Reproduced in this format with permission from the British Geological Survey, 2024. Originally published: Stone, P (editor). 1996. Geology in south-west Scotland: an excursion guide. (Keyworth, Nottingham: British Geological Survey.)
In memory of Byron Charles Lintern 1948-1993
By R.P. Barnes
OS 1:50 000 Sheet 83 Newton Stewart 6- Kirkcudbright
BGS 1:50 000 sheets 2 Whithorn, 4E Wigtown
Route map:
Main points of interest D1 deformation styles and post-D1 structures in Silurian turbidites, characteristics and stratigraphical relationships of the Cairnharrow, Kirkmaiden, Carghidown and Ross formation, syntectonic dykes.
Logistics All localities are easily reached via the A75 from Newton Stewart or Glenluce and small roads on the Whithorn peninsula; Locality 4 requires vehicular access across private land and permission should be gained. The localities are described anticlockwise around the coastline
This excursion aims to demonstrate the relatively complex deformation of the Hawick Group in the coastal exposures of the Whithorn promontory. Relationships can be demonstrated between the widespread, but here relatively variable, D1 deformation, the later folding and faulting, and the intrusion of suites of syntectonic dykes.
The Hawick Group crops out over the southern part of the Whithorn promontory
Tectonostratigraphy follows the regional pattern of the Southern Uplands. The younging direction of the strata is typically northwards yet the limited biostratigraphical evidence indicates that the formations become older in this direction. This paradox can only be resolved by inferring the presence of reverse faults parallel to the ENE strike of the rocks. These subvertical structures, spaced 2–5 km apart, define a series of parallel-sided tectonostratigraphical tracts. The faults are thought to have formed as thrusts in association with D1 folding and S1 cleavage formation but have since been rotated into a near-vertical attitude. The thrust front, detaching the turbidite sequence on the underlying Moffat Shale Group, effectively migrated southwards into progressively younger strata; the D1 event is thus diachronous, becoming younger southwards. This concept is discussed more extensively in the Introduction (see especially
D1, represented by gently plunging folds, was the only phase of deformation to have affected many of the rocks now preserved in the northern half of the Southern Uplands (Leadhills and Gala groups), later phases of deformation being apparent only rarely. To the south, in the Hawick Group, D1 was more complex and produced a wide range of fold plunge including steeply plunging sinistral folds. A second phase of deformation D2 is also relatively widespread, forming inclined, south-verging open folds and smaller recumbent folds associated with a locally well-developed subhorizontal cleavage (S2). Numerous lamprophyre and felsite dykes were emplaced into the Hawick Group at various times during the deformation history. The early, bed-parallel dykes are locally cut by the S1 cleavage, whereas other dykes have various relationships with D2 folds suggesting emplacement before or during D2 deformation. Examples of these relationships can be examined at Locality 2 of this excursion and at Locality 1 of Excursion 16.
Steeply plunging folds also occur elsewhere in the Southern Uplands, commonly adjacent to the tract-bounding faults. The Orlock Bridge Fault, for example, shows evidence of major sinistral strike-slip reactivation (Excursion 17). Many of these structures may have formed in response to a single sinistral shear event. At Locality 2 (below), folds of this nature can be shown to deform S2 and therefore to represent a third phase of deformation (D3). However, bearing in mind the diachronous nature of D1, these D3 structures may have formed at the same time as the steeply plunging D1 folds in the younger part of the Hawick Group
Extensive coastal exposure through the Hawick Group on the Whithorn promontory allows study of its turbidite sedimentology and the effects of the deformation it has undergone. The tectonic structure is particularly well displayed. A study of the area by Rust (1965) was followed by British Geological Survey mapping (BGS, 1987; 1992c; Barnes et al., 1987; Barnes, 1989). The structure is dominated by DI, the effects of which are separated into two domains
From Newton Stewart follow the A714 to Port William (just over 30 km) then turn south on the A747 coast road for 5 km, parking on the seaward side of the road
Thin- to thick-bedded greywacke, often parallel-laminated even through thick beds, is interbedded with silty mudstone. These strata were formerly regarded as part of the Kirkmaiden Formation (BGS, 1987; Barnes, 1989) but are now assigned to the Cairnharrow Formation (BGS, 1992b; 1992c). They are deformed by steeply plunging folds, varying from minor folds with short limbs a few centimetres long, locally intensely developed in very thin-bedded units, to larger sinistral folds in more thickly bedded greywacke. The folds are associated with a steep, east-striking, near-axial-planar cleavage. The gently dipping S2 cleavage, which will be seen associated with recumbent D2 folds at Locality 2, is also developed in places here and the relationship between the various structures is critical to understanding the timing of the fold phases. The gently dipping S2 cleavage appears to be folded by the steeply plunging folds but, because these are at a high angle to one another, it is difficult to be certain. However, S2 is also crenulated by the steep cleavage, confirming that the steeply plunging structures are later. They can therefore be referred to the D3 phase of deformation.
