Breamish Valley, Coldlaw Cairn, Shiel Cleugh and High Cantle

River Breamish adjacent to Low Bleakhope

The River Breamish at Low Bleakhope

Map showing the excursion route, locations and igneous rock types.

A map showing the route, locations , and igneous rock types in the area of Linhope Spout
Key for the excursion map

We have a number of aims on this excursion. A profusion of dykes associated with the intrusion of the Cheviot pluton and the Biddlestone laccolith have been located along the Breamish valley – in the banks and bed of the river and in the road cuttings. Currently, these dykes are classified into four types; porphyry (recorded on some survey maps as mica-porphyry on account of the biotite that appears in the rock) quartz-porphyry (on account of its quartz phenocrysts), felsite and andesite (recorded on some survey maps as pyroxene-porphyry on account of the pyroxene in the rock). We hope to find exposures of all four types along our route today and, with the kind permission of Mr. & Mrs Nelson who farm much of the land that we will be walking over, we aim to locate examples of each type. The BGS maps show porphyry and quartz-porphyry dykes down-river of their farm at Low Bleakhope, andesite dykes upstream of High Bleakhope and also at the mouth of the Ainsey Burn and felsic dyke a short distance up the same burn.
We also want to take a look at Coldlaw Cairn. It is a rocky outcrop that appears to have been ignored by geologists – at least we haven’t come across a description of the type of rock found there or seen records of samples and thin sections. The maps of Robson and Al Hafdh suggest it is the coarser-grained Standrop type of granodiorite – but we want to make sure. Furthermore, Al Hadft records contact between his ‘Standrop’ and ‘Dunmoor’ types below the nearby Shielcleugh Edge, and contact between his ‘Dunmoor’ and ‘Marginal’ types near Rig Cairn on High Cantle so we want to take in those locations too.

The excursion begins with the 3 mile walk from the car parking space at Hartside, along the private road past Alnhammoor and up the Breamish valley to Low Bleakhope farm. It’s quite a hike but there’s plenty to see up here on a fine day.

Location 1. The Breamish valley near Low Bleakhope


The porphyry dyke in the River Breamish below Low Bleakhope

Porphyry dyke in the River Breamish below Low Bleakhope

Quartz-porphyry dyke crossing the Breamish at Low Bleakhope

Quartz-porphyry dyke crossing the Breamish at Low Bleakhope

We begin our investigation of the dykes in the half mile straight before Low Bleakhope between NT938158 and NT935154. The broad, pink to red porphyry and quartz-porphyry dykes are clearly visible in the river and, in the case of the former, in the road cutting.

Altered granitic rock  below Low Bleakhope. Prepared hand specimen in reflected light (45mm across)

Altered granitic rock below Low Bleakhope
We first took this for altered mica-porphyry but a thin section suggests otherwise. Prepared hand specimen in reflected light (45mm across)

 Porphyry dyke below  Low Bleakhope. Prepared hand specimen in reflected light (45mm across)

Mica-porphyry dyke rock, below Low Bleakhope
Prepared hand specimen in reflected light (45mm across)

The rock from a porphyry dyke is characterised by the presence of phenocrysts of albite (and occasionally phenocrysts as calcium-rich as oligoclase and even andesine) along with biotite in an haematite-stained, devitrified quartz-orthoclase groundmass.The plagioclase phenocrysts are often turbid and sericitised. Feldspar can also be present as perthite and anorthoclase.
Fresh or altered biotite (chloritised) is common but augite, or a calcite pseudomorph after augite, is seen more rarely and hypersthene and it’s replacements are very rarely seen.
The groundmass is usually a sub-crystalline or micro-crystalline quartz-feldspar matrix – often with some granophyric texture. Sometimes free quartz crystals can be identified in the groundmass but biotite only rarely.
Occasionally, large crystals of apatite can be found. Larger iron-titanium ore crystals are sparse occurring mostly around relict biotite crystals.

