June must be my favourite month. There is the promise of some fine weather, the hours of daylight are long, and the countryside and gardens still retain that fresh green appearance of spring. It is time to get out and enjoy it all.
I have always loved maps. Nothing better than spreading them out on the kitchen table and planning a new walk or cycling route. Every kilometre square telling a different story and waiting to be discovered. Something you see time and time again marked on ordnance survey maps of the Yorkshire Dales, is the mention of ‘shakeholes’. What are they, what do they look like, and how do they form?
You have probably walked past some and not even noticed. They are large 2-10 metre circular depressions, looking much like ‘dimples’ dotted across the countryside.
Shakeholes, are to give them their scientific name, a type of doline, known as a subsidence doline. This is where it gets complicated, so I am going to try and keep it simple. There are other types of dolines, but I am just going to discuss shakeholes, the commonest type found in the Dales. These form in the soil wherever there is reasonable depth of overlying glacial till or alluvium covering a bedrock of limestone.
The slightly acidic rainwater percolates through the soil and reacts with the limestone beneath, forming calcium bicarbonate, which being soluble gets washed away, widening rock fissures within the limestone in the process. Over time the soil just above the limestone, will fall into the widening fissures and likewise be washed away, allowing a void or subterranean cavity to develop. This gradually increases in size, and eventually the arch above the developing cavity will no longer support its own weight and collapses. Cows and sheep have been known to become trapped when this happens suddenly. The vertical sides are then gradually weathered, and the hole becomes less steep and covered in vegetation, forming the shakeholes we see today in the Dales.
Finally, I wish you all happy walking or cycling this summer, wherever you may wander.