Royanne Wilding, a member of UWFS until she moved to Giggleswick, and a retired Open University lecturer in Geology, led a group of 9 of us to Combs Quarry in good weather. 

The rocks quarried until the late 1880s were the Horton Formation, known as Horton Flags and W R Mitchell of The Dalesman  wrote an interesting article on their widespread use.

The Horton Flags are Silurian rocks formed from muddy deposits in deep seas when what was to become England was south of the Equator 450-400 million years ago. Intermittent volcanic eruptions occurred which left layers of fine ashes on the sea surface to then sink down and form different intermediate layers. 

Rocks of the Horton Formation -layers of lightly metamorphosed mud deposits with volcanism ash between

Tectonic Plate collision and the closure of the Iapetus Ocean caused these deposits to significantly fold and later compression altered the nature of the rock to change or partially metamorphose into the harder rock present today. Local faulting caused smaller changes which can be seen in the rocks with displacement across the gap. 

A break in the layers, a Fault

The two ends of the walking stick show the amount of displacement caused by the fault

Further Tectonic plate activity meant the crust that was to become Yorkshire had moved to just north of the Equator in Carboniferous time and Limestone was formed from shell debris in warm shallow seas, the horizontal layers of rock now seen above the steeply dipping Horton Formation.  The Devonian Rocks, formed when the crust was above the sea level  between Silurian and Carboniferous times, in hot windy dry desert conditions,  have since been removed by massive erosion thus leaving a large time gap between the Horton Formation and the Limestone known as an Unconformity. Because the horizontal rocks are lain on a dipping layer of rock this is known as an Angular Unconformity. 

The horizontal layers of Carboniferous Limestone resting on top of steeply dipping Silurian Rocks

After a well earned rest over lunch we headed back towards Arcow Tarmac active quarry which shows much more of the bedding of the lower rocks, again with local folding forming “hills” or anticlines and “hollows” synclines.

Red arrow at the top of Anticline
Blue arrow at the bottom of Syncline

In deep seas with shelves and slopes avalanches of mud occur,  known as Turbidity Flows. These were discovered when the first Telegraph lines laid in the Atlantic were broken, that is the force of Turbidity Flow.  Outcrops of rock above the Austwick to Helwith Bridge road were examined to see examples of rock formed by Turbidity Currents. The Bouma Sequence is the name given to the layers of deposition forming the rocks.

Layers of rock forming more than one Bouma Sequence

A Base of Bouma and high energy deposits
B Layering as energy drops
C cross stratification and ripple marks

When the force of flow begins to lessen it first drops the larger grains which also gouge the underlying surface creating eddies and hollows so the lowest layer of Bouma is uneven. As the forces lessen finer particles are lain in layers, these too might be affected by flow of water forming ripples, later things quieten to allow layering and finally the finest of debris in water settles over time.

Here in the rock outcrops we were able to see magnificent Flute Marks, the casts of the hollows created by the first powerful flow. Then the graded layering of particles as the flow lessens followed by some ripples. Nowhere was the full sequence seen but it was possible to recognise the different layering.

The base of the Bouma showing Flute Marks

Royanne had done a splendid job in educating and entertaining us on what one can learn from careful observation of rocks about past environments. A very big thank you. 

Josephine Drake

Photos  mostly Ian Hughes occasional Jo Drake