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Understanding Reef Knolls in the Yorkshire Dales

Updated: Sep 19, 2020

Whist walking in the Yorkshire Dales recently I pointed out to a friend, some strangely round conical hills just outside Grassington and explained that they were the “Cracoe Reef Knolls”. Having no interest in my geographical factoid, I was met with the response “Great Name for a Band!”.

But, for those people who maybe interested in learning a bit more about the Yorkshire Dales landscape, here is a bluffers guide to the Reef Knolls of the Dales. The weird cone shaped hills you may have spotted whilst driving on the B6265 between Grassington and Skipton ….

During the Carboniferous period, about 330 million years ago, some of Yorkshire was covered by a shallow tropical sea that covered much of the Dales. The water was teaming with small sea creatures and as they died, their shells and lime rich skeletons built up on the sea bed and as they compressed they formed what we know now as Shelf Limestone.

Sugar Loaf Hill Reef Knoll - by Stephen Oldham

On the edges of the shallow sea - reefs started to form. The reefs were made up of corals, algae and other micro organisms whose shells were made up of calcium carbonate. The corals and algae being living organisms needed light to live and grow. Along the edges of the reef, as the sea floor began to sink and fall away on the margins into deeper water, the coral and algae became further away from the surface and hence the light.

The corals which included species such as crinoids and rugosa as well as the algae, had to work harder and grow faster to get closer to the suns light source. Gradually these large conical structures formed on the fast growing reefs. Research has shown that most of the knolls were formed where a mud mound had existed originally where the coral had colonised and which probably accounts for their conical/round shape. The diagram below is a simplified version showing Reef Knolls formation based on Richard Bell's original illustration

The knolls were also home to other primitive creatures such as brachiopods, sponges and molluscs too and so today offer palaeontologists a rich source of coral and shell fossils.

The knolls had to be able to withstand sea currents so grew and formed atolls which can still be seen in tropical reefs in the Pacific and Indian Ocean nowadays.

The reef knolls can mainly be seen along the edge of the mid craven fault and as well as around Cracoe (now designated a Site of Special Scientific Interest), they can also be seen close to Malham and Settle. Swinden Quarry was established on a reef knoll due to it being a source of quality limestone.

In Cracoe the knolls include Thorpe Kail, Elboton, Myrah Bank Stebden, Skelerton and Butter Haw hills and in Malham the Reef Knolls include Cawden, which can be seen from the village as well as Wedber & Burns Hill.

Picture above below Cawden Reef Knoll, Malham

So, I hope you have learnt a bit about this distinctive geological feature and if not you can always go and form a prog rock band called the Cracoe Reef Knolls!

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Sep 17, 2020

Tim never ceases to amaze not just with his knowledge, but light touch and humour to help us learn about what we see in this stunning landscape- called Yorkshire. Looking forward to his next blog on the Mid Craven Fault; sadly although I'm a little older, I wasn't around in the carboniferous period myself ! Great piece here Tim Barber.

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