The Chatsworth Grit at Curbar Edge

 

The Chatsworth Grit at Curbar Edge

My first visit to Curbar Edge, back in November 2015 with the Sheffield U3A Geology Group, took place on a day that started with winds in excess of 65 km/h and finished with heavy rain that forced us to abandon the walk. 

 

A view back towards Baslow Edge

 

This time, in late August 2021, the temperature was in the early 20s and there was no sign of rain, which gave me plenty of time to explore this spectacular gritstone edge at leisure. Having briefly encountered the Chatsworth Grit at Wellington's Monument and Baslow Edge, I took a good look at the various rock formations here before having some lunch.

 

A bed of quartz pebbles in the Chatsworth Grit

Given its major contribution to the character of the landscape in the Peak District National Park, surprisingly little is written about the  general characteristics of the Chatsworth Grit in my Geological Survey of Great Britain memoirs.The coarse grained upper part, which is separated from a lower finer grained sandstone by shale, is described as coarse grained, often pebbly and in places conglomeratic, cross-bedded and highly feldspathic. 

 

A detail with a 21 mm diameter £1 coin for scale

 

Petrographic examinations were undertaken of the sandstones from various locations in the further describes them as being poorly sorted subarkose, composed predominantly of quartz with an igneous origin, orthoclase, microcline-microperthite, very minor acid plagioclase and the primary micas - muscovite and biotite.

 

A detail of an unusually blackened bed of pebbles

 

The binding matrix is mainly granular kaolinite, which forms by the chemical weathering of feldspar, mixed with illite and the average feldspar/kaolinite is about 15%. The assemblage of heavy minerals, which help to determine the provenance of the sediments, have granitic affinities.

 

 Coarse gritty sandstone with occasional pebbles

The various sediments in the Millstone Grit Group were laid down as part of a vast river delta system, with palaeocurrent measurements showing that the sediment was being brought down from the Caledonian mountain chain to the north-east, which included the area that now forms much of modern Scandinavia.

 

Cross-bedding at Curbar Edge

The Chatsworth Grit here is generally massive and the cross-bedding is well developed, with graded bedding also being very common. As a geology field trip locality, there are many examples of how differential weathering and erosion has sculpted the Chatsworth Grit into various tor like features, where a range of sedimentary structures can be seen.

 

Differential weathering and erosion in the Chatsworth Grit

Having seen enough of the Chatsworth Grit at close range, I just carried on along the path northwards and stopped several more times to photograph further examples of the varying lithologies and sedimentary structures from a distance.

 

A view along Curbar Edge to Baslow Edge

In places, the bedding of the Chatsworth Grit is strangely contorted, which is usually associated with slumping on the underwater front of the delta, turbidity currents, dewatering and sometimes seismic activity, but I didn't stop to examine them closely.

 

Convoluted bedding in the Chatsworth Grit

 

The number of visitors to Curbar Edge, which includes mountain bikers, exerts a lot of wear on the main path and great care is needed in traversing some sections; however, despite the management problems this causes, this erosion creates some excellent sections through the soil horizon, where heather passes down into peat and into the sandy and stony subsoil. 

 

An exposure of the soil and sub-soil

 

Having had a good look at Curbar Edge, I carried on along the path until at some arbitrary point I reached Froggatt Edge, which coincided with my first view of the Stoke Hall Quarry – located on the west side of the River Derwent on the Kinderscout Grit.

 

The geology of the area around Froggatt Edge