by John Russell
The rocks of Swaledale belong to the Carboniferous Period, 354-290 million years ago. Not all rocks of the Carboniferous Period can be seen to outcrop on the surface in Swaledale. Other Pennine Dales give a better understanding of this geology.
The Carboniferous Period spans a time when plant and animal life exploded onto the land from the sea. The northern dales of Swaledale, Wensleydale, Wharfedale and Wensleydale 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 start of the Carboniferous period 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 rocks can be found in southern areas of the region e.g. Thornton Force (SD 695 754) and White Scar Caves (SD 716 746). The photograph of Arcow Quarry, near Horton-in-Ribblesdale, reveals the following features:
a) shows the rocks mined at Arcow Quarry 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-stone walls.
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 a 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 Orogeny (a mountain building period). 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.
See also:
Overview: The Geology of Swaledale and Arkengarthdale
