A walk rich with varied geological and geomorphological interest

A short walk in the limestone country around Grassington explores the history of the rocks and the landscape. Rocks are visible all around: cropping out as “scars” on the hill-sides; exposed in the bed of the River Wharfe and in many quarries, both old and modern; and seen especially in the drystone walls and buildings. Understanding the rocks helps to explain the natural features of today’s landscape, but can also reveal hidden landscapes when the rocks were formed many millions of years ago and also much later, just a few thousand years ago, when the valleys and hills were shaped during and after the lce Age.

The geology of the upper Wharfedale area is recorded in the layers of rock laid down as sediments over immense periods of time. The oldest strata are not exposed here but were reached by a borehole at Kilnsey, at a depth only a few metres below the floor of the valley; they are sandstones and mudstones of Silurian age; a few boulders of these rocks may be found on our walk. Above them comes the dominant rock of the area – the Carboniferous Limestone. The lower, older part of this is called the Great Scar Limestone and forms many of the dramatic cliffs and gorges of the Yorkshire Dales.This massive limestone gives way, higher up the valley sides, to the Yoredale beds, consisting of thinner layers of limestone with intervening beds of shale and sandstone. Finally at the top are thick beds of coarse-textured sandstone, here known as the Grassington Grit.

The walk starts from a viewpoint in the Yorkshire Dales National Park car park overlooking the Wharfe valley. Looking west are two contrasting landscapes, separated by the line of the North Craven Fault, which runs westward to Malham and Settle. North of the fault are the limestone uplands, with pale-coloured rocky outcrops and green pastures; to the south brown moorland formed on the down-faulted Grassington Grit. To the left are seen the rounded limestone hills of the Cracoe reef knolls which are considered to be fragments of a reef built up in the Carboniferous sea at the boundary between a wide shelf with clear, shallow, life-rich water and deeper more muddy water in the Craven Basin to the South.

Downhill from the viewpoint we reach the river at Linton Falls where the intricate erosion of the Great Scar Limestone by the flowing water and its entrained load of gravel and sand can be seen. A short distance downstream in the left bank an outcrop of dark, organic-rich shale can be seen, part of the Bowland Shales, a rock abundant in the Craven Basin and of great interest to would-be frackers. lts presence suggests that, walking between the Falls and this outcrop we have crossed the North Craven Fault.

The walk then continues upstream with more exposures of limestone, notably in the rapids of Ghaistrills Strid but the chief interest centres on the Wharfe valley itself and the processes that have shaped it: erosion by ice and then by water, and also deposition of sediment in the valley bottom, from both the glacier and the river. The valley has been mainly formed by glacial erosion and this is most clearly seen upstream from the narrows near Netherside Hall. Here the valley has a flat floor and steep sides and is almost straight, the glacier having trimmed off any spurs. The undercut cliffs of Kilnsey Crag are attributed to powerful glacial erosion, just downstream from the confluence of the Wharfedale and Littondale glaciers. Low hills in the valley bottom near Chapel House Farm probably formed as a moraine during a pause in the wasting away of glacier; the wide, flat valley floor at Kilnsey is thought to mark the site of a temporary lake formed after further retreat of the glacier. The glacier was also able to transport large boulders, called erratics, plucked from the valley floor. A few of these can be recognised as coming from the Silurian basement, and are known to form a trail downstream from Kilnsey, as far as Threshfield and Linton.

 

Since the end of the ice age the river has further deepened the valley as well as depositing gravel and sand in its channel and silts over its floodplain. Four episodes of down-cutting and erosion have left behind relics of the old valley floor as terraces, the first of these about 15m above the present river level; several can be recognised during the walk, especially near the northern edge of Grass Wood.

The walk now turns away from the river and up into Grass Wood and then Bastow Wood. Limestone is close to the surface, cropping out as scars and as moss-covered pavement, with ferns and trees growing in the grikes. The pavements owe their origin to the glacier; strong beds of limestone were stripped of weaker overlying strata and debris but resisted attack by the moving ice. At the top of the woods a short diversion leads to a view of Dib Scar, a limestone cliff at the head of a steep narrow valley draining down towards the Wharfe, which is thought to have been eroded by glacial meltwater at the end if the ice age. Looking beyond Dib Scar, the hillside above has a strongly terraced structure, reflecting repeated cycles of deeper – to shallower-water conditions in the Carboniferous seas. Such cycles are common in higher parts of the Carboniferous and may be due to world-wide repeated sea-level changes caused by ice caps alternately forming and then melting on a large continental mass around the south pole.

The return to Grassington is along the wide upland shelf of Lea Green, its surface roughly coinciding with the top of the Great Scar Limestone. The path crosses several lines of shallow pits and spoil heaps, signs of very old lead workings, probably little more than prospection pits. On the spoil heaps it is possible to find small crystals of creamy white barytes and colourless, transparent fluorspar, minerals that commonly accompany the lead ore, galena. These failed mines are of course mere outliers of the once rich and productive ore field of Grassington Moor.

David Turner

Text by David Turner, images by Ian Hughes