Identical steeply plunging fold structures will be seen farther SE (Localities 3 to 6) associated with the late stages of D1. Given the diachronous nature of the D1 deformation, the same phase of sinistral shear may have been responsible for the formation of steeply plunging D1 folds in the youngest strata being deformed at any one time, contemporaneous with steeply plunging refolding (D3) structures in the older, previously deformed rocks of the thrust hinterland.
Proceed through Monreith village on the A747 and about 1 km farther on turn right for Monreith golf course. The narrow road turns sharply to the left after 500 m, at which point a bronze otter, a memorial to the author Gavin Maxwell, is situated on a cup and ring marked rock to the right of the road. Drive through the golf course to the public car park at the shore
The turbidites exposed in the cliff are typical of the Kirkmaiden Formation: thin-to medium-bedded greywacke with some thicker beds, and interbedded silty mud-stone. Several D1 folds are well displayed. Two sets of later folds are also present, with steeply and gently dipping axial planes and opposed vergence. Their overall conjugate geometry forms open box folds (
At low tide it is possible to walk around the headland into the next bay, the deep embayment in the southern part of which is called Callie's Port. Alternatively, from just above the high water mark in Back Bay, follow the narrow path which winds up on to the headland then walk around the cliff top to the SE side of Callie's Port where the beach is easily reached. Several lamprophyre dykes are present in the north of the bay, particularly in the west-facing cliff where critical relationships with fold structures indicate post-D1 but pre-D2 intrusion. One large, irregular intrusion cross-cuts an anticlinal D1 fold hinge. Several bedding-parallel intrusions occur in the adjacent strata, a 1.5 m-thick dyke being folded by a small recumbent D2 fold and cut by fractures parallel to the S2 cleavage (Barnes, 1989, plate lc). Immediately south of Callie's Port, a wide wave-cur platform presents easily accessible exposure in which both sets of D2 folds cause variations in the dip of bedding. About 100 m south of Callie's Port, a 1 m-thick lamprophyre dyke is exposed parallel to bedding but divided into a number of offset segments. At least one of the offsets is related to a recumbent D2 fold closure (Barnes, 1989, plate lb), suggesting syn-D2 emplacement.
Proceed a further 50 m south, to the end of the wave-cut platform where deep gullies have been eroded along NW-trending fractures, and look at the cliff face immediately SE to the east. Thin-bedded turbidites folded around an upright D2 fold include a bedding-parallel lamprophyre dyke, a few centimetres thick, which also passes around the fold. It is difficult in such circumstances to determine whether the dyke was intruded before or after folding. However, here the anticlinal hinge is fractured and the dyke is offset, suggesting that the dyke predates the folding. At low tide it is possible to proceed along the section to the SE where, over the next 120 m, D1 folds are deformed by recumbent D2 folds and related thrust faults.
To return to the car park, regain the cliff top and follow it almost to the golf course. After crossing a wire fence a path slants down over the steep grassy slope into the northern corner of Back Bay.