We took our first sample as an example of altered mica-porphyry. A thin section revealed that it is altered – all plagioclase having been changed to sericite and opaque oxides, but it is a relatively equigranular rock lacking the fine matrix associated with dyke material. Also there is practically no biotite and no chlorite . the common alteration product of biotite. If this is an example of the granite that the BGS map shows extending out into the andesite at this point, then it is surprisingly different from the mafic mineral-rich Marginal type that we would expect to see.

Quartz-porphyry dyke at Low Bleakhope. Prepared hand specimen in ordinary reflected light (40mm across)

Quartz-porphyry dyke at Low Bleakhope
Prepared hand specimen in ordinary reflected light (40mm across)

Quartz-porphyry dyke at Low Bleakhope. Thin section viewed in plane polarised light. (40mm across)

A thin section from the same sample viewed in plane polarised light

Quartz-porphyry dyke at Low Bleakhope. Thin section viewed with crossed polarising filters. (40mm across)

The same thin section viewed with crossed polarising filters

Quartz-porphyry is characterised by the presence of quartz phenocrysts and usually plagioclase and biotite in an iron-stained, devitrified quartz-orthoclase groundmass. Generally, the quartz phenocrysts are rounded and free from inclusions or cavities and the plagioclase phenocrysts are usually albite or sometimes of a composition between albite and andesite. The plagioclase is frequently turbid and sericitised and sometimes the feldspar phenocrysts may be perthitic.
The groundmass is generally quartz-orthoclase and may be sub- or micro-crystalline – granophyric texture is commonly associated with the latter.Fresh or altered biotite is common but hypersthene and augite, or a calcite pseudomorph after augite, are rare. Iron-titanium ores are sparse occurring mostly around relict biotite crystals.

Quartz with altered feldspars and chlorite PPL (FoV 4.6 x 3.0 mm)

Quartz with altered feldspars and chlorite in quartz-porphyry dyke at Low Bleakhope
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Quartz with altered feldspars and chlorite in quartz-porphyry dyke. Viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

The same crystals viewed with crossed polarising filters

Perthitic texture in turbid feldspars with chlorite, quartz crystal, muscovite and carbonate. Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Perthitic texture in turbid feldspars with chlorite, quartz crystal, muscovite and carbonate
Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Perthitic  texture in turbid feldspars with chlorite, quartz crystal, muscovite and carbonate. Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

The same section viewed with crossed polarising filters

Perthitic texture in feldspar  with secondary muscovite in quartz-porphyry dyke, Low Bleakhope. Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

Perthitic texture in feldspar with secondary muscovite in quartz-porphyry dyke, Low Bleakhope
Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

All along the river here between the two dykes, pink-to-red rock mingles and merges with dark grey rock and we are interested to learn more about them. Thin sections reveal that the black rock is always the much finer-grained and mafic-mineral rich of the two It always appears to be a later, quite fluid intrusion into the red rock, so that crystals of the latter have become embedded in the former. A sample from the east end of the sampled stretch shows that the red rock here is granitic. Quartz crystals appear in the mass of the red rock and as xenocrysts. presumably torn from the host rock, in the black rock. This black rock is an extremely fine-grained mix of what looks like feldspar, opaque oxides and what were mafic minerals now altered to chlorite. Further west, quartz is absent in both the red rock and the black rock. The red rock is haematite-rich andesite that has been extensively carbonatised and the black rock has been extensively chloritised. Many tiny, clear granules of clinopyroxene are scattered through the dark rock as well as occasional yellow to brown granular clusters that are associated with opaque material.

