulting caused by the closure of the Iapetus ocean. The brown colouration
is caused by iron which dissolves out of the pyrites in the slate.
At the base of the rock which Gylen Castle is built on, is this
wadi formation. It is made of Old Red Sandstone, lying unconformably over
the slate on the beach, and is evidence of a flash flood in a hot desert, with
all the pebbles in the conglomerate being rounded, having been tumbled a
distance by water. The red bits are iron oxide which is red when freshly
exposed but eventually weathers to black. The RH pic shows stratification
in the sandstone. At the top are pebbles, then lines of coarse grains,
with fine grains from silty soils at the bottom, the lowest level being mud.
This suggests that it was all laid down in layers over a period of time in a
fairly gentle process.
A natural arch through the same mass of sandstone
Rocks like these fascinate the eye and the mind when all you bring to them is ignorance, but when you begin to understand how they extend in time as well as space they inspire still more awe. This is slate on the beach below Gylen Castle. It was originally mud, deep under the Iapetus ocean. Under pressure it turned to mudstone and then to slate. If it had received more pressure it would have gone on to become phyllite, then schist, then gneiss and ultimately migmatite. But the limited pressure allowed it to remain as slate, which is called a low-grade metamorphic rock, as it retains features that show how it was laid down.
It was originally flat, but has been strongly folded due to sideways pressure at some point in its history. This will have split open the slate surface at the top of each fold, revealing the material beneath, which was also originally flat but is now rounded like sausages or lengths of toothpaste. This splitting of the slate at the top of each fold is known as cleavage and leaves a series of jagged edges at either side of the fold, but these wear down over time to give the sort of rough surface you see here. Indeed the slate has now weathered to below the level of the rounded material that protrudes through it.
Our leader at first thought this smooth material might be limestone, but on testing it with HCl it did not fizz, so it is probably sandstone.
Cutting through all this almost at right angles is a pink dyke, probably of dolerite, from the Mull volcano.
The next 3 pics show features from the above scene in close-up.
"Toothpaste", originally flat and lying below the slate, then
folded along with the slate and left exposed as the slate surface broke open.
A similar piece showing cleavage. As it folds, the top
layer, being unable to stretch, has to break, revealing the layer beneath, which
in this case is the same material. To the left you can see veins of quartz
in the slate. This originates as hot fluid coming up through fissures in
the slate as it's being pressurized.
The pink dolerite dyke that cut through the slate in comparatively recent times.
All photos and other content copyright © Carl Farmer