From Monteith take the A747 south for about 9 km towards Isle of Whithorn; at a cross-roads, turn right following a sign for St Ninian's Cave and after a further 2 km stop in the car park just before the farm of Kidsdale
In passing, examine the small exposure of thin-bedded greywackes at the back of the beach a few metres south of the end of the path. This exposure includes gently plunging, south-verging D2 folds with a crenulation cleavage developed in the fold hinges only. Exposures towards the south of the beach show thin slickensided quartz coatings on bedding planes. These indicate subhorizontal shearing along the bedding, shown to be sinistral where the sense of movement can be demonstrated, and probably relate to deformation associated with a strike-parallel fault along the straight valley followed by the path to the beach.
The section south of the beach is similar in its sedimentology to the Kirkmaidensequence but two features distinguish it as the Carghidown Formation: 1. thin red mudstone beds occur sparsely here but increase in frequency and thickness to the south; 2. short sections in which the bedding is disrupted are commonly interspersed with coherently bedded units. Faults form most of the boundaries to the disrupted zones which are themselves veined, probably because these units were initiated as zones of soft-sediment disruption but served to localise subsequent deformation. South of the beach, the strata are cut by several lamprophyre dykes which postdate well-developed, west-plunging folds but display variable relationships with faults.
Two fault sets, NW- and NNE-trending with conjugate geometry, are present throughout the Whithorn area, the straight coast at Locality 3 being developed along a zone of the NW-trending set. These cut the bedding and most of the bed-parallel dykes with clear evidence of dextral lateral displacement, although polyphase oblique slip is shown by several generations of slickensides. Later dykes have been emplaced into these faults. One example, exposed about 50 m from the start of the main coastal section, shows evidence of intrusion during fault movement. Bedding, including a 25 cm bed-parallel dyke, is displaced by a minor fault trending 125°. A 40 cm-thick lamprophyre dyke has been emplaced into the fault and carries a fabric in its margins consistent with dextral movement on the fault. Both dykes are folded by steeply plunging, dextral kink bands. A short distance to the south, D1 folds are well displayed in a north-verging fold pair which plunges moderately SW. A thick dyke forms a cliff immediately to the south and clearly postdates the folds.
The cliff prevents further access but a walk south along the cliff top for 100 m allows the shore to be regained in the south side of a small bay. However, before descending to the shore, walk 50 m further along the cliff top to where a steeply plunging sinistral D1 fold pair is particularly well exposed. The anticlinal hinge, plunging at about 87° at the top of the cliff, curves sharply to plunge 66° west lower down. Return along the cliff top, descend to the shore and examine the steeply plunging folds on the wave-cut platform. Cleavage is best developed in the muddy beds between sandstones and is axial planar to the folds. To the south, the same cleavage is related to several minor folds, affecting thin-bedded greywacke and silty mudstone alike; this emphasises that, despite their widely variable attitude, all the folds are the products of one phase of deformation. The cliff to the south exposes several minor thrust faults which displace the D1 folds and were probably formed during the D2 event; similar relationships were seen at Locality 2.
Return to the cliff top and walk back via Port Castle Bay to Kidsdale. Alternatively, if appropriate transport arrangements can be made, the cliff top can be followed for a further 2 km SE to Locality 4, near Burrow Head. In passing, the shore can be accessed to examine the characteristic features of the Carghidown Formation, including red mudstone beds, slumped units, folds of variable attitude and cross-cutting faults and dykes.
From Kidsdale, return to the A747 and proceed 4 km SE to Isle of Whithorn. Turn right shortly after entering the village, following the sign for Burrow Head caravan park. Stop at the reception office to gain permission for access, then drive to the south part of the site and park near the cliff top
From where the vehicles are parked, walk round the coast to a stream at the east margin of the caravan park. The stream is incised into the raised beach and enters the sea along a small north-trending sinistral fault. Descend to the shore west of the stream close to two lamprophyre dykes, one north-trending and the other parallel to bedding. Bedding dips steeply SW and can be shown from sedimentary structures to be the right way up. The southward younging nature of the succession is consistent with a minor, north-verging fold pair plunging 10° SW, immediately west of the stream. Note that the plunge changes to about 30° NE across the fault.
