Leadhills Supergroup, Ordovician, Southern Uplands
|Stone, P, McMillan, A A, Floyd, J D, Barnes, R P, and Phillips, E R. 2012. British regional geology: South of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.|
The lithostratigraphical characterisation of the sedimentary tracts forming the Leadhills Supergroup is based on the marked variation in the compositions of the detrital material forming the constituent grains of their sandstones, a phenomenon reflecting differences in the provenance of the original sediment. So, for example, some formations prove to be rich in quartzofeldspathic material in contrast to others that contain abundant volcanic detritus (P912383). These compositional variations may be accompanied by differences in the directions of the palaeocurrents deduced from the sedimentary features of individual beds, emphasising the geographical separation of the different provenance regions. Some tracts feature the interbedding of differently sourced sandstone units.
The Ordovician sandstone groups and associated volcanic rocks that make up the Lead-hills Supergroup mostly crop out between the Orlock Bridge Fault and the line of the Stinchar Valley Fault/Southern Upland Fault. The one exception is the Ettrickbridge Formation, which occupies a slightly anomalous position farther south, interbedded with mudstone of the Moffat Shale Group. The Leadhills Supergroup incorporates the volcanic Downan Point Lava Formation and other lavas in the Tappins Group, as well as the substantial pile of lower Caradoc lavas and pyroclastic rocks of the Bail Hill Volcanic Group.
The Tappins Group encompasses the most northerly structural tracts in the Southern Uplands. It lies immediately south of the Southern Upland Fault, and is the oldest division in the Leadhills Supergroup (P912325, P912326 and P912327). It comprises the Traboyack, Downan Point Lava, Currarie, Dalreoch, Corsewall and Marchburn formations, each of which is a discrete tract, separated from its neighbours by major strike faults. The southern boundary of the group is formed by the Glen App and Carcow faults. Limited biostratigraphical (graptolite and conodont) evidence indicates an early Caradoc age (gracilis to apiculatus-ziczac biozones) for all of the component formations but, unlike the other units in the Northern Belt, they do not rest on Moffat Shale Group strata. Hence, no formational bases can be defined. Most of the sandstones were derived from the north and north-west and are immature; they contain a high proportion of igneous detritus, much of it likely to have been derived from ophiolitic rocks similar to those forming the Ballantrae Complex (P912383).
Red and purple mudstone and sandstone characterise the Traboyack Formation (c.1000 m?), which forms the northernmost sedimentary tract in the Southern Uplands terrane and so most probably contains the oldest strata. The formation crops out on the south side of the Stinchar valley to the west of Barr, between the Stinchar Valley Fault to the north and the Dove Cove Fault to the south, but westwards it is structurally imbricated with the Downan Point Lava Formation. All of the margins to the outcrop are faulted so that no base or top to the formation is known. The Traboyack Formation is rich in ophiolitic detritus and as a result is the most highly magnetic of the sedimentary tracts in the Southern Uplands.
Basaltic pillow lavas of the Downan Point Lava Formation (c.1000 m?) crop out on the coast at Downan Point, south of Ballantrae, where the pillow structures, confirmation of submarine eruption, are strikingly well developed (P005827). At the coast, the formation occupies all of the ground between the Stinchar Valley and Dove Cove faults, but eastwards the outcrop narrows and the lavas are structurally intercalated with the Traboyack Formation. Although formerly assumed to equate with the superficially similar lavas of the Ballantrae Complex, the Downan Point lavas are now regarded as a separate unit within the Southern Uplands Terrane. Apart from a rather imprecise Sm-Nd age date of 468 ± 22 Ma there is little direct evidence for the age of the Downan Point succession. The geochemistry of the lavas suggests their eruption at a within-plate, ocean island volcano. Lavas at Currarie Port and Portandea, previously correlated with those at Downan Point, are of slightly different composition and so could represent an entirely separate phase of volcanic activity.
