The Solid Geology and The Physical Landscape From Ingleton to Thornton Force

Aim:

To interpret the geology/landscape relationship.

Objectives:

To explore and investigate the routes between Ingleton and Thornton Force;

To measure and record features of bedding, jointing and dip strike;

Look closely at rocks and the different vegetation.

The Ingleton area is located in the Western Yorkshire Dales and the main feature of the region is the Askrigg Block which is immediately North East of Ingleton. Resting on this block is the 200m thick, almost horizontal Great Scar Limestone of Lower Carboniferous age.

Just to the North of Ingleton, the floors of the main dales cut down through the limestone to reveal tightly folded sediments of the older Lower Palaeozoic rocks which form the block itself. These inliers are stopped to the Southwest by the Craven Faults which trend Northwest to Southeast and have substantial downthrows on their southern side.

South of the faults exposures in the Craven Lowlands are not so abundant although around Ingleton the presence of the productive Coal Measures and some Red Beds suggest the stratigraphic level reached is Upper Palaeozoic.

This area has a long and complex tectonic history. It was established in Lower Carboniferous times and it marks the facies boundary between clean shelf limestone in the North and dirty basin limestone to the South. Movement occurred in this area throughout the Carboniferous and are considered to be of the Hercynian Orogency. There may have been further movements in the Tertiary and recently there have been weak earthquakes at Skipton and Settle; these may well have originated on the fault lines.

During the Pleistocene period, ice from the North moved over the entire region except possible the summits of Ingleborough and the other Peaks. The dales are great glacial valleys; a legacy of the action on this area. The landscape displays an abundance of morphological features attributed to the glacial action and the limestone pavements and cave systems testify to this.

The waterfalls trail follows a route through the Ingleton Glens first opened to the public by the Ingleton Scenery Company in 1885. The famous walk has some of the most spectacular waterfall and woodland scenery in the North of England; it begins at Ingleton village, then continuing upstream through Swilla Glen there are many changes in the solid geology and the physical landscape of the land, then on to the impressive cascades and waterfalls of Pecca Falls and Thornton Force.

The walk is 4.5 miles through ancient oak woodland and magnificent Dales. In fact, the natural history of the Glens is of such national importance that English Nature has designated a large part of them a Site of Special Scientific Interest (SSSI) in order to safeguard their future.

The best place to begin this investigation is location five; the car park at NGR 693731, located on the wide flat floodplain of the river on the Carboniferous coal measure series. The large rocks and boulder clay in the river suggest that the valley has been glaciated. Further upstream the geology of the land is interrupted, which suggests that there has been a fault as there is evidence of the limestone being faulted against the carboniferous coal measures (North Craven Fault).

En route to location four (Swilla Glen) showed a change in the type of rock which lies closest to the surface. This area is made of Carboniferous limestone and dips 25� to the south/southwest. There is a clear difference between the true dip and the apparent dip. This is due to the two faults close by, which has interrupted the original position of the limestone. At this point the fold of the limestone dips quite sharply, towards the bed of the River Twiss. These damp shaded conditions provide the ideal environment for ferns to survive. Here, rich moss can be found covering the walls of the limestone. Lighter patches, which tend to be dry, are colonies of lichens, containing algae and fungi. The route tracks over an anticline at the crest; there is no dip and then the limestone begins to dip gently to the north by up to 5�.

To the north of Manor Bridge, a cave marks the line of the North Craven Fault. This is location three and here, the Carboniferous Great Scar Limestone dips to the north at a gentle angle of 5� and is observed to the left of the cave. To the right of the cave is the dark shale of late Ordovician age. The youngest rocks can be found on the down throw of the fault.

For a short distance above Manor Bridge the Twiss follows the line of the North Craven Fault. This is the more conspicuous of the great earth fractures in the region. On the left, Carboniferous limestone remains clearly exposed while the right hand bank is underlain by dark shale which is about one hundred and eighty-five million years older than the limestone. Again at this point, the vegetation is very similar to that described at Swilla Glen, as the conditions in this gorge enables ferns, algae and fungi to develop and survive. The presence of this fault provides strong evidence as to why the Upper Ordovician shale gives way to the Lower Ordovician slates and sandstones.

Moving on to location two, The quarry exposes a thick sequence of lower Ordovician slates, but just upstream the slates alternate with beds of sandstone – these rocks have been folded up into the vertical position. On examination, the green slate is a greywacke; part of a turbidite sequence and laid down horizontally on the bed of the ancient Iapetus Ocean. Many ripple marks can be seen on the surface of the rock. The Coniston Slates and the Ingleton Slates (Lower Palaeozoic) are exposed. They have no steep dips and are faulted and folded into a syncline. Exposed by erosion they are uncomformably overlain by the younger Carboniferous Limestone. It was found that the changes in the rock type influence the scenery at this site.

At this same site, there is Pecca falls which consists of five main falls and they drop 30 metres over sandstone interbedded with slate. By further investigating the geology of Pecca Falls, the conclusion as to why there are five falls was quite simple; the falls were formed because this area was made from Ordovician slates and sandstone. In addition to this the beds of rock are nearly vertical and the slates are more easily eroded than the hard sandstone. The River Twiss therefore tumbles over the steps of the sandstone, while the plunge pools have been eroded from the slate.

It was found that the vegetation changed above Pecca Bridge from typical limestone woodland to that associated with more acidic conditions to an environment of bracken and heather which now indicate the change of rock to Ordovician slates and sandstones.

Finally, location one; here, the younger Carboniferous limestone lays uncomformably on the older Ingletonian beds and the slates first seen at Pecca Quarry are exposed again, but now they dip about 65� downstream. Originally laid as horizontal beds, the slates and sandstones were down folded in Silurian times, forming a syncline structure.

The Carboniferous limestone is the more resistant rock, which lies over the less resistant Ordovician slate; therefore the river Twiss leaps 14 metres over a rock structure which encapsulates the whole fascinating geology of the area. The upper part of the cliff consists of horizontally bedded Carboniferous limestone, whereas the lower part is of vertically bedded lower Ordovician slates. The junction between the two is interesting as the slates have been planed off and the limestone rests on the level surface. The beds above and below the erosion surface have an unconformable angle of dip. The vegetation of this high upland area is completely different from that not so far downstream; in the acidic limestone conditions (photograph 1).

After investigating the relationship between the solid geology and physical landscape from Ingleton to Thornton Force, it was found that over the last few hundred million years, many important changes to the landscape had occurred, with respect to folding and fault lines.

All of the evidence collected appeared to be quite reliable as it corresponded really well with the historical and geological background which has already been analysed. Comparisons between diagrams/photographs and the documented work from the field study, with the evidence, already collected from a geologist, there are certainly some similarities.

For example at Pecca Quarry/Falls, the presence of the North Craven fault provides strong evidence as to why the Upper Ordovician shale gives way to the Lower Ordovician slates and sandstones. The evidence can be in the form of a photographic image (see insert), actual geological case studies or hand-drawn diagrams. However, the hand-drawn diagrams have little reliability as the illustrator may miss out important and complex details.

Other factors, which may not have been completely reliable in the field study are, the distinguishing between true and apparent dip which was quite difficult at times, as the dip values and directions varied markedly over just a few metres. Not only that, but finding the height, width and depth of rocks proved to be difficult, which again, is why the evidence can only be a close approximation. Also it was difficult to observe the bedding planes and joints in the rocks. Finally, an important factor which should be considered is the fact that the students undertaking the investigation were somewhat inexperienced due to it being one of their first field studies.

The time of year may affect the reliability of the research as; if it had been completed in the winter – plant vegetation was at a minimum, therefore information would have been clearer and gathered easier.

When experimenting to find out how many faults were present in the area, it may have been more effective if some tests were taken in a diagonal line, (across the first fault found), and in an East-west direction.

The validity and completion of the field studies could have been measured more accurately with qualified personnel or an in depth understanding of the general area beforehand. Another way to improve the field study next time is to allow the students to experiment for themselves. For example; taking a bottle of acid, would enable the students to test and identify the different rock types (i.e. limestone reacts with acid).

Finally, by taking a clinometer, the students could collect their own data and at the same time improve their field study skills.