Nature Notes from Argyll
(and occasionally other places)
Fri 28 May 2010 Kerrera - part 2 (back to part 1)
Moving west a bit we came to this spectacular unconformity, for
which Kerrera is famous. You can see the Old Red Sandstone conglomerate
directly above the slate which is 150 million years older.
Here's a close-up of the unconformity: conglomerate above and
Contorted beds of quartz in slate.
Turbidites - laminated silts produced by turbidity, in slate.
Graded beds with coarse sediments at the bottom, fine at the top, so that you
can tell the younger layers are to the left. The pattern is repeated as
other events formed new sequences of layers. This is a cross-section of
the Iapetus ocean bed, now on its side. There must have been slopes nearby
to deposit all this material on the ocean floor.
From the same spot, the sandstone layer above the unconformity here takes the form of breccia, rather than conglomerate. This means that the stones embedded in the sandstone matrix are angular rather than rounded, and therefore had not been transported any great distance. This suggests that there was something like scree slopes adjacent to the beds where the sandstone formed.
The second picture shows people examining a slab of sandstone
that had fallen from the rock above into a convenient cavity. The
following photos show what was found on the slab.
These ripples were formed 400 million years ago when the slab
was part of a sandy lake bed.
Living creatures had also left their mark on the lake bed.
The LH pic shows insect trails and the RH pic shows worm burrows. Trust
me, they really do.
My notes on this say only "razorbacks" and I can't find any
info about this as a geological term.
And my notes on this say only "imbrications".
This is the surface left where a large chunk of rock broke off,
by natural process, about 10 years ago. It is covered with glittering
flakes of Muscovite Mica. The rock containing the mica must be from a lake
in the desert with volcanoes around it. This is because mica dissolves in
sea water and cannot be transported. It must have come from mountains
nearby. The stones in the breccia just above this level must have come
from screes on these mountains.
Where the chunk of rock mentioned above fell away, it sheared
right through the stones in the breccia. Here is part of the breccia with
the sheared off stones embedded in it. The large stone near the middle is
of interest, as it is a piece of layered sedimentary rock, perhaps from some
earlier lake bed, which became distorted and solidified by pressure and then perhaps after
further upheaval ended up on the mountainside, from where it fell into the lake
to become a constituent of a new sediment in turn.
On the way back to the ferry we saw this dolerite dyke from the
Mull volcano. It has been slightly built up into a wall and is shown as
one on the map.
Here the same dolerite dyke had intruded through the slate, baking
the slate and cooling the dolerite where they touched. The dolerite is in
the LH half of the pic and the slate in the RH half.
Vesicles where gas has escaped from the dolerite. In the RH pic the vesicles are elongated because the dolerite was still moving when the gas escaped.
A great thing about this walk was that because it was near home and because I noted the grid ref of every feature we looked at, I'll be able to go back and look at these things again at my leisure and hopefully learn to recognise them when I see them elsewhere. Many thanks to John and Zoe for an enthralling day out.
All photos and other content copyright © Carl Farmer