Chevin Forest Park GeoTrail #13: Variscan Orogeny
The 13th cache in the series, and the 6th of 8 EarthCaches, is located at the eastern edge of the abandoned quarry north of the Yorkgate car park.
Please refer to GCB7RA7 for the background information on the geology of The Chevin, which provides explanations of how and when the various rock formations were formed.
To access the cache location: make your way to N 53 53.576 W 1 41.925 on the main path heading north from the car park. Fork right here to head up a rocky path as far as N 53 53.603 W 1 41.889. Turn left and descend the grassy slope to the trail T-junction @ N 53 53.619 W 1 41.896 near the partially obscured Geology Trail carved marker stone #7, which depicts a stonemason’s hammer and chisel representing the long-past quarry activities. Turn right here and head up towards the cliff face and cache location.
Historical background:
Yorkgate Quarry was the last piece of land on The Chevin to be actively quarried. Like East Chevin Quarry, it has changed hands several times, finally finding itself under the ownership of Leeds City Council.
In 1891, farmers John and Joseph Hird gave permission to quarrymen William Clapham of Guiseley and William and Thomas Maston from Otley. The contract to extract sandstone from the quarry was for 2 years ending 1 October 1893 and included the requirement to keep the road in repair.
In 1949, planning permission was granted for the surface excavation of sandstone. Then the company employed 7 quarry workers, and blasting for the extraction of stone occurred about once a fortnight.
However, by 1967, there were local concerns about Aberford Quarries, the then owners, extending their activities. They had requested that the public footpath, which runs along the top of the quarry, be diverted to enable further extraction of stone. They wanted to extract stone over the lip of the summit and 150m down the slope. However, in 1949 permission to extend their quarrying was made with the proviso that they should not affect the outline of The Chevin.
A local ‘Save The Chevin’ campaign developed and funds were sought to oppose this request. From Between 1967-1973, local respected men mounted a successful campaign to prevent this happening. Fears were that further incursion into the hillside would destroy the outline of The Chevin and that blasting operations could affect West Chevin Road, water courses and cause a landslip. After a long and bitter battle, the Minister of Transport decided that the footpath would not be diverted due to its amenity value. This overturned the agreement made by West Riding County Council for the expansion of operations and diversion of the footpath. This probably led to it ceasing to be used in 1969 and its eventual handing over in 1987 to be added to the Park.
It has been restored since then and has a pond and some mature trees. It is hard to see the original quarry faces, but the cliff at Geology Trail Marker Stone #7 shows you what the rock was like and why it was so excellent for building purposes.
Maps from 1800 show many small-scale quarries across much of the scarp slope of The Chevin. Most of the stone was described by stonemasons as Bramley Fall Stone, although it is not as good as the gritstones that came from Bramley, Horsforth or Guiseley, which were favoured for finer-quality masonry work. The main use of Chevin stone was railway embankments, bridge footings, defensive sea walls, local houses and civic buildings, roofing slates and millstones.
Now for the fascinating geology . . .
Throughout the Carboniferous Period, the earth’s crust under what was to eventually become the British Isles, was being affected by earth movements caused by the shifting of two crustal (tectonic) plates. This reached a peak at the end of this Period 290 Mya when compressive forces caused the rocks deposited in the Pennine Basin to be tilted, folded, broken (faulted) and uplifted. Many earthquakes would have been generated as the rocks buckled and broke, and the resulting folds and faults vary in scale from centimetres to kilometres.
This mountain building period is called the Variscan orogeny and culminated in the uplift of a high mountain range across southern Europe, which affected south-west England more than the rest of Britain.
However, the brittle crust underneath northern England was also under tension, and Variscan plate movements caused it to fault and fold and be uplifted into the Pennine anticline (upfold). So, after the end of the Carboniferous period, the present Pennines were a range of hills trending north-south from the Midlands to southern Scotland.
At the same time, Lower Wharfedale was affected by this crustal pressure, so older, deeper layers of rock were arched upwards and younger overlying layers of Millstone Grit sloped away southwards.
The tilted and folded rocks formed an anticline which runs east-west across the Wharfe valley - the long escarpment running from Harewood (in the east) to Addingham (in the west) seen today.
Otley Chevin lies on the south side of the fold so the rocks slope southwards towards Airedale as seen in the sandstone bedding planes at Marker Stones #6 and #7. Later, across the higher parts, the agents of erosion began to strip away Coal Measures rocks, exposing the older, Millstone Grit Group rocks that lay below.
On the approach to the large quarry face, which (apart from the northern end where GZ is) is inaccessible from the footpath, the dip of the sandstone can be clearly seen, as can the change from very massive Doubler Stones sandstone in the lower part of the quarry to the more flaggy High Moor Sandstone at upper levels.
To Log the EarthCache
Please complete the following tasks and send your answers to me via the GC website messaging service or by email to forshaw.chris@gmail.com - thanks!
a) Look at the quarry face and the exposed rock beds. Are they horizontal or tilted? If tilted, what is the angle, and what might have caused this?'
b) Examine the rock face closest to the path. What does it look like and how does it feel?
c) You will see parts of the face where the outer surface of the rock has broken off, revealing the internal rock surface. How does this differ from that observed in b)?