Herbert Kynaston remarked on these rocks in his ‘Notes on Contact Metamorphism round the Cheviot Granite. paper. He describes them as, ‘… peculiar banded rocks, which
are well exposed in the River Breamish, a few hundred yards below Low Bleakhope. The exact nature and relations of these rocks are very difficult to make out. They have a banded and streaky appearance, pink and dark greyish bands alternating with one another. Under the microscope these rocks show the effects of very decided contact alteration; but it is difficult to say what was the true nature of the original unaltered rock, the exposure having doubtless undergone considerable decomposition from surface agencies. The rocks must have varied a good deal in composition originally, and may possibly represent altered tuffs. One slide ….has an extremely confused and streaky appearance under the microscope, being in some places fairly clear; sometimes greenish from the predominance of numerous small flakes of a chloritic mineral; probably altered biotite; and sometimes of a dense reddish brown colour from the predominance of turbid felspar.
Throughout the whole slide are scattered numerous grains of the characteristic secondary pyroxene, biotite and chlorite flakes, and magnetite granules.
Small veins occur filled up with a granular aggregate of secondary augite, probably representing veins of calcite. The clear bands in the slide consist of a microcrystalline mosaic of quartz and felspar, scattered throughout which are numerous granules of secondary pyroxene and magnetite. Occasionally the felspar and quartz have the appearance of being inter-grown, after the manner of micro pegmatite, the turbid felspar being traversed in different directions by narrow bars and canals of quartz, and the quartz is also included in the felspar as simple grains.
Another slide, which was prepared from a strongly banded rock, shows the same structure on a far more extensive scale. The slide consists partly of a fairly coarse granular
aggregate of felspar, intersected here and there by a curious network of quartz, and including grains and irregular patches of quartz, and partly also of a microcrystalline mosaic of quartz and felspar. A few corroded phenocrysts of plagioclase also occur.
The grains of secondary pyroxene are more numerous and larger than in any of the other rocks examined. They are unevenly distributed, being frequently dispersed in irregular bands or concentrated into compact groups. Flakes of biotite and grains of secondary sphene are also occasionally present.’

Kynaston’s description and speculations are clearly of their age and our current understanding of mineral formation would not allow for secondary pyroxene appearing here as a metamorphic product of calcite and the like. However, we would agree with him that, ‘these rocks are decidedly of interest; but it would be unsafe to pronounce any decided opinion upon their exact nature before a more thorough investigation of their relationships can be made.’
Oh for the resources to explore the chemistry of these rocks!

Intermingled pink and grey rock by the River Breamish below Low Bleakhope

Intermingled and grey andesitic rock by the River Breamish below Low Bleakhope

The range of colour in the andesitic rocks along the straight stretch of river below Low Bleakhope.The samples are arranged from east (left) to west (right).

The range of colour in the rocks along the straight stretch of river to the north-east of Low Bleakhope
The samples are arranged from east (left) to west (right). The redder, more granular rock in the first sample is granitic.

Intermingled red and black andesitic rock. Low Bleakhope (NT935154) Prepared hand specimen in ordinary reflected light (44mm across)

Intermingled red and black andesitic rock
Low Bleakhope (NT935154) Prepared hand specimen in ordinary reflected light (44mm across)

Intermingled red and black andesitic rock. Low Bleakhope. Thin section viewed in plane polarised light (44mm across)

A thin section from the same sample viewed in plane polarised light

Intermingled red and black andesitic rock.  Low Bleakhope. Thin section viewed with crossed polarising filters (44mm across)

The same thin section viewed with crossed polarising filters

Granular yellow-brown mineral with associated clear v. high birefringent mineral. Viewed in reflected light with crossed polarising filters (FoV 0.5 x 0.3 mm)

Granular yellow-brown mineral with associated clear v. high birefringent mineral
Viewed in reflected light with crossed polarising filters(FoV 0.5 x 0.3 mm)

Granular yellow-brown mineral with associated clear v. high birefringent mineral viewed in plane polarised light (FoV 0.5 x 0.3 mm)

The same area viewed in plane polarised light
The clear crystals have the look of zircon and the yellow tinted crystals, pyroxene. It is evident that the very fine-grained black andesite has been heavily chloritised.

Granular yellow-brown mineral with associated clear v. high birefringent mineral that we think is Zircon. Viewed with crossed polarising filters (FoV 0.5 x 0.3 mm)

The same area viewed with crossed polarising filters

At the eastern limit of the quartz-porphyry dyke, the hornfels is brecciated and cemented with quartz. Perhaps there was a pre-existing fracture here that permitted the dyke’s intrusion or possibly the fracture was a result of the intrusion.

