Curious rock features on Penyghent

5 September 2010
Leader - Mike Slater
 North Craven 
 Heritage Trust 

David and Hilary Holdsworth who proposed this event were unfortunately unable to conduct the walk. However, a replacement leader who knew the objective had been arranged. The article ‘Curious rock features on Penyghent’ written by Helen Sergeant for the 2009 Journal was the inspiration and provided the challenge to determine what was the nature of the set of holes in a large rock found on the first platform on the southern approach to the mountain. Were they man-made or natural, and if natural what was the process involved?

A party of fourteen set out from Horton Church on a pleasant day and took the usual route via Brackenbottom to the southern nose of Penyghent. Fortunately one of the party, David Shore, had a good understanding of the geology of the area. He pointed out the glacial deposits on the approach over the fields, lying on acidic peaty ground below one of the first exposed limestone edges. The walls are made of limestone at lower levels but change to gritstone higher up the approach slope.

One of the very many sedimentary layers on which Penyghent sits is one in which the fossil Girvanella occurs. (It is visible in Larch Tree Hole (SD 830 720)). This fossil-bearing band of rock is found also in North America, showing that Europe and North America were once joined (as Pangea). A report of a field meeting on Penyghent made by the North Yorkshire and Lancashire Groups of the Geologists’ Association in 2000 gives more details. Their expedition divided into two groups - The Contourists and the Ascentionists. Our group became ascentionists and made its way up the main path, climbed the short rock section and emerged onto the first platform, the top of a hard erosion-resistant limestone band. On this platform lies a great tumble of massive gritstone rocks (Grassington Grit). David drew our attention to one rock by the path which contained small circular depressions lined with an iron oxide shell. These are remains of nodular concretions formed in sedimentary sandstone by migration to a growth point by the element iron and its subsequent oxidation.

We then crossed an unstable wall on the left of the path to locate the stone found earlier by Helen Sergeant. A very large flat-surfaced gritstone boulder lies near the cliff edge and is notable for many circular depressions of 10 to 20 cm diameter (see the photograph in the 2009 Journal). Iron-staining is very evident in the rock and particularly around and in these holes. One small hole is lined with iron oxide. It therefore seemed to us that the most likely explanation is the loss by dissolution of iron-bearing nodular concretions when the rock surface became exposed to the weather after its fall. Some of the large holes are surrounded by a ‘halo’ of small quartz pebbles presumably exposed during the dissolution process of the iron concretion and the mineral in the surrounding iron-rich sandstone. It was perhaps a matter of chance that the rock originally was split by frost on the plane of a layer of iron concretions and quartz pebbles and then fell with the concretions uppermost and the boulder near horizontal so that weathering could occur. No similar stone has yet been found but the rock tumble is so chaotic and the the cliff face so close that it is cautioned that exploration is somewhat hazardous.

We returned the same way after a very satisfactory expedition.

The curious rock

The curious rock