Rather than walking over to Bellyside Hill via the Hawsen Burn – Lambden Burn route, or via the track between upper Cheviot and Goldscleugh farm that goes by Woolhope Crag, we take the easy option of buying a day permit for ten pounds from Savills, Glendale Road, Wooler and driving up the College Valley to Dunsdale.
Our main aim on this excursion is to investigate the bedrock that outcrops above the 550m contour on Bellyside Hill shown on current BGS maps to be part of the northern limit of the Cheviot pluton.
Back in 1932, Carruthers and Anderson classified these rocks as lava writing, ‘South of the Lambden Burn on Bellyside Hill, above the 1,500-ft. contour, there are several fairly distinct features which maintain a rude parallelism round the hill. Though exposures are few, these all seem to be in good, black andesite and contrast sharply with the even slopes of the neighbouring and higher granite ground on The Cheviot.’ Later, Jhingran (1943) identified these exposures as the ‘Marginal’ type of plutonic rock that also appears close to the pluton’s edge on Dunmoor Hill and in the Breamish Valley. The work of Robson (1976) and Al-Hafdh (1985) supported the ‘Marginal’ classification, Al-Hafdh also identifying an area of his felsic, sub-equigranular ‘Woolhope’ type on the slopes of the neighbouring Mid Hill.
A second aim of our excursion is to find and examine the andesite inlier that is mapped at the head of Goldscleugh.
Location 1. Outcrop of ‘Marginal’ rock
The first outcrop of Marginal rock on Bellyside Hill that we sample, NT910218
The slopes of the Cheviot peat bog are in the background.
Having parked the car out of the way near Dunsdale, we set off on a distinct track up Bellyside Hill. It makes an enjoyable change to take a route in the Cheviots that has all the character of a ridge walk and soon we are enjoying views cross the Lambden and College Valleys to the hills beyond.
The rock at location 1 has a high pyroxene content and, as we have seen in the ‘Marginal’ rock adjacent to the lavas on Dunmoor Hill, an virtual absence of primary biotite. Secondary, inclusion-free biotite occurs in small flakes that fringe the many altered pyroxene crystals. The plagioclase content is low compared to the ‘Marginal’ rocks at Cunyan Crags.
‘Marginal’ rock, Bellyside Hill, NT910218
Prepared hand specimen in ordinary reflected light (48mm across)
A thin section from the same sample viewed in plane polarised light
The same thin section viewed with crossed polarising filters
General view of the rock in thin section
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)
Plagioclase with orthopyroxene and clinopyroxene phenocrysts with micrographic interstitial quartz and alkali feldspar, secondary biotite and amphibole, magnetite and limonite.
The same area viewed with crossed polarising filters
Orthopyroxene with plagioclase phenocrysts
Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)
An orthopyroxene crystal at 45o
Section viewed in plane polarised light (FoV 1.2 x 0.8 mm)
The same orthopyroxene crystal rotated to 90o viewed in plane polarised light
In this position relative to the direction of polarised light, the crystal takes on a red hue.
The same crystal viewed with crossed polarising filters
Comparison with other rocks of the same type and a contrast with the ‘Central Belt’ type
Like the ‘Marginal’ rocks to the south of the pluton, crystals in these rocks vary from the very fine- to an almost medium-grain size. Some of our samples have phenocrysts of plagioclase and pyroxene in a fine-grained matrix whilst others are more equigranular. All them share a low biotite content, primary biotite being virtually absent, a feature that helps to distinguish the ‘ Marginal’ type from the ‘Central Belt’ type that does contain large primary biotite crystals. The mafic content is around 10-15% with opaque iron-titanium oxides making up about a half of that. Quartz is between 10-20% and surprisingly, the plagioclase content is very low making up less than 35% of the feldspar content.
‘Marginal’ rock at this location
This rock has some much finer-grained crystals as well as micrographic quartz and feldspar indicative of a period of more rapid cooling.
General appearance of the ‘Marginal’ rock at Cunyon Crag
Its biotite has very few inclusions and appears in close association with altered pyroxene.
Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)
Central Belt rock from Dunmoor Hill viewed at the same magnification.
This rock is coarser-grained and its biotite is inclusion-rich which suggesting it is a primary mineral.
Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)
Location 2.
Our second sampled outcrop of Marginal rock, Bellyside Hill, NT910216
We take a couple of samples here. The first looks identical to the rock at location 1 but a thin section reveals some coarse and fine micrographic textures in the quartz and feldspar whereas we only saw coarse micrographic texture at location 1.
The second sample we take is of a red, felsic vein – one of a number that have been intruded into these rocks at the edge of the pluton. Thin sections reveal xenocrysts and xenoliths in the felsic material – small pieces of ‘Marginal’ rock that have been broken off and transported in the flow. We also find small amounts of tourmaline.
