Muiravonside Country Park - an excursion

thumbnail From: Browne, M.A.E. and Gillen, C. (editors) 2015. A geological excursion guide to the Stirling and Perth area. Edinburgh: Edinburgh Geological Society in association with NMS Enterprises Limited. Buy the book EGS.

Figure 4.1 Geological map of the area around Muiravonside Country Park, showing localities for Excursion 4.

Figure 4.2 Vertical section of the Passage and Upper Limestone formations in the Muiravonside area.

Plate 4.1 Locality 4.1. Base of quartz-dolerite sill in River Avon.

Plate 4.2 Locality 4.5. Adit to Castlecary Limestone, just upstream in River Avon from Union Canal Aqueduct.

Plate 4.3 Locality 4.6. Weir built on sandstone at base of Passage Formation above Castlecary Limestone, River Avon, downstream of Union Canal Aqueduct.

Excursion 4 Muiravonside Country Park

By Mike Browne

Purpose:	To examine marine rocks of the Upper Limestone Formation (Clackmannan Group) including the Orchard Beds, the Calmy Limestone and Castlecary Limestone; to inspect marine and fluviodeltaic rocks of the Passage Formation; to view a component of the Midland Valley Sill-complex, consisting of quartz-dolerite and related felsic veins.
Logistics:	Accessible by coach or car and with adequate parking, Muiravonside Country Park is on the south side of the B825 midway between the Bowhouse Roundabout junction with the A801 and the turn off to the village of Whitecross, just west of Linlithgow. The entrance to the park is on the south side of the road, at [NS 9595 7590]. It is reached from Stirling by leaving the M9 Motorway at Junction 4 east of Polmont to join the A801. Toilet facilities are available. This is an easy half-day walk. Locality 4.3 on the south bank of the Avon is accessible by fording the river at low water only. Wellingtons are essential if wading the river; boots with good grip are required.
Maps	OS 1:50,000 Sheet 65 Falkirk and West Lothian; OS 1:25,000 Sheet 349 Falkirk, Cumbernauld & Livingston; BGS 1:50,000 Sheet 31E Grangemouth; locality map (Figure 4.1).

The postglacial incision of the River Avon has cut the Carribber gorge and allows the examination of exposures of some of the upward-coarsening, cyclic marine and deltaic sedimentary rocks of the Namurian Upper Limestone Formation. The Orchard Beds, Calmy Limestone and Castlecary Limestone cycles are accessible, and within them marine fossils may be found. The section beneath the Union Canal Aqueduct displays marine and fluviodeltaic sedimentary rocks of the overlying Passage Formation. Intruded into the Carboniferous sedimentary rocks is a component of the

Midland Valley Sill-complex. There are many exposures of this quartz-dolerite on the trails beside the river. It forms the walls of Carribber Glen below the ruins of Rab Gibb’s Castle. Grain-size variations, jointing and felsic segregation veins are well displayed.

Locality 4.1 [NS 9638 7536] Carribber Glen, west: Midland Valley Sill-complex

From the northern entrance to the park follow the main access road southeastwards towards the ruins of Muiravonside House and the visitor centre at the Home Farm. Past the bridleway, take the footpath on the right and follow it down to the riverside. First, turn right to head westwards up the river. In a short distance, quartz-dolerite is encountered in the bed of the river, just before where the valley opens out to a floodplain and rock exposures cease. Here fine-grained, amygdaloidal quartz-dolerite may be seen. The white calcite amygdales are usually less than 2mm across. The rock is jointed, the closer spaced striking around 185° and dipping steeply west. Less important subvertical joints strike about 090° and 142°. Pink or grey, banded, microgranitic veins usually from 1–6mm across are present and strike about 180°, 135° and 230°. Next, head eastwards down the river (along Carribber Glen). About 50m downstream is an exposure of pegmatitic dolerite in which the white, rotted feldspar crystals exceed 10mm in length. According to Robertson’s zoning scheme for dolerite sills (Robertson & Haldane, 1937), such coarse-grained rock is found in the upper third of the sill’s thickness. Following the upper of the two paths downstream from just south of the last exposure, jointed, commonly spheroidally weathering, usually medium-grained dolerite (2–3mm) is exposed discontinuously in the cliffs. The sill may be about 60m thick. Radiometrically dated elsewhere as being about 306 million years old, the most recent German studies on the ages of Carboniferous successions indicate that the sill should be regarded as being latest Carboniferous.