The Currarie Formation (90–300 m) has a restricted outcrop on the coast north of Currarie Port (NX 056 780). It consists of red and grey-green mudstone and chert with interbedded masses of lava breccia, whilst irregular blocks of pillow lava set in a red siliceous mudstone matrix, that occur at several localities, have been interpreted as olistostromes. Graptolites obtained from the green mudstone are referred to a level near the base of the gracilis Biozone. The formation rests on lavas, previously correlated with the Downan Point Lava Formation, but of apparently different composition as mentioned above. Sandstone turbidites overlying the formation on the coast near Currarie Farm are thought to correlate with the Dalreoch Formation (see below). Although the presence of red mudstone in the Currarie Formation is reminiscent of parts of the Traboyack Formation, the magnetic signatures of the two formations are quite different.
The Dalreoch Formation (c.1000 m) is a major unit of dark green, turbiditic sandstone with its type section in the Water of Gregg near Barr (NX 294 930). Its outcrop lies to the south of the Dove Cove Fault and to the north of the Glen App Fault, and it has a magnetic signature intermediate between the adjacent Traboyack (to the north) and Corsewall (to the south, see below) formations. Poorly exposed red mudstone and basalt lava which crop out along the southern margin of the formation may correlate with the lithologically similar Currarie Formation and its underlying lavas.
Formerly known as the the Corsewall Group, the Corsewall Formation (c. 1700 m) has its type section at the northern tip of the Rhins of Galloway (NW 982 728) where the spectacular Corsewall Point Conglomerate Member is well exposed on the coast (P008481). It contains boulders up to 1.5 m across and was derived from the north. The clast suite in the conglomerates is very varied. Granitic lithologies are the most abundant, with porphyritic microdiorite, spilitic lava, sandstone and chert also well represented. Various ages have been obtained from radiometric dating of the granitic boulders, some as old as Mesoproterozoic, but with the most reliable suggesting intrusion in the Early Ordovician. This would require rapid uplift and erosion in the provenance area, to the north, and invites comparison with the situation seen in the slightly older Benan Conglomerate Formation of the Girvan– Ballantrae succession (see Chapter 2). Elsewhere, much of the Corsewall Formation consists of sandstone turbidites, but there are numerous other coarse rudite horizons, including the named Finnarts and Glen App Conglomerate members. The latter unit forms the north-eastern limit of the formation at the Water of Tig, 5 km east of Ballantrae, beyond which the tract pinches out so that eastwards the Dalreoch Formation is faulted against the northernmost tract of the Barrhill Group. Graptolites collected from the Corsewall Formation on the coast north of Finnarts Bay (NX 048 728) indicate that the strata lie within the gracilis or apiculatus-ziczac biozones.
The Marchburn Formation (c. 1300 m), with a type area about 5 km east of New Cumnock (NS 674 130) mostly comprises dark green quartz-poor sandstone turbidites, but with a heterogeneous mixture of other, associated lithologies including red and grey bedded chert, lava and the distinctive ‘Haggis Rock’. The latter consists of a green sandy microconglomerate containing a colourful mix of red, grey, green and black lithoclasts of chert, mudstone and dacitic and basaltic lavas. At Ruddenleys (NT 2025 5067) near Leadburn, red cherty mudstone within the Marchburn Formation has yielded early Caradoc conodonts, whilst closely associated dark mudstone yields gracilis to apiculatus-ziczac Biozone graptolites of about the same age. The red cherty beds almost certainly correlate with the lithologically similar cherts of the Kirkton Formation within the Leadhills Fault Zone.
In the Leadburn area and along strike to the south-west, pillow lavas are interbedded with the red cherts and cherty mudstones of the Marchburn Formation. Although it is rarely possible to demonstrate clear relationships, these lavas have been defined as the Noblehouse Lava Member (c.100 m), with a type section at Grassfield Quarry (NT 1947 4990), 7 km southwest of Leadburn. Geochemically, the lavas have the characteristics of within-plate volcanic rocks, showing affinity to both alkaline and tholeiitic oceanic island basalts.
