Friday, 12 July 2019

A Geology Field Trip to Monsal Dale


Gigantoproductus brachiopods in a dry stone wall

After a couple of visits to Barnsley, to explore the villages of Worsbrough and Cawthorne, and their mediaeval churches, my next day out in April was to Monsal Dale in the Peak District National Park with the Sheffield U3A Geology Group.

Gathering at Monsal Head

Sixteen members of the Group convened at Monsal Head for the usual 10:30 start and, after a brief discussion of the formation of the horse shoe shaped valley that we could see below, we headed off to find the path that would take us up to Hob’s House.

Arriving at Hob's House

Having prepared this trip a couple of weeks earlier, with my colleague Paul May, I did have some concerns that this first part of the walk – to explore the Hob’s House landslip – might be demanding for some members of the Group, given the average age of over 70; however, the whole Group easily overcame all of the obstacles that I had perceived as a potential problem during our reconnaissance.

The Monsal Dale Limestone Formation at  Hob's House

When at our destination, the Group did sit down for a brief rest but we followed this with a good examination of the numerous corals and the very distinct beds of chert. Having provided the Group with an explanation of the chemical difference between the limestone and the chert, Paul May then applied a hydrochloric acid test to samples of both of these rocks.

Undertaking a hydrochloric acid test

Making our way back down to the River Wye, again without incident, we had a good look at the exposure of tufa that had been exposed by a fallen tree. Again, hydrochloric acid was used to test for carbonate minerals and those that had brought their hand lenses were able to examine the very porous nature of this rock.

Examining an exposure of tufa on the river bank

Taking lunch on the old river terrace, we took the opportunity pass around a few geological maps that had been brought along, in preparation for the next part of the filed trip - to try and find some exposures of basalt along the lower reaches of Monsal Dale.

Examining sediments in the river bank

The afternoon session commenced with an examination of the river bank sediments and then we took a leisurely walk along the River Wye, where we encountered various riffles in the river, springs with tufa deposits and remnants of industry.

The Group stops to have a look at riffles in the bed of the River Wye

Arriving at the boggy ground underlain by the Lees Bottom Lava Member, a few of the Group decided to walk back to Monsal Head along the route we had already traversed, instead of tackling another moderately steep hill that I had previously found quite demanding.

Examining a stream that passes over the Lees Bottom Lava Member

Carrying on, we discovered a species of plant/fungus that nobody could identify, found a dry stone wall, where the coping stones were packed full of Gigantoproductus brachiopods, and then rummaged through an old waste tip along the Putfield Hill vein where several good examples of calcite and barytes were found.

Exploring an old waste tip for minerals

Finishing our return journey on the path that descends to Headstone Viaduct, now knowing where to look, we encountered a few basalt boulders, one of which must have been previously discovered by a group of geologists, as it had been split in half. Although very heavy, we didn't have far to go until the end of our field trip and it now forms an essential part of my growing rock collection.

A split boulder of basalt

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