Locality 4.2 [NS 9686 7533] Carribber Glen: Orchard Beds

Follow the riverside path downstream (continuing on the upper of the two ‘low’ paths), the exposures of dolerite cease and poor exposures of sandstone may be seen if the vegetation permits. At the duckboards and just before the path curves markedly northwards, exposures of at least 4m of bedded, grey and dark-grey, heavily weathered mudstone are readily apparent. Decayed irony or limy nodules are present. At and below the level of the raised duckboards, the gently dipping mudstones contain marine fossils. It would be necessary to shift large quantities of dead leaves and rock debris to expose two beds of grey fine-grained limestone. The upper bed, about 5cm thick, is shelly and is separated by 19cm of mudstone from the slightly crinoidal lower bed, which is about 33cm thick (at the east end). Just below the level of the path and about one metre below the limestones is an exposure of off-white, rooty, fine- to medium-grained sandstone. The mudstones and limestones are marine shelf deposits. The Orchard Beds are one of nine major marine transgressions in the Upper Limestone Formation and can be recognised widely throughout central Scotland. The fauna collected at this locality includes ‘Zaphrentites’ sp., Fenestella sp., Buxtonia sp., Eomarginifera sp., Orbiculoidea sp., Productus cf. carbonarius, Rugosochonetes sp., Spiriferellina sp., Euphemites ardenensis, Myalina sp., Palaeoneilo mansoni.

Locality 4.3 [NS 9689 7513] Carribber Mine: Calmy Limestone

Retrace the low path to the riverside and, assuming low water, cross the Avon at the west end of the island. On the south side, follow the river bank eastwards until a major cliff exposure of bedded, weathered, grey and dark-grey mudstone is reached. The Calmy Limestone is easily identified because of the presence of an adit driven into the hard limestone. The Calmy Limestone is usually fine grained, pale grey or grey in colour and, as the section below indicates, in two (or more) leaves.

	Thickness (m)
Siltstone grey, thinly bedded, sandstone bands 1–5cm thick, carbonaceous micaceous layers, planty layers, some septate burrows	5.00
Gap (much mudstone in soil)	4.00
Mudstone, silty, grey, thinly bedded, siltstone bands, irony and calcareous nodules, some plant remains, rare marine shells	4.00
Limestone, medium-grey, thickly bedded, silty in part, shelly	0.12
Mudstone, calcareous, bedded, dark-grey	0.15
Limestone (of mine), medium-grey and paler grey, in bands up to
25cm thick, thinly bedded in places, rare shells, pyritic nodules, some crinoid columnals	0.68

The Calmy Limestone is part of another major marine sedimentary cycle that can be recognised throughout much of central Scotland. The geographical extent of these and other major marine units suggests that the contemporary sea level rose rapidly by many metres, drowning terrestrial sediments beneath. Major marine transgressions of this nature have been attributed to the melting of major ice caps in Gondwanaland, a megacontinent then situated at the South Pole. The Calmy Limestone is a stratigraphical marker used in exploration for the Upper Hirst Coal, which was formerly mined at Castlebridge Colliery [NS 9405 9260] to supply high-ash coal to Longannet Power Station. The station [NS 9530 8530] is on the north shore of the Forth near Kincardine Bridge. The Upper Hirst (Wood) Coal underlies the mudstones containing the Calmy Limestone. High sulphur content in coals is usually attributed to the close association between the original coal-swamp peat and the overlying marine muds (oxygen-starved sea-bottom sediments) immediately above. The fauna at this locality includes Serpuloides sp., Eomarginifera sp., Lingula cf. straeleni, Rugosochonetes celticus, Schizophoria sp., Spiriferellina sp., Aviculopecten sp., Palaeo-neilo?, Parallelodon cf. semicostatus, Schizodus aff. aequilateralis, Streblochondria sp.

Locality 4.4 [NS 9681 7559] Old Mill: baked sedimentary rocks

Return to the Country Park by fording the river at the same point. Follow one of the paths northwards, passing the Gardens Cottage as far as the Old Mill. Just downstream of the Mill, a riverside cliff exposes about 5m of dark-grey siltstone passing up into sandstone. The siltstones are somewhat micaceous, thinly bedded, nodular and contain graphitised plant remains. These rocks have therefore been somewhat thermally metamorphosed by the intrusion of the quartz-dolerite sill. The metamorphic aureoles above and below the quartz-dolerite sill are much thicker than those above the earlier Carboniferous ‘teschenitic’ alkali olivine dolerite ones. This is because the host sedimentary rocks were much more mature by latest Carboniferous times, having lost most of the pore water originally present in the unlithified sediments. The prior loss of this pore water appears to have promoted metamorphism (Raymond & Murchison, 1991).