The Barrhill Group consists of the turbidite sandstone units that crop out in a series of tracts between the Glen App/Carcow and Leadhills faults: the Kirkcolm, Galdenoch and Blackcraig formations. Together with differences in age, there are marked differences in the compositions (and hence provenance) of the three formations (P912330). The Kirkcolm Formation sandstones consist mainly of quartzo-feldspathic grains, with the Proterozoic and Archaean ages of accessory zircon grains suggesting their original derivation from an ancient Laurentian basement; garnet is another important detrital accessory. The Galdenoch and Blackcraig formations contain much additional igneous detritus, but with a background of quartzo-feldspathic material similar to that forming the Kirkcolm Formation, and with a similar Proterozoic–Archaean population of detrital zircons. Palaeocurrent indicators from the Kirkcolm Formation show that turbidity flow was highly variable (P008425 and P008463) but with a weak overall bias for derivation from the northern quadrant; the evidence is more ambiguous for the Galdenoch and Blackcraig formations.
The thick and extensive Kirkcolm Formation (c. 3000 m) occupies an across-strike outcrop width of up to 16 km in the western part of the Southern Uplands (P912325). It was originally erected as the Kirkcolm Group in its type area on the Rhins of Galloway and can thence be correlated north-eastwards along strike for almost 170 km as far as the Leadburn area. The formation is dominantly composed of well-bedded quartzose sandstone turbidites (P008483) that are sporadically interbedded with thick units of grey laminated siltstone; the sandstone becomes conglomeratic in a few places. Two distinct tracts comprise the formation, separated by the Glaik Fault. There is a considerable difference in age between the north-western and south-eastern tracts, ranging from the apiculatus-ziczac Subzone in the north-west to the clingani Biozone in the south-east (P912326 and P912327). However, though the major change in age occurs across the Glaik Fault, in the absence of Moffat Shale Group inliers this structure cannot be accurately delineated along the full length of the Kirkcolm Formation’s outcrop. The resulting uncertainty is compounded by the composition and general lithology of the sandstone being very similar in both the north-western and south-eastern tracts despite the difference in their ages.
Where inliers of Moffat Shale Group strata are seen in association with the Glaik Fault, graptolite faunas prove only the apiculatus-ziczac Subzone, i.e. the Glenkiln Shale: examples occur at Knockingarroch (NX 5574 9666) near Carsphairn and at Cowieslinn (NT 2354 5126), 4 km south of Leadburn. At several localities in the 12 km wide zone between the Glaik and Glen App faults, black laminae within the grey siltstone units of the Kirkcolm Formation also yield a graptolite fauna referable to the bicornis Biozone, though at Dounan Bay (NW 965 688), at the northern end of the Kirkcolm Formation’s type section on the Rhins of Galloway, the fauna in graptolitic interbeds near the top of the Kirkcolm Formation may be from its wilsoni Subzone. In contrast, at the southern end of the Rhins of Galloway section (also in Nithsdale and the Peebles area), the Kirkcolm Formation rests on mudstone of the Lower Hartfell Shale Formation ranging in age up to the clingani Biozone. The mudstone and underlying Moffat Shale Group strata form inliers in the hanging wall of the Leadhills Fault Zone, which forms their southern margin. No graptolitic interbeds are known in the overlying turbidite succession, which is truncated to the north by the Glaik Fault. However, conglomerates within the Kirkcolm Formation in the vicinity of Kilbucho (NT 056 336) contain derived shelly faunas with ages equivalent to a range from the upper apiculatus-ziczac to the lower clingani graptolite biozones. The fossils — trilobites, brachiopods, gastropods and corals (P521144) — are shallow-water species carried down from the continental shelf in masses of slumped sediment. Similar forms are seen in situ to the north of the Southern Upland Fault in Upper Ordovician successions at Girvan (see Chapter 2) and in Northern Ireland, which might imply that any lateral movement on the fault — and substantial sinistral displacement is a much-discussed possibility — amounted to no more than a few hundred kilometres.
