Wednesday, 4 September 2019

A Geology Field Trip to Rushup Vale

Bowland Shale (left) and Bee Low Limestone (right) in Rushup Vale

A trip to the Peak District National Park is always extremely refreshing and, although I had spent a productive month investigating Nottingham and parts of Barnsley and West Yorkshire that I hadn’t known before, I was looking forward to visiting Rushup Vale. 

Meeting at the layby on the Sheffield Road

Meeting at the layby on the Sheffield Road, approximately 6 km west of Castleton, our itinerary for the day was to explore the interface of the White and Dark Peaks, where the sequence of the Upper Carboniferous Shale Grit, Mam Tor Beds and Bowland Shale overlie the Lower Carboniferous Bee Low Limestone

The geology of the area around Rushup Vale

Walking down Rushup Lane to the No Car Cafe, now part of the commercially developed outbuildings to Rushup Hall, whilst various members took advantage of the toilet facilities before our walk, I was struck by the numerous swallows that had just arrived from southerly latitudes – having previously seen only a pair that have been regular visitors to Treeton. 

Boggy ground along the line of a stream

Once everyone was ready, we then set off on our walk and crossed numerous watercourses that flow from the spring line that has developed at the junction between the Shale Grit and Mam Tor Beds and then form boggy ground on the Bowland Shale, which occupies the shallow valley bottom. Along this section of the walk, there are various changes in the slope leading up to the road that reflect the underlying bedrock. 

A temporary exposure of the Mam Tor Beds

We then proceeded to take a quick look at a temporary exposure that had been exposed with drainage work to one of the farms sited on the spring line, from where we had very good views of the apron reefs, which produce rounded landforms that rise above the valley to the south. 

A view across Rushup Vale with apron reefs in the middle distance

The next stage was to briefly explore the valley bottom into which a least 14 water courses flow from the north. Although the landforms indicate that an east flowing river probably once occupied the shallow valley, at the junction of the Bowland Shale and the Bee Low Limestone, there is no sign of it today. 

The entrance to Little Bull Pit

All of the streams that drain the north side of the valley disappear into swallow holes, as we discovered at Little Bull Pit. Here, Paul May undertook a hydrochloric acid test on the surrounding rock, which demonstrated that it was Carboniferous limestone. 

An outcrop of the Bowland Shale Formation

To the north of the stream that disappears down Little Bull Pit, there are exposures of the Bowland Shale Formation, which I had never seen before in outcrop and had expected to be highly laminated and very friable; however, the rock here is in the form of a dark grey, dense and brittle mudstone. 

A specimen of Bowland Shale

Continuing our walk up the apron reef limestone to Bull Pit, another very large swallow hole that is hidden by a copse and fenced off for safety reasons, we then paused for lunch before heading off down to the southern end of Rushup Lane, where we could clearly see the mound like apron reef of Gautries Hill

A view of Bull Pit

Also, although on private land, there is another example of a swallow hole here and another south flowing watercourse terminates in a small pond, where its extremely lush vegetation contrasts strongly with that seen earlier in our walk.

A swallow hole at the southern end of Rushup lane

This proved to be the last stopping point on the walk, which was originally intended to be longer, and we then walked back up Rushup Lane to the No Car Cafe, which had been especially opened for us.

A relatively short walk around Rushup Vale

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