Two red mudstone beds, about 20 cm thick, occur on the foreshore either side of the stream; the intervening greywacke beds contain reddened mica which is conspicuous (particularly with the aid of a hand-lens) on freshly broken surfaces. These features indicate that the strata are part of the Carghidown Formation, which makes up the coastal section for 4 km to the NW (see Locality 3).Return to the cliff top and walk on to the small headland about 50 m to the SE. Here the turbidite sequence is essentially similar to that seen previously, but instead of red mudstone beds it includes grey laminated siltstone, indicating that these strata are part of the Ross Formation. The first siltstone, about 30 cm thick, crosses obliquely through the centre of the headland; two more beds, each about 45 cm thick, occur close together about 20 m to the SE. Locally, the intervening sandstones contain red mica. The southward younging of the sequence is again shown by grading in the sandstone beds, congruent with a north-verging D1 fold pair which plunges 32° NE on the east side of the headland.
The transition between the two formations is exposed in the west side of the headland. However, the transition zone is confused by a fold pair with a hinge separation of about 25 m and some faulting. The occurrence of red mica in strata which are clearly part of the Ross Formation suggests that the disruption is minor.
The shore can be accessed from the cliff top at various places to west and east of this locality, allowing a wider examination of the Carghidown and Ross formations. Grey siltstone can be found in various places and may yield graptolites if carefully split along the lamination. Graptolites of the early Wenlock Cyrtograptus centrifigus and Monogmptus riccartonensis biozones have been collected (Barnes, 1989; White et al., 1992). Apart from the frequent disruption of bedding in the Carghidown Formation, the structure of the two formations is essentially the same, with beds dipping steeply south and frequent folds varying from moderate SW- to moderate NE-plunging. Steeply plunging sinistral D1 fold pairs are also common, related to the S1 cleavage which transects the gentle to moderately plunging folds but is axial planar to the steeply plunging folds.
From the Burrow Head caravan park return to Isle of Whithorn; turn right into Main The Isle of Whithorn (really a peninsula) and the area to the north are largely formed of Carghidown Formation turbidites like those at Localities 3 and 4, with common red mudstone beds. The south end of the Isle includes a tectonically complex sliver of Ross Formation. Several folds and strike-parallel faults cause changes in the younging direction of the strata. Lamprophyre dykes up to 3 m thick are commonly bed-parallel or fault-parallel, but include a few of ESE trend which are not related to any obvious structural weakness.
Well-bedded turbidites, dipping steeply SE but overturned to young NW, are exposed in the shore below the car park. About 70 m to the south these pass into a 20 m wide zone disrupted by slumping. Massive sandstone blocks and disorientated bedded slabs lie in a mudstone matrix with patches of red mudstone. Units of very thin-bedded greywacke and silty mudstone contain small slump folds with widely variable axial orientations. The mélange is cut by lamprophyre dykes which lie parallel to bedding in the adjacent strata, indicating that this attitude is tectonically controlled, not simply constrained by bedding as the easiest path for intrusion. The mélange passes southwards back into steeply SE-dipping turbidites, initially southward younging indicating the presence of a fold or fault either within or at the southern edge of the melange. After 50 m there is a return to north-younging across a bedding-parallel fault.
Hereabouts a narrow concrete path leads NE across the foreshore; follow it to where it crosses the high water mark. Immediately south of the path, a small anticlinal fold pair in very thin-bedded greywackes shows extreme variation in the plunge of the prominent anticlinal hinge, from 81° NE to 45° SW over a distance of 1–2 m along the axial plane. Cleavage, approximately axial planar, is continuous throughout the fold.
