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| A quarry exposure of the Carboniferous-Permian unconformity |
Continuing the Sheffield U3A Geology Group field trip in Knaresborough on Abbey Road, having walked along Waterside to Low Bridge via Knaresborough Castle, our next stop was the old Bunker's Hill Quarry in the Sprotbrough Member of the Cadeby Formation, which is now occupied by the Low Bridge caravan park.
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| Bunker's Hill Quarry |
Looking from a distance, the form of the massive, cross-bedded oolites can be clearly seen in the south-west face of the quarry, which the 1934 edition of the 1:25,000 Ordnance Survey map - where it is named Low Bridge Quarry - shows as having lime kilns.
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| The House in the Rock |
Passing the House in the Rock, which can be seen at the top of the cliff, the Grade I Listed Chapel of Our Lady of the Crag is excavated into yellow sandy limestone in the lower part of the Sprotbrough Member, which is composed of large sub-aqueous, oolite sand waves.
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| The Chapel of Our Lady of the Crag |
This chapel dates from the early C15, with late C17 and early C18 alterations and, although we could only see this at a distance from the roadside, with my zoom lens I could see the figure of a knight to the right of the door, which was probably made between 1695 and 1739.
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| A cliff exposure on Abbey Road |
A little further down Abbey Road, we gathered in the layby to have a closer look at the large scale cross-bedding and the very sandy lower part of the cliff, where I prised off a very small specimen of yellow limestone, which in places has distinegrated into an extremely fine grained orange silty material that I scooped into a sample bag.
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| Samples of yellow limestone and orange silt |
Taking care on the narrow pathless road, we followed Abbey Crags and, as instructed by the version of the original walk by Anthony Cooper we were using - provided by Bill Fraser of the Leeds Geological Association - we looked for the Carboniferous-Permian unconformity behind No. 51a Abbey Road and, although the vegetation covered most of this, we did get a glimpse of the Upper Plompton Grit and the mainly obscured Cadeby Formation above.
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| An exposure of the Upper Plompton Grit |
Our next stop was the first of two old quarries on the east side of Abbey Road, opposite The Abbey, which was overgrown in places but there were some rock exposures where both the variegated yellow and red Upper Plompton Grit and the overlying Cadeby Formation could be seen - separated by the unconformity
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| Exposures of the Carboniferous-Permian unconformity |
Retracing our steps back to Abbey Road, we continued for a short distance until our leader Dave, who had not found it easy to find some of the sites on his recce, took us to another quarry where there are more extensive quarry faces, which expose the unconformity between the Upper Plompton Grit and the Cadeby Formation.
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| Upper Plompton Grit overlain by the Cadeby Formation |
Just beyond this quarry, a roadside building is part of a group that is marked on old Ordnance Survey maps as a smithy, which is built out of red angular and poorly sorted Upper Plompton Grit that is presumably from one of these quarries. In one place, a block with pebbles displays graded bedding that shows that it has been placed upside down and not on its natural bed, as when removed from the quarry face.
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| Upper Plompton Grit at the smithy |
The next part of the walk passed modern houses and, except for an excellent view of the yellow limestone of the Cadeby Formation at Grimbald Crag across the River Nidd, there was nothing of interest to see until we reached Saint Robert's Cave, where we had lunch.
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| A view of Grimbald Crag from Abbey Road |
The Grade II* Listed cave and the foundations of an adjoining or outbuildings are associated with Saint Robert the Hermit (1160-1218), but there are no datable features. The guide that we were using suggests that the cross-bedded, oolitic dolomite exposed here probably represents the Wetherby Member of the Cadeby Formation and not the Sprotbrough Member, which we had seen elsewhere on our walk to date.
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| Saint Robert's Cave |
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