GEOLOGY

Leader: Derek Brumhead

Visit to Lathkill Dale.

Monday, 25 June 2012

[Click on images to enlarge]

On 25 June, 25 members of the two geology groups led by Derek Brumhead visited Lathkill Dale, a mile east of Monyash. The dale is over 4 miles long so a coach dropped us at the beginning and picked us up at end of the trail at Over Haddon. A selection of localities is described here.

Ricklow Dale (1). The party walked about l km down Lathkill Dale, with low crags of Monsal Dale Limestone (Carboniferous age) on either side, and then made a steep climb up the north side which took us up and over a hill made of a limestone mud mound  (base of Eyam Limestones).

In Ricklow Dale, tilted beds of bedded limestone can be seen lying banked against the margins of this mudbank, dipping about 20 degrees. These formed a fossil reef best seen in the adjacent Ricklow Quarry.

Section showing mud mound. Peter Gutteridge, Mercian Geologist, 15 (4), 2003

Ricklow Quarry (2). This quarry is famous for its ‘figured marble’ a highly fossiliferous limestone consisting particularly of crinoids (fossil sea lilies) which polish to a decorative stone (white fossils on a darker background). In some crinoid stems (seen in cross section) the central cavity has been partially infilled with sediment so that the top of the infill acts as a spirit level. Such partial infills are known as geopetals. Geologists use them for ascertaining the original horizontal direction in a rock which has been tilted later.

Lying around the quarry are many loose blocks of the limestone which have parallel drill holes which enabled the breakage of the rock along straight lines so providing a surface for polishing. Gill Westall reminded us that there are fine examples of this polished stone inside the Crescent at Buxton (Another field excursion when it opens !). Apparently, it also forms the doorstep of the Bull’s Head in Monyash.

Drill holes in limestone blocks in Ricklow Quarry

Large amounts of waste stone were tipped over into Lathkill Dale making vast screes characterised by the largest boulders being at the bottom. There is a superb view of Lathkill Dale from the top of one of these screes, which tumble down into the dale.

Lathkill Cave (5). The first part of the dale consists of an extraordinary narrow, rocky limestone gorge, very difficult under foot, the sides of which show evidence that it was eroded by water. The date of this is uncertain. Beyond the gorge, the path led past beautiful patches of ‘Jacob’s Ladder’ (the dale is famous for this blue wild flower) leading to Lathkill Cave. As a result of the recent very wet weather, a huge stream of water (the source of the river Lathkill) was pouring out of this cave draining the limestone upland for miles around. In dry weather, however, when the water table is low, this cave is dry and water does not then appear until further down the dale at Sheepwash Bridge (6). David Sims gave us an account of exploring this cave and its system.

Tufa Waterfall (7).This waterfall is encrusted with a thick bed of tufa formed by the precipitation of calcium carbonate from lime-rich water. The tufa encrusts the leaves, stems and moss growing on the waterfall. Tufa is a form of travertine which is often found forming banded deposits in freshwater lakes, and can be used as attractive ornamental stone (eg the entrance floor of Manchester Art Gallery in Mosley Street).

Mill Dale (9). This is a small tributary valley from the north and along it are fine quarried outcrops of the ’dark’ Monsal Dale Limestones. Derek explained that these limestones are among the youngest in the Peak District, the limestones, for instance, of Millers Dale and Tunstead being underneath us. The ‘dark‘ refers to the fact that the limestone has layers of chert, a black, extremely hard substance made up entirely of silica. Its origin is somewhat obscure, but it is thought that it is probably derived from the breakdown of millions of tiny siliceous microscopic animals called Radiolaria which lived in the Carboniferous seas. Somehow, this silica migrated into nodules or, as here, in bands, being deposited after the formation of the limestone. It is very similar to the flint of the Chalk, which is distinguished by the fact that it fractures in a coinchoidal nature, whereas chert has flat fracture.

Bateman’s House (13). Bateman was the agent for the Lathkill Dale Mining Company and this building was built in 1830 over a shaft which had an unusual water turbine pump (a disk engine patented by the Dakeyne brothers) for draining the mines upstream. It worked until the 1860s. So much water is still removed today from the limestone that it contributes to the drying out of the river. Climbing down into the shaft, with the aid of a ladder, one can turn the handle of a generator which lights up the sough (tunnel). The pump has long since been removed but there is an interesting board describing what it was like and how it operated. The site has been cleared and re-opened for public access by English Nature.

Mandale Lead Mine (15). Shortly before reaching the mine, the stone pillars of an aqueduct crossing the river once had a wooden trough which carried water to the Mandale Lead Mine where it drove a 35 feet diameter water wheel (erected in the late 1830s) used for pumping water from the mine. The leat leading to the aqueduct was taken off the river some distance upstream at Cowgate Weir (10). The water wheel was soon replaced by a Cornish-type steam pumping engine and the impressive bob wall on which the beam of the engine was supported is still standing. The mine closed in 1851, having lost a large amount of money. Near the mine entrance is the exit of a sough (tunnel) built to drain the mines. This feature chiselled through the tough limestone in the early nineteenth century took the miners 23 years (between 1797 and 1820) to drive this 2 kms into the dale side and one can only reflect on the huge labour and discomfort involved. But at least it was employment!

Two boulders (19). After passing two trial mine adits, the end of the trail is marked by two large boulders on which surfaces there is a fine display of many Gigantoproductus a fossil brachiopod. Derek explained that this shell bed was a death assemblage and in some cases, when seen in section, the shells are piled up on top of each other in a concave-up position like a pile of saucers.

Words by Derek Brumhead. Photographs taken by Pat Stanway and Derek Brumhead.