Excursion to the Bathgate Hills
By David Stephenson
|For:||Open University Geological Society (East of Scotland) Sunday 22nd September 2013|
|Leader:||David Stephenson, British Geological Survey, Edinburgh|
The Bathgate Hills owe their elevation to resistant late-Visean to early-Namurian (Brigantian to Arnsbergian) basaltic lavas and latest Stephanian (or earliest Permian, depending on your timescale) quartz-dolerite sills and dykes. The intrusions are very well exposed and the lavas are moderately well exposed in places. Thick sequences of pyroclastic rocks and volcaniclastic sedimentary rocks, mostly below the lavas, are rarely exposed but are known from boreholes.
The volcanic rocks constitute the Bathgate Hills Volcanic Formation. They are thickest (up to c. 600 m) and have their widest stratigraphical range to the west of the Bathgate Hills, where their extent coincides with large, high-amplitude, positive gravity and magnetic anomalies — the Bathgate Anomaly. However, although they undoubtedly contribute to the anomaly, the volcanic rocks alone cannot account for such large amplitudes and it is thought that there must be a large mass of underlying basic intrusions extending to a depth of about 8 km.
Sedimentary strata intercalated with the volcanic rocks, between the top of the West Lothian Oil-shale Formation and the top of the Upper Limestone Formation, are exposed only in limestone quarries, some of which are extensive, but during the summer in particular many are overgrown.
The regional dip is 20–30° westwards on the eastern limb of the Central Coalfield Syncline. Dominant faults trend E–W and many of those are followed by dykes of quartz-dolerite. This imparts a dominant E–W trend on the topography, which has been emphasised by west to east ice movement during the Main Late Devensian (Dimlington) Stadial.
Unfortunately the overall quality of exposure in the Bathgate Hills seems to have deteriorated considerably since I worked there in the early 1980s and since publication of the field excursion by Stephenson and Monro in Lothian Geology in 1986.
1 East Kirkton Quarry
(SSSI so no hammering please)
Park on the road, on the edge of the housing scheme, at NS 990 689 and walk north, across the grass, into the overgrown quarry (better than it looks at first).
This sequence, dominated by freshwater limestone, is undoubtedly one of the most internationally significant geological localities in the British Isles as a result of the spectacular, largely terrestrial fauna, which includes a scorpion, millipedes, a harvestman, a mite, eurypterids and four groups of amphibians that are the earliest known tetrapods adapted to living on land and hence are of great evolutionary importance.
The exposed sequence is 9 to 19 m thick and its fault-bounded known extent along strike is only 200 m. It lies in the upper part of the Hopetoun Member of the West Lothian Oil-shale Formation and is thought to have been deposited in a temporary and localised lake within a low-lying, heavily vegetated tropical landscape, close to sea level. It comprises mainly finely laminated limestone, with some nodular, spherulitic and massive limestone, interbedded with black carbonaceous mudstones, thin ironstones and thin beds of ‘tuff’. Stromatolites, pseudomorphs after gypsum and thin lenses of coal occur locally. Although pyroclastic components have been recognised, the majority of the ‘tuffs’ are actually volcaniclastic sedimentary rocks; hence the volcaniclastic debris was washed into the lake along with diverse plant remains and possibly at least some of the animal remains. There is little evidence of contemporaneous volcanic eruptions (e.g. ash-fall deposits) but siliceous laminae and lenses of chert might be the products of hot springs and fragments of charred wood suggest forest fires, possibly resulting from volcanic activity. Were the animals driven to their deaths in the lake by the fires?
At the top of the limestone sequence is a fissile lacustrine mudstone, up to 1.85 m thick, with horizons of ironstone nodules. It contains a variety of fish (notably absent from the underlying limestone sequence), ostracods and bivalves, suggesting a connection with a larger lake that is known to have existed to the north (Lake Cadell). The mudstone is still exposed, but the overlying volcaniclastic rocks are probably no longer seen.
The tetrapods and other terrestrial fossils were first found in 1984 by the commercial collector, Stan Wood and subsequent bed-by-bed systematic logging and sampling of the site was carried out between 1987 and 1992 by a team from the National Museums of Scotland, who distributed the finds among internationally recognised experts in other institutions. Their combined works were published in a special issue of the Transactions of the Royal Society of Edinburgh: Earth Sciences (Rolfe et al., 1994). Although overgrown and degraded to some extent, the type sections are still visible.