A short distance to the south a narrow bay called Chapel Port West has been deeply eroded along a strike-parallel fault. Cross to the opposite side of the Isle where a comparable bay (Chapel Port East) south of the ruins of St Ninian's Church marks the trace of the same fault. South of the fault the turbidite dominated sequence contains thick red mudstone beds. Irregular ESE-trending mica-lamprophyre dykes cut obliquely across bedding and show sinistral displacements ranging from a few centimetres to 3 m on faults lying nearly parallel to bedding. A red mudstone 6 m thick contains several such faults; a lamprophyre dyke is repeatedly offset sinistrally by 30 to 50 cm. Horizontal slickensides can be seen on some of the fault surfaces and are also commonly seen in thin veins along bedding, indicating that this late (post-lamprophyre) sinistral shear was mainly taken up by movement on discrete faults and by slip along bedding.
About 50 m SE of the bay on the east side of the Isle, a major anticlinal fold, plunging about 30°NE, is well exposed across the foreshore. The same fold is also exposed on the west coast, although here it is associated with minor folds and plunges more gently. Follow the western shore south; after about 40 m, a very thick sandstone bed with large flute casts on its base is well exposed above high water in an open, east-trending fold pair. A further 40 m south, several strike-parallel faults are apparent in an intensely deformed zone, 50 m wide, marked by discontinuous and contorted sandstone beds and blocks, with some gently plunging coherent folds, in a sheared matrix. Discontinuous beds of laminated, grey siltstone occur in the deformed zone and yield a sparse lower Wenlock graptolite fauna. These features are characteristic of the Ross Formation as seen at Burrow Head, although here at Isle of Whithorn the precise stratigraphical relationships are unclear.
Return to the car park by walking north around the shore or by following one of several paths across the Isle.
From Isle of Whithorn, take the B7063 north towards Wigtown. After 1.5 km, shortly before the road reaches the coast, the farm of Cairnhead is visible set back east of the road. Park at the road and, with permission from the farm, follow the track through the farmyard for about 700 m until a small east-facing bay is reached. Cairnhead Mote
Bedding within the Carghidown Formation is here disrupted in a manner consistent with soft sediment deformation. However, two folds with axes plunging steeply west are clearly seen crossing the foreshore ENE of the mote; they form a sinistral fold pair and have minor folds locally in the short limb. Close inspection of bedding will reveal earlier tight folds which, despite poor preservation, can be seen to be refolded. A tectonic fabric is apparent in the early fold hinges, suggesting that they are not simply slump folds. This fabric is not discernable elsewhere, possibly due to the intense cleavage associated with the steeply plunging folds, or perhaps because it formed only in association with the early fold hinges. This evidence for two phases of folding was first described by Rust (1965). Elsewhere, throughout much of the Hawick Group outcrop, 'early' folds are characterised by widely variable hinge plunge and a mildly transecting cleavage; some isolated, steeply plunging sinistral folds appear to be related to the same cleavage. It is only at Cairn Head, where the two fold styles are superimposed, that one can be seen to postdate the other, but even here it seems likely that both are part of the D1 event. The variable relationships are probably due to a change in the stress regime during deformation, from one dominantly orthogonal to one involving a major component of sinistral shear.
Return directly to the road along the track, or follow the coast north around Cairn Head and then round the south side of Portyerrock Bay, meeting the road just north of Cairnhead Farm. If the latter route is taken the foreshore exposures of the Carghidown Formation can be examined. Changes in the direction of younging of the near-vertical bedding, shown by sole markings and grading in the sandstone beds, indicate the presence of major folds, although many of the fold hinges have been replaced by bed-parallel faults. Many minor fold pairs are present, the hinge plunges varying from moderate east to moderate west. This variation is sometimes seen in the same fold, producing markedly arcuate hinges. Several lamprophyre dykes are also present, usually bed-parallel and less than 2 m thick but including an 8 m-thick dyke through Cairn Head.