Quartz vein and brecciated rock by the River Breamish below Low Bleakhope

Quartz vein and brecciated rock by the River Breamish below Low Bleakhope


Quartz vein in brecciated rock below Low Bleakhope. Prepared hand specimen in reflected light (43 mm across)

Quartz vein in brecciated rock below Low Bleakhope
Prepared hand specimen in reflected light (43 mm across)

The dark rock in the sample shown above has a profusion of altered, grey-brown ortho- and clinopyroxene crystals that show only a white to yellow interference colours when viewed in thin section with crossed polarising filters. Along with these are occasional embayed granular clinopyroxene aggregates that contain so much opaque oxide material that, at first sight, they appear to be entirely magnetite crystals. It is only with more thinning that sufficient ‘crud’ is removed to reveal their transparency and their higher order interference colours. They are surrounded by a good deal of chlorite that appears to wrap around them. There is no sign of twinned plagioclase crystals.
It is difficult to say whether the quartz or the mafic material preceded the other. The mass of quartz has tiny veinlets running through it composed of extremely fine-grained quartz.

The junction of the mafic material and quartz. Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

The junction of the mafic material and quartz
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

The junction of the quartz and mafic material. Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

The same crystals viewed with crossed polarising filters

Quartz in contact with the mafic material. Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Quartz in contact with the mafic material
Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Quartz and mafic material. Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

The same section viewed with crossed polarising filters

Mafic material in the sample from Low Bleakhope. Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

Mafic material in the same sample from Low Bleakhope.
Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

Pyroxenes in the same sample. Section viewed with crossed polarising filters (FoV 1.2 x 0.8mm)

Pyroxenes in the same sample
Section viewed with crossed polarising filters (FoV 1.2 x 0.8mm)

Pyroxenes in the sample.  Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

Pyroxenes in this sample
Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

Granular clinopyroxene rimmed with chlorite. Section viewed in plane polarised light (FoV 0.5 x 0.3 mm)

Granular clinopyroxene rimmed with chlorite
Section viewed in plane polarised light (FoV 0.5 x 0.3 mm)

Granular clinopyroxene rimmed with chlorite. Section viewed with crossed polarising filters (FoV 0.5 x 0.3 mm)

The same section viewed with crossed polarising filters

Granular clinopyroxene and chlorite. Section viewed with crossed polarising filters (FoV 0.5 x 0.3 mm)

Granular clinopyroxene with chlorite
Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

Location 2. The Breamish near High Bleakhope

Location 2. Pyroxene porphyry dyke crossing the River Breamish

Location 2. Pyroxene porphyry dyke crossing the River Breamish

From Low Bleakhope, we continue on the farm track that takes us over the river, past the house and out-buildings at High Bleakhope and through the trees to the edge of the open land at NT920161 where it is joined by the footpath to High Cantle that we will take later.
Maps show a pyroxene-porphyry dyke crossing the river here – but access isn’t easy. All the rock looks suspiciously like ‘Marginal’ granite and although the steep rocky banks to the river suggest a tougher, perhaps different rock, we don’t identify the intrusion. Once cleaned up, the rock sample proves to be ‘Marginal’.

Marginal granitic rock on the Breamish near High Bleakhope. Prepared hand specimen in reflected light (45 mm across)

Marginal granitic rock on the Breamish near High Bleakhope
Prepared hand specimen in reflected light (45 mm across)

Location 3. The Ainsey Burn

It’s a short walk along the track to the mouth of the Ainsey Burn, a small stream that flows from the south-west, down through Cheviot lavas and tuff to join the Breamish at NT918163. Here, the survey shows two intersecting pyroxene-porphyry (andesite) dykes along with a quartz-porphyry dyke. The latter is fairly clear to see in the south-eastern bank of the burn and, on account of its regular jointing, in the track that crosses the burn – but the pyroxene-porphyry dykes elude us; we see only Marginal granite.

Dyke at the mouth of the Ainsey Burn

Quartz-porphyry dyke in the bank at the mouth of the Ainsey Burn

Pressing on up the burn, we enjoy the little waterfall at the point before the water flow takes an abrupt turn from the east to the north, perhaps on account of its encounter with much more resistant rock. We continue for a short distance to see the felsic dyke that outcrops in the bank of the burn at NT917162.

