Recently I attended a geological walk lead by Mike Allen from the South Peak Estate of the National Trust to have a look at a new geological section that has been opened upon the side of Ossum’s Hill above the Manifold Valley in the Staffordshire part of the Peak District. The geology of the area is shown in Figure 1.
Figure 1: Geology around Ossum's Crag after The Hamps and Manifold Geotrail, Staffordshire RIGS 2007. Grey = Mixon Limestone Shales; Pale Blue = Ecton Limestones; Dark Blue = Milldale Limestone; Purple = Milldale Limestone Knoll Reefs
The walk departed from the car park opposite Wetton Mill which was the site of the shaft, dressing floor and smelter for the Botstone Lead Mine that operated until around 1850. Machinery was driven by a waterwheel from the River Manifold. The assent of Ossum’s Hill was via the footpath that follows the path of the Hoo Brook, a small misfit river in a wide valley carved by meltwater at the end of the last Ice Age. Part way up, a large Ash tree on the right marks the site of one of the entrances to the Botstone Mine (Figure 2).
Figure 2: Adit Entrance to Botstone Mine
The Hoo Brook valley sides show evidence of landslipage, probably the result of over-steepening of slopes by meltwater undercutting them. On the climb up the valley, many of the pauses for breath were used to discuss different facets of global climate change, both during the Ice Age and during the Carboniferous Period. The route then took us up a side valley to the left up beyond Ossum’s Hill Farm and then left again to the top of Ossum’s Hill where the depressions from numerous old mine shafts can be seen. The local farmer estimates the shaft of one of them to be at over 50 metres deep.
Over the hillcrest, the path then follows a farm track slanting down the hillside back towards Wetton Mill. It is here that the farmer, in widening the track, has opened up some new exposures of the Carboniferous Limestone. The upper part of the section displays the mid-grey coarse bioclastic crinoidal Ecton Limestone (Figure 3). This was deposited on the lower part of the shelf slope as turbidites from the shallower shelf areas.
Figure 3: Bioclastic crinoidal Ecton Limestone
As the path descends the hillside the transition can be seen to the underlying finer bedded and laminated micritic Milldale Limestone with thin beds of chert deposited under quieter conditions. Here is also displayed some tectonic deformation with a small fault and some local small-scale folding, probably related to the fault (Figure 4).
Figure 4: Small-scale folding and faulting in Milldale Limestone
The view across the Manifold Valley clearly displays the reef-knolls within the Carboniferous Limestones as upstanding mounds in the topography (Figure 5), but, with the light fading fast, we didn’t linger too long and descended the hillside back to the Manifold Valley floor and returned to Wetton Mill.
Figure 5: Cross-section showing topography controlled by reef-knolls. Ossum's Hill is on the left. After The Hamps and Manifold Geotrail, Staffordshire RIGS 2007
Thanks go to the National Trust for organising the walk and Mike Allen leading it.
Upcoming National Trust events, including guided walks can be found here: Peak District National Trust Events.
The Hamps and Manifold Self Guided Geotrail leaflet is available from local tourist venues and downloadable from the Staffordshire RIGS website.