On the north-east side of Bail Hill, the northern, older outcrop of the Kirkcolm Formation includes two unusual variations which have been separately defined as members: the conglomeratic Spothfore Member and the partly volcaniclastic Stoodfold Member. The conglomeratic Spothfore Member (300–800 m) can be traced for about 5 km along strike. All clasts of pebble size (4 mm) and upwards are of intrabasinal origin, with sedimentary rock types constituting 95 per cent of the total and volcanic rocks, including porphyritic lavas, the remaining 5 per cent. The largest proven clasts are boulders of sandstone and siltstone up to 2 m in diameter, though large discontinuous bodies of chert, possibly up to tens of metres across, may represent gravity-emplaced olistoliths. The Spothfore Member conformably overlies laminated siltstone of the Kirkcolm Formation but no upper contact is seen since the member is truncated to the north by a strike-parallel fault. The Stoodfold Member consists of at least four units of lithic and crystal-rich volcaniclastic sandstone related to the Bail Hill Volcanic Group (described below) but interbedded with sandstones and siltstones of the Kirkcolm Formation. Whilst there is little doubt that their origin was volcanic, they are interbedded within a turbidite sequence and themselves exhibit the classical suite of turbidite characteristics such as graded bedding, scoured and eroded bases, mudstone rip-up clasts and parallel and cross lamination. The volcaniclastic beds probably aggregate to no more than a few tens of metres in total thickness.
Another conglomeratic facies variation of the northern Kirkcolm Formation is the substantial Carsphairn Conglomerate Member (40–450 m). It has a matrix that is petrographically similar to the enclosing Kirkcolm Formation and crops out in the vicinity of Carsphairn as a series of lenticular bodies that represent cross-sections through original channel-fill sequences. Angular pebbles of vein quartz, up to 6 cm in diameter, are the most abundant clasts in the conglomerates, but most of the larger blocks consist of calcareous siltstone or sandstone, which weather to give a carious ‘rottenstone’ appearance. Well-rounded rhyolite clasts are also quite common and although most are less than 10 cm across, the largest boulder seen (38 x 20 cm) is of this lithology. Local development of similar, albeit less-coarse, conglomerate is seen at approximately the same stratigraphical level elsewhere in the Kirkcolm Formation outcrop.
The Galdenoch Formation (up to about 500 m) is a turbidite unit, the component sandstone of which is characterised by a significant content of andesitic detritus. The andesitic material occurs both as lithoclasts of pyroxene and/or hornblende andesite, and as individual detrital grains of pyroxene and hornblende. The combined volcaniclastic contribution may form as much as 30 per cent of the total grains, the remainder being mainly quartzofeldspathic. Magnetic susceptibility is uniformly high relative to that of the Kirkcolm Formation sandstone. The Galdenoch Formation interdigitates with the older, northern division of the Kirkcolm Formation to form three main members each consisting of several sequences of volcaniclastic beds ranging up to an aggregate thickness of several hundred metres. One such sequence, though only about 25 m thick, can be reliably traced for over 10 km along strike in the Barrhill area. Although most extensively developed to the west of the River Nith, the characteristic andesite-rich sandstones of the Galdenoch Formation also crop out near Abington (NS 9182 2203), and south of Leadburn (NT 2491 5296). These eastern outcrops confirm that the formation is represented across the full width of the region.
The Blackcraig Formation (c. 1500 m) forms a lenticular wedge interfingering with the older, northern division of the Kirkcolm Formation and is best seen in Glen Afton, south of New Cumnock (NS 628 055). There, spectacular massive gritty sandstone and boulder conglomerate form one of the most distinctive turbidite units in the Southern Uplands. Although a conformable junction with the underlying Kirkcolm Formation sandstone can be seen in Craig Burn (NS 6398 0562), the upper boundary to the Blackcraig Formation is obscured by faulting. The sandstone and the sandy matrix of the conglomerate have a distinct green colour due to abundant detrital epidote and other ferromagnesian minerals. In the conglomerate, boulders of granite, gabbro and both acid and basic lavas are supported in a sandy matrix and the formation probably represents a large channel-fill deposit on the upper reaches of a submarine fan.