Small waterfall on the Ainsey Burn

Small waterfall on the Ainsey Burn

This is a lovely, secluded valley, and it is tempting to carry on up to investigate the tuffs and/or agglomerates in relation to the andesite lava and still more dykes – but we have to leave it for another day in order to retrace our steps to the track that will take us up to Coldlaw Cairn.

Location 4. Coldlaw Cairn

Coldlaw Cairn (NT 914 180) from the unmarked track to the east

Coldlaw Cairn from the track to the east

At NT920162 we rejoin the track from High Bleakhope and walk up onto the hillside below Shielcleugh Edge. Below to our left, the river flows along the line of the Breamish fault.
The OS maps show the track terminating at NT912172. There’s a gate here and the track continues, circling up and around the south and west side of Coldlaw Cairn to the summit. This is a good place to enjoy some food and take in the views of the long flank of Cheviot up to the north and below to the west, the head of the Breamish, King’s Seat and the Scottish Borders beyond.

Cheviot from Coldlaw Cairn

Cheviot from Coldlaw Cairn

A boulder field constitutes most of the rock at Coldlaw Cairn (NT914180) but granite bedrock rises out of it. It is a grey, relatively coarse-grained rock, well jointed in places, with prominent feldspar phenocrysts and dark crystals of biotite and pyroxene: the ‘Standrop’ type of Central Belt granitic rock.

Course-grained Central Belt granitic rock, Coldlaw Cairn (NT 914 180). Prepared hand specimen in ordinary reflected light (44mm)

Course-grained ‘Central Belt’ granitic rock, Coldlaw Cairn (NT 914 180)
Prepared hand specimen in ordinary reflected light (44mm)

Course-grained

Thin section from the same sample viewed in plane polarised light

Course-grained

The same thin section viewed with crossed polarising filters

Altered clinopyroxene phenocryst in Central Belt granitic rock at Coldlaw Cairn. Viewed with crossed polarising filters with crossed polarising filters (FoV 4.6 x 3.0 mm)

Altered clinopyroxene phenocryst in ‘Central Belt’ granitic rock at Coldlaw Cairn
Viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

Secondary biotite and chlorite at the edge of clinopyroxene phenocryst. Viewed in plane polarised light (FoV 1.2 x 0.8 mm)

Secondary biotite and chlorite at the edge of a clinopyroxene phenocryst
Viewed in plane polarised light (FoV 1.2 x 0.8 mm)

Secondary biotite and chlorite at the edge of clinopyroxene phenocryst. Viewedwith crossed polarising filters (FoV 1.2 x 0.8 mm)

The same area viewed with crossed polarising filters

Zircon in Central Belt granitic rock at Coldlaw Cairn. Viewed with crossed polarising filters (FoV 0.5 x 0.3 mm)

Zircon in Central Belt granitic rock at Coldlaw Cairn
Viewed with crossed polarising filters (FoV 0.5 x 0.3 mm)

Location 5. Shielcleugh Edge

Shielcleugh Edge with Coldlaw Cairn behind viewed from the Breamish Valley

Shielcleugh Edge with Coldlaw Cairn and Cheviot behind

From Coldlaw Cairn we retrace our steps along the track across the hillside below Shielcleugh Edge. At NT916166 we leave the track and head directly up the hill to the low-lying but prominent outcrop at NT918166. The rock here is finer-grained than that on Coldlaw Cairn and much pinker in colour. Al Hafdh records this along with most of the rock on this hillside as his ‘Dunmoor’ variety, an earlier intrusion than the ‘Standrop’ type. As mentioned earlier, he also records the contact between the two types and, significantly, the discovery of ‘Standrop’ chilled against ‘Dunmoor’ a little higher up the hill at NT918168 and NT920167.