Locality 4.5 [NS 9664 7579], [NS 9650 7579] and [NS 9650 7589] Mine adit, shaft and limekilns

Continue to follow the bridlepath until the adit is reached (Plate 4.2). The adit is below the level of the path and is stone arched. Although the entrance is open, the roof of the mine has been brought down deliberately. The adit drained water from mine workings in the Castlecary Limestone. There is no surviving plan of these workings, which were almost certainly by stoop and room methods like those that can be seen underground in the Birkhill Fireclay Mine (McAdam & Clarkson, 1996). From the adit, follow the bridle path westwards to the site of the old mine shaft (also to the Castlecary Limestone) and to the limekilns where the limestone was burnt. The finished products were probably shipped out on the Union Canal, which is just to the north of the limekilns (at the dry dock). Return to the adit.

Locality 4.6 [NS 9672 7590] Union Canal Aqueduct: Castlecary Limestone, Passage Formation

From the adit, follow the riverbank and ascend the valley side via a wooden staircase and continue along the top of the bank to the field edge on the east side of the aqueduct. Descend again to the mill lade somewhat downstream of a weir. Just downstream of the weir (on sandstone) the Castlecary Limestone crops out in the riverbank and bed (Plate 4.3). This part of the excursion is only accessible during low flow conditions. The section observed hereabout is as follows:

Above Mill Lade	Thickness (m)
Sandstone, medium-grained, off white, carbonaceous-micaceous layers and wisps in places throughout, sharp erosive base	1.20
Sandstone, fine-grained, off white, siltstone laminae and thin bands and carbonaceous-micaceous layers decreasing in frequency from base to top, some roots, up to	0.81
Siltstone, bedded, grey, rooty, up to	0.14
COAL, very inferior, dull- and bright-banded, carbonaceous mudstone bands and lenses	0.07
Seatrock, grey, mainly silt but also clay grade, grey	0.54
Sandstone, medium-grained, off white, traces of cross bedding, rare carbonaceous-micaceous parting. Sharp flat erosive base	3.14
Mudstone, dark-grey bedded, very weathered, soft, scattered marine fossils include Productids, coaly base	0.75
Below Mill Lade	Thickness (m)
Siltstone, bedded, dark-grey, muddy and coaly, rooty	0.15
Seatrock, clay and silt grade, some sandy bands, roots throughout, blocky fracture except in sandy bands; pale-grey weathered	4.20
Sandstone, medium- to fine-grained, off white, rooty, bedded, carbonaceous-micaceous layers. Sharp erosive base	0.45
Siltstone, grey, bedded, rooty, large 15cm irony nodules	0.45
Mudstone very silty, dark-grey, bituminous and carbonaceous in parts, hard, thinly bedded, fish debris, coprolites	0.80
Limestone, dark-grey, medium-grained, sparry, sugary, crinoid columnals	1.00
Gap, probably mudstone	0.30
Limestone, dark-grey, medium-grained, sparry, sugary, crinoid columnals	0.60

As elsewhere, the Castlecary Limestone is in two leaves. It is not very fossiliferous, although small (<1cm) algal nodules may be found. They appear as pale nodules with a dark rim. The presence of such nodules indicates a near-shore shelf environment. The roof beds above the limestone are typical of much of the Central Coalfield. In some places they are bituminous, silty mudstones (and siltstones) which elsewhere have been analysed to determine their paraffin content (oil-shale). They are not marine, which is surprising considering the thickness of the marine limestone below. Only fish debris has been found at this locality, but the non-marine bivalve Curvirimula sp. has commonly been retrieved from this horizon elsewhere. These mudstones must be of lagoon or lake-bottom origin. The top of the Castlecary Limestone is the top of the Upper Limestone Formation.

The strata above the Castlecary Limestone are typical of the base of the Passage Formation. They are exposed in the adjacent cliff but are not currently easy of access because of fallen trees and minor landslipping (Plate 4.4). They consist of marine mudstone passing up into deltaic siltstones and fluvial sandstones. Overall, fluvial deposits dominate the Passage Formation succession. The fauna from the mudstone (No.1 Marine Band) includes Brochocarina sp., Composita ambigua, Crurithyris urii, Linoproductus sp., Orbiculoidea nitida, Productus carbonarius, Schellwienella sp., Schizophoria resupinata, Euphemites cf. hindi, E. urii, Naticopsis sp., Retispira sp., Edmondia sulcata, Koninckopecten scotica, Palaeolima simplex, Palaeoneilo laevirostrum, Paleyoldia macgregori, Parallelodon semicostatus, Polidevcia attenuata, Posidonia corrugata, Promytilus sp., Sanguinolites cf. striato lamellosus, S. tricostatus?, Schizodus cf. portlocki, S. taiti, Epidomatoceras sp.

At all times follow: The Scottish Access Codeand Code of conduct for geological field work