A Geology Field Trip in Cressbrook Dale

 

Silicified Lithostrotion coral in Cressbrook Dale

Setting off from Trinity United Reformed Church with Stuart and his friend Cheryl, having spent 15 minutes having a look at the Greenmoor Rock on Ecclesall Road while waiting for my lift, we arrived at the Tideswell Dale car park to find that only 8 others had turned up for the Sheffield U3A Geology Group June 2024 field trip. 

 

Spheroidal weathering at Tideswell Dale dolerite quarry

 

Quickly walking up to the Tideswell Dale dolerite quarry, which I have already described in my report on the recce undertaken by Stuart and I on the last day of April, the very bright back lighting didn’t make it easy to see the large scale variations of the physical characteristics and weathering of the dolerite sill that is well exposed here. 

 

A lump of highly weathered dolerite

 

Nevertheless, we could see good examples of the spheroidal weathering of the dolerite, which is well developed here and, using a lump of dolerite that I had collected from the small quarry near to the car park entrance, I demonstrated its highly weathered nature by crushing it in my hand. 

 

A specimen of dolerite from the chilled margin of the sill

 

After collecting a sample from a small quarry excavated into the floor of the main quarry, we all examined this with our hand lenses but none of the essential mineral components - olivine plagioclase feldspar and augite - could be seen as distinct crystals. This usually distinguishes basalt from dolerite, but the Lower Miller’s Dale Lava Member into which the dolerite has been discordantly intruded is typically vesicular and it is probable that this indicates a chilled margin. 

 

Weathered calcite with rhombohedral cleavage

Returning to our cars and driving the short distance to Litton, where we started our circular walk, everyone successfully negotiated the stiles on the way to Tansley Dale, where Stuart explained the lead mining history of the area – a Scheduled Monument – and I obtained specimens of weathered calcite with rhombohedral cleavage, which is associated with the lead rakes here. 

 

Taking lunch in Tansley Dale

 

Taking advantage of the relatively level top of an old waste tip to have our lunch, everyone admired the wide varieties of flowers and plants – identified by a member with considerable botanical knowledge - which now replaced the early purple orchids that had been seen on the recce. 

 

The east end of Tansley Dale

 

Continuing down to the end of Tansley Dale, where the Monsal Dale Limestone Formation (MDLF) changes from light grey calcarenites laid down on a shallow shelf to very fine grained and often argillaceous calcisiltites, which were deposited in deeper water and is described as the Dark Lithofacies on the GeoIndex Onshore map viewer. 

 

An outcrop of the vesicular Cressbrook Dale Lava

Making our way north along Cressbrook Dale, we noted the small outcrops of the vesicular Cressbrook Dale Lava along parts of the public footpath, before walking up the steep unmarked path to Peter’s Stone, where we had a good discussion about the formation of the landslip and noted the Lithostrotion corals that are exposed in one of the smaller slipped blocks. 

 

A view from Peter's Stone towards Wardlow Mires

Rummaging through the scree beneath Peter’s Stone, to find brachiopod fossils and further examples of colonial corals, and taking in the views along the valley to Wardlow Mires, which was flooded when undertaking the original recce in February and led to the field trip being postponed. 

 

Retracing steps from Peter's Stone to the low level public footpath

 

Retracing our steps down to the low level public footpath, we carried on until a place where we could cross over to the west side of the valley, where we explored the well exposed beds of the upper part of the MDLF that dip north-east down to the valley bottom. 

 

Examining a 'death bed' in the Monsal Dale Limestone Formation

When I first visited Cressbrook Dale back in 1993, when assessing the Geotourism potential of the RIGS (Regionally Important Geological Sites) in the Peak District National Park, I spotted some fine exposures of Lithostrotion coral and blocks of fossil scree, but Stuart and I only encountered a ‘death bed’ – a layer with a concentration of brachiopod shells that records a catastrophic event. 

 

A 'death bed' full of brachiopod shells

Having encouraged the group to split up and have a good look at the mainly unfossilferous beds that are exposed in this locality, someone finally discovered the location of the coral that had previously jumped out at me.

 

Another view of the 'death bed'

 

The fossils here have been silicified and differential weathering has left them proud of the surrounding limestone - a spectacular example of which the group had seen along with solitary corals at Hob’s House in Monsal Dale a few years earlier. 

 

Cheryl and Stuart admiring the silicified Lithostrotion coral

Returning to the east side of Cressbrook Dale, we discovered a place where the ephemeral stream flows underground and, cutting across this to avoid the busy A623 road where another public footpath starts, we headed back to Litton.

 

The stream at the head of Cressbrook Dale