Low-lying but prominent outcrop and boulders at NT918166 below Shielcleugh Edge

Low-lying but prominent outcrop and boulders at NT918166 below Shielcleugh Edge

Medium grained Central Belt granitic rock below Shielcleugh Edge (NT918166) Prepared hand specimen in ordinary reflected light (50mm across)

Medium grained Central Belt granitic rock below Shielcleugh Edge at NT918166
Prepared hand specimen in ordinary reflected light (50mm across)

Medium grained Central Belt granitic rock below Shielcleugh Edge (NT918166) Thin section viewed with crossed polarising filters (50mm across)

A thin section from the same sample viewedwith crossed polarising filters

There are no outcrops of bedrock here, only boulders. Amongst them we do find contact between different types of granitic rock. One is much redder and finer grained than the other but it is impossible for us to determine if the redder rock was part of the pluton or if it is dyke material. Thin sections show that it is the redder, finer-grained rock that is chilled against the ‘Standrop’ type – the reverse of Al Hafdh’s observation.
If the redder material is from a dyke, this chilling effect would be as we would expect. However, if as Al Hafdh maintains, it is a chilled version of the same ‘Dunmoor’ rock that we saw outcropping at NT916166, and if this rock really does represent a separate injection of magma into the pluton, it’s chilling against the ‘Standrop’ type indicates that it was injected later, not earlier than the ‘Standrop’ magma. This contradicts Al Hafdh’s account of events.
Of course, he may have collected a specimen with the coarser rock chilled against the finer, but the fact that this isn’t outcropping bedrock would call into question the significance of such a find in relation to the theory and mapping it supported.

Boulders on Shielcleugh Edge (NT918169) with Coldlaw Cairn and Cheviot behind

Boulders on Shielcleugh Edge (NT918169) with Coldlaw Cairn and Cheviot behind

Contact between coarser- and finer-grained granitic rock on Shielcleugh Edge( NT918166) Prepared hand specimen in ordinary reflected light (48mm across)

Contact between coarser- and finer-grained granitic rock on Shielcleugh Edge at NT918166
Prepared hand specimen in ordinary reflected light (48mm across)

Contact between courser and finer-grained granitic rock on Shielcleugh Edge at NT918166. Thin section viewed in plane polarised light (45mm across)

A thin section from the same sample viewed in plane polarised light

Contact between courser and finer-grained granitic rock on Shielcleugh Edge ( NT918166) Thin section viewed with crossed polarising filters (45mm across)

The same thin section viewed with crossed polarising filters

Boulder carrying contact between coarser-grained (top left) and finer-grained (bottom right) granitic rock on Shielcleugh Edge( NT918166)

Boulder carrying contact between coarser-grained (top left) and finer-grained (bottom right) granitic rock on Shielcleugh Edge at NT918166

Contact between coarser-grained and finer-grained Central Belt granitic rock in a boulder on Shielcleugh Edge NT918166. Prepared hand specimen in ordinary reflected light (50 mm across)

Contact between coarser-grained and finer-grained ‘Central Belt’ granitic rock in a boulder on Shielcleugh Edge NT918166
Prepared hand specimen in ordinary reflected light (50 mm across)

Contact between coarser-grained and finer-grained Central Belt granitic rock in a boulder on Shielcleugh Edge NT918166. Thin section viewed in plane polarised light (50 mm across)

A thin section from the same sample viewed in plane polarised light

Contact between coarser-grained and finer-grained Central Belt granitic rock in a boulder on Shielcleugh Edge NT918166. Thin section viewed with crossed polarising filters (50 mm across)

The same thin section viewed with crossed polarising filters

Detail of contact between coarser-grained and finer-grained Central Belt granitic rock in a boulder on Shielcleugh Edge NT918166. Thin section viewed with crossed polarising filters (25 mm across)

Detail of contact between coarser-grained and finer-grained ‘Central Belt’ granitic rock in the same thin section
Section viewed with crossed polarising filters (25 mm across)

Clinopyroxene and zoned plagioclase in the finer-grained rock in the same sample, Shielcleugh Edge. Section viewed with crossed polarising filters

Clinopyroxene and zoned plagioclase in the finer-grained rock in the same sample, Shielcleugh Edge
Section viewed with crossed polarising filters

Chilling of finer-grained rock shown at the top against coarser-grained rock below it, Shielcleugh Edge. Section viewed with crossed polarising filters

Chilling of finer-grained rock shown at the top against the coarser-grained rock below it
Section viewed with crossed polarising filters

