Those who are familiar with geology will immediately
associate Arran with 'Hutton's Unconformity'.
This is one of the sites at which James Hutton (1726 - 1797),
first identified this type of geological structure. He was then
able to put forward a theory about the geological history of the
earth that was to have as profound an effect upon society as did
Charles Darwin's 'Origin of the Species', as he was the first to
propose that the earths' surface had evolved over an immense period
of time.
Hutton's Unconformity - near Lochranza
Visiting Lochranza in 1787, Hutton explored the coast to the
north of the village. Here, he noticed that one rock strata which
was inclined nearly vertically was overlain by another rock strata
which was almost horizontal. On seeing this, he concluded that the
layers of the lower strata (schist) had been tilted and eroded over
a long period of time before the upper strata (sandstone) had been
deposited on top of it. In fact, the vertical layers of schist were
deposited during the Silurian Period. Over the next 100 million
years these rocks became tilted by movements within the earth's
crust and were eroded away. The eroded remains of these rocks were
then covered with deposits of sandstone during the Carboniferous
Period.
From his observations at Lochranza and alsewhere, Hutton
formulated the Uniformitarian theory of geology, which suggested
that processes such as sedimentation, erosion and volcanic activity
caused changes in the surface of the earth which have been acting
in the same manner and at the same rate over the whole of
geological history. Hutton proposed that the earth was much older
than had been previously thought. It was at Lochranza where the
length and complexity of the Earth's history was first fully
appreciated.
Arran was also one of the sites where Hutton found evidence that
led to his theory that igneous rocks were produced by the cooling
of molten magma within the earth's crust, rather than being laid
down by water, as was believed by the `Neptunist' geologists of the
time
Formation of Schist
Schist is medium grade metamorphic rock, formed by the
metamorphosis of mudstone / shale, or some types of igneous rock,
to a higher degree than slate, i.e. it has been subjected to higher
temperatures and pressures. The resulting foliation is coarser and
more distinct than that of slate due to the higher degree of
crystallisation of mica minerals (biotite, chlorite, muscovite)
forming larger crystals, and is often referred to as schistosity.
These larger crystals reflect light so that schist often has a high
lustre, i.e. it is shiny. Porphyroblasts are common in schist, and
they provide information on the temperature and pressure conditions
under which the rock formed. Due to the more extreme formation
conditions, schist often shows complex folding patterns. There are
many varieties of schist and they are named for the dominant
mineral comprising the rock, e.g. mica schist, green schist (green
because of high chlorite content), garnet schist
To log the cache please upload a photo of the Unconformity and
E-Mail me through my profile the answer's to the the following
questions
1) Using your compass tell me the direction that the
Carboniferous sandstone dips
2) Estimate the angle of the sandstone
Any logs without a photo may be deleted