Location: Arcow Quarry Grid Ref: OL2 803 706
Camera Direction: NW
Click Images to enlarge
|a. Road Stone.
b. Silurian Slates, grits and
c. Carboniferous Limestone – Great Scar
d. Unconformity – all
Devonian rocks missing.
e. Almost horizontal bedding planes.
f. Steeply dipping rocks due to
rocks of Swaledale belong to the
354-290 million years ago. Carboniferous is derived from the
Latin word carbo meaning coal.
We will aim to look at all of the rocks of the Carboniferous. In
doing so we’ll encounter our first problem and find that not all
rocks of the Carboniferous outcrop on the surface in Swaledale.
We cannot look at the geology of Swaledale in isolation, so
other Pennine Dales will be visited to give a better
understanding of Swaledale’s geology.
The Carboniferous Period spans more than 60 million years.
During this period of time, in evolutionary terms, plant and
animal life exploded onto the land from the sea. Sixty million
years is a vast period of time. To put this into context, we
need to go back from the present day to the time, sixty million
years when the last of the Dinosaurs would have been roaming
over Swaledale. The
Cretaceous rocks that once
covered the Pennines in a huge arch have been removed by
erosion, so that there is no trace of them today.
The northern dales of Swaledale, Wensleydale, Wharfedale and
Uredale have remarkably similar geological histories. They are
dominated by carboniferous limestone, sandstone and shale. There
is a huge gap in time and the next most recent features in
Swaledale are caused by two million years of glaciations. The
geology of the Dale will be developed through the medium of
Photograph 1. Arcow Quarry. Grid Ref: 803 706. Direction: NW.
The start of the Carboniferous is not seen in Swaledale due to
the tilting of the whole of the Askrigg Block to the north at a
low angle. The base of the Carboniferous can be found in
southern areas of the region e.g. Thornton Force (OL2 695 754)
and White Scar Caves (OL 716 746).
The photograph of Arcow Quarry reveals the following features
marked a) – f):
a) shows the rocks mined at Arcow Quarry. These rocks were mined
for road stone.
b) shows Silurian rocks that are very hard, metamorphosed
(changed by heat and pressure) greywackes, slates and grits
found in the sea. A greywacke is a rock made of sand grains and
gravel particles in a fine grained matrix of fine clay and silt
particles. It has been used as road stone due to its very hard
nature. Locally, it has been used in house building and dry
c) These rocks are part of the Great Scar Limestone and are some
of the first Lower Carboniferous rocks on the Alston Block. They
are sedimentary rocks deposited in warm tropical seas.
Continental drift carried our country to latitude of around 20
degrees south. Notice the rocks are almost horizontal. The rocks
at b) are tilted (dip) at a very high angle. The lines marked on
e) and e1) are bedding planes separating the rock strata. The
rocks on e1) are younger than those at e) because they lie on
top of them.
d) shows a ‘line’ that is known as an unconformity. The rocks at
b) have an age of around 420 million years and those at c) have
an age of 350 million years. The rocks at b) were compressed,
folded and lifted out of the sea as two continents’ collided.
They then suffered erosion for millions of years, right through
the Devonian period. At the start of the Carboniferous the sea
flooded in from the south and created the limestone of c) in
warm tropical seas. Between these two rocks it is possible to
find the remains of beaches. These can be seen at Thornton
Force, under the lip of the waterfall.
e) shows bedding planes. These are the lines between strata.
f) shows the high angle of dip of the rocks; a result of
continental collision during the
Caledonian Orogery (a mountain
The unconformity seen in the photograph cannot be seen
outcropping in Swaledale but it is there, underground. It marks
the beginning of the Carboniferous in Swaledale and the spread
of tropical seas over the Askrigg Block.