Location 6. Shiel Cleugh

We cut across rough ground to the head of Shiel Cleugh and follow the burn down through the ravine to the River Breamish below. As we go, we inspect some of the many small outcrops and see something of a transition in the colour and texture of the rock. At the highest point, the rock is relatively grey and course-grained with many plagioclase phenocrysts and fewer iron-titanium oxides. As we move down and out towards the periphery of the pluton, the rock transitions through a redder, medium-grained apperance with fewer plagioclase phenocrysts and more opaque oxides, to the dark, medium-grained ”Marginal’ type at the bottom of the cleugh. It has fewer prominent plagioclase phenocrysts and a much higher iron-titanium oxide content. The ‘Marginal’ rock at the exit of the burn is quite altered and weak.

Transition in colour index and texture in rock from near the top of the Shiel Cleugh (left) and the bottom (right)

Transition in colour index and texture in rock from near the top of the Shiel Cleugh (left) and the bottom (right)

Location 7. High Cantle

High Bleakhope and the Breamish Valley from the lower slopes of High Cantle

High Bleakhope and the Breamish Valley from the lower slopes of High Cantle

No sooner we are down, we are up again. The Shiel Cleugh burn joins the Breamish close to the marked path that will take us over High Cantle, to Rig Cairn and back through Linhope to the car at Hartside.
The hillside above High Bleakhope is strewn with boulders, most of them are the typical ‘Marginal’ type of rock but we find one boulder that has an extremely fine-grained and intensely dark rock included in or intruded into the dark red ‘Marginal’ rock The inclusion looks similar to the extremely fine-grained hornfelsed andesite we saw intruded into the andesite and the granite in the Breamish by Low Bleakhope. Thin section reveals it is quite different. This rock has both biotite and plagioclase phenocrysts.

Medium-grained Central Belt rock at High Cantle (NT924162). Prepared hand specimen in ordinary reflected light.

Medium-grained Central Belt rock at High Cantle (NT924162)
Prepared hand specimen in ordinary reflected light

Medium-grained Central Belt rock at High Cantle (NT924162). Thin section viewed in plane polarised light.

A thin section from the same sample viewed in plane polarised light

Medium-grained Central Belt rock at High Cantle (NT924162). Thin section viewed with crossed polarising filters.

The same thin section viewed with crossed polarising filters

Marginal rock at High Cantle (NT924162) in contact with a very dark intruded or included rock. Prepared hand specimen in ordinary reflected light.

Marginal rock at High Cantle (NT924162) in contact with a very dark intruded or included rock
Prepared hand specimen in ordinary reflected light.

Marginal rock at High Cantle (NT924162) in contact with a very dark rock. Thin section viewed in plane polarised light.

A thin section from the same sample viewed in plane polarised light

Marginal rock at High Cantle (NT924162) in contact with a very dark rock. Thin section viewed with crossed polarising filters.

The same thin section viewed with crossed polarising filters

Plagioclase and pyroxene in a very fine groundmass in dark rock on High Cantle. Thin section viewed with crossed polarising filters.

Plagioclase and pyroxene in the very fine groundmass of the dark inclusion/intrusion in a boulder on High Cantle
Thin section viewed with crossed polarising filters

Very dark mafic material included or intruded in the medium-grained Central Belt rock at NT924162, High Cantle. Prepared hand specimen in ordinary reflected light

Very dark mafic material included or intruded in the medium-grained Central Belt rock at NT924162, High Cantle
Prepared hand specimen in ordinary reflected light

Very dark mafic material included or intruded in the medium-grained Central Belt rock at NT924162, High Cantle. Thin section viewed in plane polarised light.

A thin section from the same sample viewed in plane polarised light

Very dark mafic material included or intruded in the medium-grained Central Belt rock at NT924162, High Cantle. Thin section viewed in plane polarised light.

The same thin section viewed with crossed polarising filters

Kinked biotite in the dark rock on High Cantle. Thin section viewed with crossed polarising filters.