2 Petershill Quarry
(SSSI so no hammering please)
Park, with due consideration for the residents (difficult), in the small housing scheme around [NS 983 693]. Walk back to enter the SWT Nature Reserve at the SW corner of the former Petershill Reservoir. There is also a small parking area for one or two cars on the north side of the main road at [NS 986 695]. Please do not park in the entrance to the farm track at [NS 984 694].
This is the southern limit of outcrop of the marine Petershill Limestone, thought to be equivalent to the combined Main and Mid Hosie limestones of the Lower Limestone Formation, which was the subject of a PhD by Jerry Jameson in 1980. Jerry unearthed a fascinating story of facies changes around a volcanic island, which unfortunately was never published in an easily accessible journal (but see Jameson 1987 in reading list). Here we have the outer, reef facies, with an abundant fauna of solitary corals, colonial corals (Lithostrotion junceum), spinose productid brachiopods, echinoids, crinoids, bryozoa and sponges.
Bedding surfaces on the east side of the pond have abundant productids and colonies of Lithostrotion. The south wall of the quarry, beneath the road, formerly exhibited a spectacular section through a bioherm composed largely of skeletal remains in three recognisable zones that were interpreted in terms of shallowing with regression and eventual emergence. An initial transgression is/was marked by carbonaceous mudstones with Lingula near the base of the section and two periods of subaerial exposure are/were marked by erosion surfaces. This section was so spectacular that ?SNH (or was it the National Museums of Scotland?) paid for it to be sand-blasted in order that it could be better studied and displayed. Unfortunately most of the section is now obscured by piled-up debris from dredging of the pond and little now remains to be seen. This is the subject of ongoing negotiations with SWT but with little hope of resolution.
Farther north is an almost continuous line of quarries in the Petershill Limestone, which can be seen on the left as we drive to the next locality. These quarries record a progression northwards from the reef into a near-shore turbulent zone and then more-argillaceous limestones, representing a back-reef lagoon environment with anaerobic conditions that favoured syngenetic mineralization.
3 The Knock
Park in a parking area, below The Knock at [NS 990 711].
The Knock is part of an outcrop of quartz-dolerite that trends NNW and dips steeply ENE at 60°. This appears to be part of an inclined dyke, up to 40 m wide, which extends from Boghall on the eastern edge of Bathgate to Wairdlaw in the north. It is, in fact, a major step-like transgression in a major sill of the Midland Valley Sill-complex. The sill is generally concordant with the bedding of the sedimentary and volcanic succession and hence dips west at 20–30°, so the dip of 60° eastwards at e.g. the Knock is at right angles.
In the back wall of the car-park quarry, the western chilled contact of the ‘dyke’ is well exposed; it cuts basalt lava, dipping NW at about 20°.
The summit of The Knock, with orientation table, is a splendid view point. To the north and south we can see continuations of the ‘dyke’ across the eastern edge of Cairnpapple Hill and Raven Crag respectively, and we can discuss the dominant E–W features of the Bathgate Hills in relation to faults, quartz-dolerite dykes and ice movement. More-distant views are full of even more-varied geological features.
4 South Mine and Silver Mine quarries
Walk north from the Knock and cross the road to go down into South Mine Quarry. Here the argillaceous facies of the Petershill Limestone (the Reservoir Member of Jameson, 1987) is overlain by mudstones, siltstones and sandstones (the upward-coarsening, deltaic Silvermine Member of Jameson, 1987). In the west wall of the quarry, a thin N–S-trending dyke of tholeiitic basalt is heavily altered to ‘white trap’.
In the west wall of the flooded Silvermines Quarry to the north are the entrances to two adits, which mark two branches of a lead vein that was worked first in the late 18th Century and then by these adits in the 19th Century. Boreholes in the 1980s in this area and to the north at North Mine Quarry have shown that the argillaceous facies of limestone is enriched in syngenetic lead and zinc. The mineralization is particularly concentrated in the lower part of the limestone, in a 1 m-thick carbonaceous mudstone at its base and in seat rocks developed in the top 2 m of tuffaceous rocks below.
5 Hilderston Mine and Cairnpapple Hill
Little now remains to be seen of the Hilderston Mine. It was in operation initially from 1606 to 1614 as a silver mine but made little or no profit after the first two years (i.e. after it had been ‘nationalised’ by King James VI but, to be fair, the silver-bearing vein had almost certainly been worked out by then). Three depressions just west of the road mark the site of the main group of early 17th Century shafts but their preservation is the result of re-excavation in the mid and late 19th Century. In the 18th Century, the vein was reworked at deeper levels for lead and zinc and a second, much longer vein, some 60 m to the north was also worked until 1772. A large mound east of the road marks the main shaft on the northern vein. The original workings were re-excavated during the period 1865 to 1873 and again from 1896 to 1898 but no further economic deposits of silver or lead/zinc were discovered. The 1873 shaft is probably marked by a pile of debris, crescentic in plan, to the south of the burn and 35 m east of the road.
The vein was worked first in the siltstones and sandstones above the Petershill Limestone and on the margin of a thin E–W-trending dolerite dyke. There the silver occurred as filaments of native silver in a gangue of barite and niccolite. It extended for only 80 m E–W and for only 18 m below the surface. At deeper levels, exploited later, where the vein cut the Petershill Limestone, lead and zinc occurred within a barite and calcite gangue.
The full list of known minerals from the mine is: barite, calcite, dolomite, quartz, galena, sphalerite, niccolite, erythrite (‘nickel bloom’), annabergite (‘cobalt bloom’), bravoite, pyrite, chalcopyrite, albertite (solid hydrocarbon) and native silver. Most of these, with the exception of the nickel, cobalt and silver minerals, have been obtained from the waste heaps.
A diversion could be made to the Neolithic burial mound on Cairnpapple Hill (entry fee for non Historic Scotland members), which is also a good viewpoint.
6 Climbing wall near Beecraigs Reservoir
From the car park west of Beecraigs Reservoir, at [NT 007 742], walk south along the road for 300 m to an old quarry on the west of the road, which has been developed as a climbing wall. The face exposes a section across an E–W dyke of quartz-dolerite with some horizontal columnar jointing. Look for a finer grained margin on the north side.
7 Hillhouse Quarry
Return to the main Beecraigs Country Park car park at [NT 007 746]. Walk 300 m NNW up the road to a T-junction.
The quarries, on both sides of the road, are horribly overgrown and potentially dangerous but just north of the road the Hillhouse Sill can be seen overlying and in contact with the top of the Hillhouse Limestone.
The sill is very fresh and is the type locality for the ‘Hillhouse type’ of basalt as defined by A.G. Macgregor in 1928 in his local classification of basaltic rocks, which plagued maps and publications throughout central and southern Scotland almost until the end of the 20th Century. The ‘Hillhouse type’ contains microphenocrysts (less than 2 mm) of olivine (brown pseudomorphs) and clinopyroxene (black). The rocks are commonly nepheline-normative (i.e. they are Si-undersaturated) and some are basanite in composition.
The Hillhouse Limestone, 9–12 m thick, is considered to be equivalent stratigraphically to the Petershill Limestone, though their outcrops are probably not continuous along strike, having been deposited on opposite sides of a volcanic island. It is characterised by beds rich in crinoids, solitary and colonial corals and productid brachiopods. It dips west at 30 – 40° and was formerly worked both by quarrying and by stoop and room in underground galleries. At the north end of the quarries the entrances to the galleries can be seen but access is precarious owing to the steep dip and slippery bedding surfaces. There have been roof collapses, some of which have caused subsidence in the overlying road. DO NOT ATTEMPT TO ENTER.
Cameron, I.B., Aitken, A.M., Browne, M.A.E. and Stephenson, D. 1998. Geology of the Falkirk district. Memoir of the British Geological Survey, Sheet 31E (Scotland).
Jameson, J. 1987. Carbonate sedimentation on a mid-basin high: the Petershill Formation, Midland Valley of Scotland. 309-327 in European Dinantian environments. (Geological Journal Special Issue No. 12). Miller, J., Adams, A.E. and Wright, V.P. (editors). (Chichester: John Wiley and Sons Ltd.)
Rolfe, W.D.I., Clarkson, E.N.K. and Panchen, A.L. (editors) 1994. Volcanism and early terrestrial biotas. Transactions of the Royal Society of Edinburgh: Earth Sciences Vol. 84, parts 3 & 4.
Smith, R.A., Stephenson, D. and Monro, S.K. 1994. The geological setting of the southern Bathgate Hills, West Lothian, Scotland. 189-196 in Rolfe et al. op cit.
Stephenson, D. and others 1983. Polymetallic mineralisation in Carboniferous rocks at Hilderston, near Bathgate, central Scotland. Mineral Reconnaissance Programme Report, Institute of Geological Sciences, No. 68.
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