Bail Hill Volcanic Group
In the area north of Sanquhar, the Bail Hill Volcanic Group crops out as a heterogeneous sequence of lavas, pyroclastic and intrusive rocks. It forms the largest single area of volcanic rocks in the Northern Belt (c. 4 km2) and probably represents the remains of an oceanic seamount that built up on a floor of Glenkiln Shale Formation strata in early Caradoc times. Laterally, the group interfingers with sandstones of the Kirkcolm Formation.
Although it superficially resembles a pyroclastic rock, the Cat Cleugh Formation (c. 150 m) consists of autobrecciated basaltic lavas containing characteristic large euhedral pyroxenes. It represents the basal unit of the volcanic complex and appears to be in conformable contact with underlying black mudstone of the Moffat Shale Group that contains an apiculatus-ziczac Subzone graptolite fauna. The Peat Rig Formation (c. 1000 m) is lithologically diverse and forms the larger part of the complex. It includes highly porphyritic autobrecciated lava, lithic tuff and agglomerate of hawaiite/mugearite composition. Phenocrysts in the lava include feldspar (oligoclase/andesine), amphibole (pargasite), apatite and biotite/phlogopite. The Cat Cleugh Formation and the lower part of the Peat Rig Formation are cut by the Bught Craig Breccia, which is interpreted as the fill of a small volcanic vent that fed into the upper part of the Peat Rig Formation.
The volcanic rocks of the Bail Hill Volcanic Group are alkaline in character, ranging from alkali basalt to trachyandesite, with the whole-rock geochemical characteristics and enrichment patterns of oceanic, within-plate basalt. Although geochemically distinct and the largest single component, the group forms part of a mixed assemblage of tholeiitic and alkaline, within-plate lavas that are distributed through the Northern Belt of the Southern Uplands, in places intercalated with the sandstone sequences. This assemblage is thought to record a period of extension and within-plate volcanism coincident with the early stages of development of the Southern Uplands accretionary complex.
The Scaur Group includes all of the sandstone turbidite units that crop out in the Northern Belt tracts between the Leadhills and Orlock Bridge faults. These are the Portpatrick, Glenwhargen, Shinnel and Glenlee formations. The sandstones from the Glenwhargan and Shinnel formations are mostly quartzo-feldspathic, with a similar Proterozoic–Archaean population of likely Laurentian detrital zircons to that found in the Barrhill Group. The Portpatrick Formation sandstones are quite different (P912330), as described below, with a large volcanic component and an accessory population of Neoproterozoic detrital zircon grains. The Glenlee Formation is a rather more mixed assemblage. Palaeocurrent indicators show that turbidity flow was predominantly from the north and north-east, except in the Portpatrick Formation where they show the opposite trend, indicating flow from the south and south-west.
The dominantly volcaniclastic Portpatrick Formation (c. 2000 m) is one of the most distinctive formations in the Southern Uplands with its characteristic lithology of medium- to thick-bedded, commonly massive, dark blue-grey sandstone turbidite. Detrital andesitic grains are very common throughout the sandstone sequence, usually accompanied by remarkably fresh detrital pyroxene and hornblende; these ‘mafic’ components may form as much as 20 per cent of the grain population, most of the remainder being quartzo-feldspathic. Despite the apparent freshness of the andesitic detritus, implying penecontemporaneous volcanism, erosion and deposition, radiometric dating of detrital minerals has given Neoproterozoic results. Ages derived from Ar-Ar dating of detrital hornblende, and U-Pb dating of zircon grains, cluster around 560 Ma with no sign of a younger component. This age is closer to that of widespread magmatism on the southern, Avalonian margin of the Iapetus Ocean, than it is to the ages of large-scale magmatism on the Laurentian margin to the north. Coupled with the consistent palaeocurrent evidence for derivation of the Portpatrick Formation sandstones from the south and south-west, this would imply a very different provenance to that from which most other Southern Uplands sandstones were sourced. The presence of detrital blue amphibole is another unusual feature, though the detrital garnet grains have the same characteristics as those seen in the Kirkcolm Formation.
The base of the Portpatrick Formation is defined by the inliers of Moffat Shale Group mudstone along the Fardingmullach Fault, which underlie the turbidite sandstone. The age of the mudstone generally ranges up to the linearis Biozone, although there is evidence of a somewhat older (clingani Biozone) base to the unit in the Rhins of Galloway. Graptolite-bearing mudstone interbeds occur sparingly, for example at Killantringan Bay where the linearis Biozone has been established. There is no defined top to the formation, which is truncated along its northern margin by the Leadhills Fault Zone.
Interfingering within the Portpatrick Formation, the Glenwhargen Formation (0–500 m) is a unit of highly quartzose, wacke and arenite sandstones, locally conglomeratic, which is restricted to the western half of the Southern Uplands. At many localities, the Glenwhargan Formation is represented by only a few thin beds of pale grey, quartz-rich sandstone sandwiched between much thicker sequences of darker, partly volcaniclastic Portpatrick Formation sandstone. Several excellent sections display the relationships between the two formations: near Killantringan (NW 985 555) and at Port of Spittal Bay (NX019 521), respectively 1.5 km north-west and 2 km south-east of Portpartick on the Rhins of Galloway; and at Knockville Moor (NX 355 732), 9 km north-west of Newton Stewart, in an area where the Glenwhargen Formation reaches its thickest development and is also conglomeratic in places.
The Shinnel Formation (c. 2000 m) crops out between the Fardingmullach and Glen Fumart/Orlock Bridge faults, with its type section along the Scaur Water (NS 787 003) about 10 km north of Moniaive. Although generally dominated by quartz-rich, turbidite sandstone, the Shinnel Formation is characterised throughout its entire outcrop length by thick sequences of laminated grey siltstone. In the Leadhills area where the siltstone is locally the dominant lithology, the informal name ‘Lowther Shales’ has been applied. A distinct bed-parallel cleavage is commonly developed in these siltstones and they have consequently been worked as an inferior roofing ‘slate’, for example at the now disused Stobo Quarry (NT 158 365), about 7 km west-south-west from Peebles. Graptolite faunas from mudstone interbeds at several localities within the Shinnel Formation outcrop are referable to the anceps Biozone, whilst the base of the formation rests on Lower Hartfell Shale Formation strata containing faunas up to the linearis Biozone.
In the Tweeddale area, the Wrae Limestone and the ‘Tweeddale Lavas’ are exotic breccias of, respectively, limestone and lava clasts that are interbedded within the Shinnel Formation. They are included within the Tweeddale Member, which extends for over 20 km along strike south-west of Peebles. It is about 35 m thick in its type section at Wrae (NT 1175 3240) but appears to thicken to over 400 m in the area north of Peebles. The limestone contains a variety of shelly fossils and conodonts that suggest an early Caradoc age yet the member is enclosed within the Shinnel Formation of linearis Biozone age or younger, a substantial difference in age that supports interpretation as a submarine slide deposit. The clasts of lava are of peralkaline rhyolite, an unusual lithology most likely to have been erupted from an oceanic island volcano. The shallow-water limestone may have formed in fringing reefs around the volcano, with masses of both lava and limestone intermittently breaking away and slumping down into deeper water.
The Glenlee Formation (c. 2000 m) crops out between Thornhill and New Galloway and is the youngest Ordovician turbidite formation of the Northern Belt. It is bounded to north and south by the Glen Fumart and Orlock Bridge faults respectively. Much of the formation consists of sandstone, although a substantial laminated siltstone member, with an outcrop up to 1 km wide, can also be traced for more than 20 km along strike. The sandstone-dominated parts of the formation include sequences rich in detrital pyroxene and/or hornblende as well as others in which ferromagnesian minerals are entirely absent and the sandstones are quartzo-feldspathic. The pyroxenous beds are restricted in distribution and crop out only to the south of (so probably beneath) the siltstone member. The latter contains sporadic black laminae containing graptolite faunas referable to the upper part of the anceps Biozone and possibly the persculptus Biozone.
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