Kinked biotite in the dark rock on High Cantle
Thin section viewed with crossed polarising filters

Granophyric microlith in mafic material included or intruded in the medium-grained Central Belt rock, High Cantle. Section viewed with crossed polarising filters

Granophyric microlith in the mafic inclusion/intrusion in boulder at High Cantle
Section viewed with crossed polarising filters

We return to the path and go in search of more supporting of Al Hafdh’s evidence for his multiple-intrusion thesis.
Al Hafdh wrote, ‘The Dunmoor Granodiorite is seen chilled against the Marginal Granodiorite …….at High Cantle (NT926164)’ but there is no outcrop here and after some time looking for ground- level bedrock, we give up the search and with it more credence in Al Hafdh’s thesis.

Location 8. Rig Cairn

Sadly, the same is true at the second of his key locations in this locality. He writes, ‘Standrop Granodiorite ….is chilled against Porphyritic Dunmoor Granodiorite at (938167) where the Standrop Granodiorite cuts through the Dunmoor Granodiorite.’
The outcrops at this location are sparse and small and the rock here is uniformly the coarse-grained, porphyritic ‘Central Belt’ type with quite a high mafic content. At Rig Cairn, we do find a point of contact but it is only a minor felsic intrusion into the plutonic rock.

Coarse-grained Central Belt rock N. Rig Cairn (NT940168). Prepared hand specimen in ordinary reflected light

Coarse-grained Central Belt rock N. Rig Cairn (NT940168) Prepared hand specimen in ordinary reflected light

Coarse-grained Central Belt rock N. Rig Cairn (NT940168). Thin section viewed in plane polarised light.

A thin section from the same sample viewed in plane polarised light

Coarse-grained Central Belt rock N. Rig Cairn (NT940168). Thin section viewed with crossed polarising filters

The same thin section viewed with crossed polarising filters

Felsic intrusion into coarse-grained Central Belt rock at Rig Cairn (NT940168)

Felsic intrusion into coarse-grained Central Belt rock at Rig Cairn (NT940168)

Felsic intrusion into coarse-grained Central Belt rock at Rig Cairn (NT940168). Prepared hand specimen in ordinary reflected light

Felsic intrusion into coarse-grained Central Belt rock at Rig Cairn (NT940168)
Prepared hand specimen in ordinary reflected light

Coarse-grained Central Belt rock N. Rig Cairn (NT940168). Thin section viewed in plane polarised light.

A thin section from the same sample viewed in plane polarised light

Felsic intrusion into coarse-grained Central Belt rock at Rig Cairn (NT940168) Thin section viewed with crossed polarising filters.

The same thin section viewed with crossed polarising filters

Alteration of mafic minerals giving rise to secondary biotite and amphibole, N.Rig Cairn. Section viewed in plane polarised light (FoV 2.5 x 1.7 mm)

Alteration of mafic minerals giving rise to secondary biotite and amphibole in rock just north of Rig Cairn
Section viewed in plane polarised light (FoV 2.5 x 1.7 mm)

Alteration of mafic minerals giving rise to secondary biotite and amphibole in rock just north of Rig Cairn Section viewed with crossed polarising filters (FoV 2.5 x 1.7 mm)

The same area viewed with crossed polarising filters

Al Hafdh’s thesis has provided us with structure and foci in our initial investigations but this excursion confirms our view that his multiple intrusion, ring dyke thesis does not accurately describe or adequately explain the Cheviot volcanic complex.


Return to the car

From Rig Cairn, we take the footpath east towards the north side of Ritto Hill, through Linhope and along the metalled road back to the car.

Hedgehope Hill with Little and Great Standrop viewed from near Rig Cairn

View of Hedgehope Hill with Little and Great Standrop on the walk over to Linhope.


References

H. Kynaston, Notes on Contact Metamorphism round the Cheviot Granite, Edinburgh Geological Society, 1899.

Carruthers, R G, Burnett, G A, Anderson, W, and Thomas, HH,1932. The Geology of the Cheviot Hills (Based on the work of C.T. Clough and W. Gunn) HMSO

Al-Hafdh N.M. 1985. The Alteration Petrology of the Cheviot Granite. Thesis submitted for PhD. at Newcastle University.

British Geological Survey Online geology map http://mapapps.bgs.ac.uk/geologyofbritain/home.html

No vestige of a beginning, – no prospect of an end

error: