Chevin Forest Park GeoTrail #6: Great Dib Landslip
The 6th cache in the series, a camo-taped 35mm film canister, is hidden a short distance from a trackside viewpoint from which the landslip can be seen down the steep slope through the trees. It is also near the Geology Trail carved marker stone #3.
For background info on the Chevin Forest Park and the series, including a map showing parking options, trails and cache locations, see GCB7RA1.
About 310 million years ago continental collisions crumpled and stacked up layers of rocks in this area. Older, deeper layers of rock were arched upwards and younger overlying layers of Millstone Grit sloped away southwards, forming a long escarpment from Harewood (in the east) to Addingham (in the west).
At the viewpoint you will be standing on Long Ridge Sandstone. The steep slope which drops down to Otley has been caused by land-slipping when the climate was still very cold but after the ice sheet had melted from the top of The Chevin towards the end of the last ice age 14,000-12,000 years ago.
There was probably still ice in Wharfedale, so there would have been very little vegetation to stabilise the slope. During the winters water in the rocks and subsoil would have frozen but in the summers it melted and the sandstones at the top of the slope were then able to slide down a weakness in the lubricated mudstones below.
The whole length of The Chevin was prone to land-slipping because of the steep northern slopes. In places along The Chevin large boulders have slid down-slope and the uneven ground can be seen under and between the trees – if often somewhat concealed by vegetation.
Trees reduce the likelihood of further land-slipping because they take water from the ground reducing the lubrication in the soil. Their roots also stabilise the sub-soil and penetrate joints in the solid rock further holding them in place.
This area is also the location* of the 1.1 ha Great Dib Wood SSSI (notified in 1981), a natural cliff which provides a 26m exposure of rock strata of the middle Namurian Series of Wharfedale, formed during the Carboniferous Period of geological history about 320 million years ago.
*This is given in the notification as SE 199443 (N 53 53.667 W 1 41.917) and described on the Chevin Forest website as ‘in an area not easily accessible to the public’. I have been to this location which is in the middle of a moderately steep bushy slope with not a cliff in sight! Having explored the area thoroughly (to the extent possible given the steep terrain to the north), I suspect that the site is most likely some 100m to the north-east where the lands falls off dramatically exposing substantial cliffs just visible from the slopes to the west through the trees.
Here two sandstone layers are exposed in 26m of strata, separated by a layer of mudstone and limestone known as the Otley Shell Bed which is rich in the fossilised remains of the animals that inhabited the sea at that time. This fossil-rich bed is of great geological interest principally because of the ‘extremely diverse assemblage of marine animal fossils’ it contains (including brachiopods , bivalves , gastropods , ostracods , corals , polyzoa and fish fragments), but also because it is one of the youngest rock-layers known to contain the remains of a now-extinct group of animals known as trilobites – one of the most successful of all early animals with >22,000 species, existing in oceans for almost 270 million years.
The preservation of the shelly fossils is particularly good, since the sediment here has been subjected to relatively little compaction. This is thought to have been a consequence of the early cementation of the layer.
See here for a comprehensive and fascinating (if you like such stuff!) paper on the multiple landslides on The Chevin which includes good information on the local and regional geology.