'Rock and Roll on the Otley Chevin’
Finding inspiration to write a monthly nature notes can be challenging, but on this occasion, I don’t have to look far – it’s the view from our study window. Living high on the valley side close to the Hermit pub and Burley Moor it is a view I have not tired of in the past 32 years. Eight miles away across the dale is Almscliff Crag, and if I turn to the right the steep wooded slopes of Otley Chevin come into view. Both are prominent geological features of lower Wharfedale, but it is the latter that grabs my attention.
As a teenager I was fascinated by the sciences of geology and botany, although in the end chose neither as my career. I was given a geological hammer for Christmas aged 12, how sad and geeky is that? Now retired I am rekindling those passions of my youth .
I am taking a closer look at the geology of the Chevin. There is no better way than with the help of Bill Fraser from the Leeds Geological Association Geo-trail. The rocks that make up the Chevin are 315 million years old. They were laid down in the Carboniferous period 360 to 300 million years ago, when Wharfedale lay close to the Equator.
Older Great Scar Limestone lies to the north-west forming much of the southerly Yorkshire Dales., while younger Coal Measures lie to the south-east in the Yorkshire coal field. In between these older and younger rocks are layers of Millstone Grit and sandstones, alternating with mudstones or shale which form the rocks of the Chevin, and much of lower Wharfedale. These were laid down in a large complex delta, with rivers draining from mountain ranges to the north.
Initially laid down horizontally, the sands and muds have been compressed to form the rocks we see today. In the late Carboniferous period, they have been folded and tilted by continental tectonic movements. This can be best appreciated by examining the profile of the Chevin. The steep heavily wooded north-facing ‘scarp’ slope overlooks Otley, while the gentler ‘dip’ slope faces south, running downwards to Guiseley and Yeadon. Alternatively, you can examine the tilted bedding planes more closely by visiting Surprise View overlooking Otley town.
Millstone grit is a coarse-grained sandstone. The name derives from its use in earlier times as a source of millstones for use principally in watermills. Take a closer look with a hand lens. The harder light-grey quartz crystals (silicon dioxide) are cemented together by the softer pink, white or cream coloured feldspar (aluminium silicates). Weathering and erosion removes the softer feldspar leaving the quartz crystals standing proud, feeling like old fashioned sandpaper, and ideal for use as millstones.
It is unlikely that the Millstone Grit on the Chevin was ever used for millstones but was favoured both as a building stone and aggregate for road construction. Park at East Chevin Quarry car park and head west following the geo-trail and you will pass multiple crags, from former quarrying in a layer of rock known as Addingham Edge grit. These rocks are heavily jointed, making it easier for stonemasons to carve building blocks. The red staining comes from iron deposited after the rocks had already formed.
If you follow the self-guided walk you are asked to examine rock faces en route. This will show the ever-changing environment of the delta over several million years. Sometimes rocks were deposited quickly with each tide, and there are exposures of unusual tidal laminites. The environment at the time would have looked much like Morcombe Bay does today.
On other occasions sandbanks were formed in river channels, with subsidence and sand avalanching down the steep sandbanks giving us cross-bedding, seen on multiple rock exposures. The cross-bedding is at a completely different angle compared to the parallel bedding planes.
The size of grains or crystals in the Millstone Grit can also give us an idea of speed of flow within the river at the time of deposition. Slower rates produced the finer grains of the Addingham Edge grit. When the trail takes you up to Surprise view, the rock layer known as the Doubler Stones sandstone contains large pebbles indicating it was deposited in a much faster flowing river.
The landscape is not just a result of the underlying rocks. We have already discussed the folding and tilting caused by tectonic plate activity. More recently there has been glaciation, the last ‘Devensian’ ice age reaching its peak 18,000 years ago. It not only scoured and widened Wharfedale but deposited glacial till or boulder clay in the valley bottom, which has since been excavated resulting in the four lakes (former gravel pits) to the north-west of Otley.
Following glacial recession, the steep scarp slope was not wooded like today but initially had a tundra-like vegetation. The subsequent trees have helped stabilise the slope, but 10,000 years ago when there were none this resulted in slippage between the layers of harder sandstones and grits and the softer mudstones, and accounts for the large number of old landslips. These can be seen today as you peer down the slope through the woods towards Otley.
There is much more I could tell you, geology after all is a fascinating science. It gives us the foundation for the different soil types, which in turn results in different plant habitats, which will influence the wildlife from the smallest insect to the largest mammal. Next time you are out walking, don’t just look at the wildlife and landscape, think about the topography and what rocks lie beneath, it is truly fascinating.
Note: Paper leaflets of the Otley Chevin geo-trail can be obtained from the tourist office in Otley town centre. Or alternatively it can be downloaded from the Friends of Otley Chevin Forest website – https://www.chevinforest.co.uk/visit/self-guided-walks/ (geology trail) There is also an audio version, but I think you probably still need to combine this with the print copy which contains valuable diagrams and images.