Marginal rock, Bellyside Hill NT910216
Prepared hand specimen in ordinary reflected light (45mm across)
Thin section from the same sample viewed in plane polarised light
The same thin section viewed with crossed polarising filters
Plagioclase, clinopyroxene and orthopyroxene with coarsely micrographic quartz and alkali feldspar, Bellyside Hill
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)
The same area viewed with crossed polarising filters
Orthopyroxene and clinopyroxene with coarsely micrographic quartz and alkali feldspar, Bellyside Hill
Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)
Orthopyroxene and clinopyroxene with secondary biotite and magnetite in coarsely micrographic quartz and alkali feldspar, Bellyside Hill
Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)
Specimen 2
The same crystals viewed with crossed polarising filters
Felsic intrusion, Bellyside Hill NT910216
Prepared hand specimen in ordinary reflected light (48mm across)
Thin section from the same sample viewed in plane polarised light
The same thin section viewed with crossed polarising filters
Quartz in coarse and fine micrographic association with alkali feldspar together with plagioclase and a few biotite flakes
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)
The same area viewed with crossed polarising filters
Coarse and fine micrographic texture in quartz and feldspar along with some perthite
Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)
The same area viewed with crossed polarising filters
An apatite crystal amongs other chloritised inclusions in feldspar phenocryst or microxenolith
Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)
Location 3. Felsic intrusion into ‘Marginal’ dioritic rock
Red, felsic material that is intruded into ‘Marginal’ rock at location 3, Bellyside Hill, NT910215
Veins of the red, felsic intrusive rock take our attention here. Once again, we see small pieces of the ‘Marginal’ rock torn off and carried away in the flow of the minor intrusion as well as chilling of the latter against the former confirming the ‘Marginal’ rock’s earlier crystallisation. A kinked biotite crystal, similar to those seen in the ‘Marginal’ rock above High Bleakhope, evidences the forces at work in the process of intrusion.
Felsic intrusion into Marginal rock, Bellyside Hill, NT910215
Prepared hand specimen in ordinary reflected light (50mm across)
Thin section from the same sample viewed in plane polarised light
The same thin section viewed with crossed polarising filters
Xenoliths and xenocrystals of host rock carried in fine-grained felsic dyke material
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)
The same area viewed with crossed polarising filters
Kinked secondary biotite at the edge of pyroxene
Section viewed in plane polarised light (FoV 1.2 x 0.8 mm)
The same area viewed with crossed polarising filters
Locations 4. Boulder at the head of Goldscleugh
Head of Goldscleugh, NT910209
An andesite inlier is shown here on BGS maps.
Our next location is the head of Goldscleugh where we hope to find an outcrop of the andesite inlier shown on the BGS map. As we climb to it, the path soon becomes indistinct and then disappears completely and so we make our way over rough ground to the large boulders that litter the upper part of the cleugh.
We can’t find an outcrop and not one of the boulders we inspect appears to be andesitic – so we make do with a sample from a very large boulder of the ‘Marginal’ type that we hope hasn’t travelled far.
It is interesting on account of it being similar to the ‘Marginal’ rock found at the southern edge of the pluton that contains thin veins of black, glassy to fine-grained mafic material. Thin sections reveal clear flow structures in this vein material around numerous fragments of pre-crystallised rock.
Marginal rock, Head of Goldscleugh, NT910209
Prepared hand specimen in ordinary reflected light (48mm across)
Marginal rock, Head of Goldscleugh, NT910209
Prepared hand specimen in ordinary reflected light (48mm across).
The same thin section viewed with crossed polarising filters
Vein of glassy, opaque-rich material in ‘Marginal’ rock
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)
The vein viewed with crossed polarising filters
Another part of the same vein
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)
The same area viewed with crossed polarising filters(FoV 4.6 x 3.0 mm)
Vein of glassy, opaque-rich material in ‘Marginal’ rock
Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)
Calcite in this sample of ‘Marginal’ rock
Section viewed in plane polarised light (FoV 1.2 x 0.8 mm)
The same area viewed with crossed polarising filters
Pyroxene in this sample of ‘Marginal’ rock
Section viewed in plane polarised light (FoV 1.2 x 0.8 mm)
The same area viewed with crossed polarising filters
Locations 5. Cobbles in the peat to the west of Goldscleugh
We leave the head of Goldscleugh and weave our way through the vegetation over to Bellyside Crag. As we go, we look for evidence for the andesite inlier but we don’t find any. We do sample a couple of cobbles found in an eroded area of peat. The first is a surprisingly fresh piece of ‘Marginal’ rock. The second is a thoroughly altered, yellowish rock that proves to be extensively sericitised and full of muscovite.
‘Marginal’ rock, Goldscleugh, NT910208
Prepared hand specimen in ordinary reflected light (48mm across)
A thin section from the same sample viewed in plane polarised light
The same thin section viewed with crossed polarising filters
Plagioclase, orthopyroxene and clinopyroxene with quartz and alkali feldspar, magnetite and secondary biotite in ‘Marginal’ rock
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)
The same area viewed with crossed polarising filters
Secondary biotite associated with altered orthopyroxene and clinopyroxene with micrographic quartz and alkali feldspar
Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)
The same area viewed with crossed polarising filters
Secondary biotite in association with altered orthopyroxene and clinopyroxene with micrographic quartz and alkali feldspar
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)
Sample 2
The same area viewed with crossed polarising filters
Altered rock, Goldscleugh, NT910208
Prepared hand specimen in ordinary reflected light (50mm across)
A thin section from the same sample viewed in plane polarised light
The same thin section viewed with crossed polarising filters
Muscovite (sericite) in intensely altered rock with quartz and feldspar
Section viewed in plane polarised light ((FoV 2.3 x 1.5 mm)
The same area viewed with crossed polarising filters
Muscovite (sericite) in intensely altered rock with quartz and feldspar
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)
The same area viewed with crossed polarising filters
Location 6. Bellyside Crag
Bellyside Crag, NT905211
We head west over rough ground to Bellyside Crag.
The crag is a small one but with fine views over the College and Lambden Valleys and the andesite hills to the north. The ‘Marginal’ rock here has the characteristics of a quartz syenite. Its mafic content is around 10% with opaque materials making up about a half of this. Quartz, both coarsely and finely granophyric, makes up less than 20% of the rock and the plagioclase makes up less than 35% of the feldspar content.
There is very little, if any primary biotite in this rock. Secondary biotite occurs as small flakes with iron-titanium oxides around and within altered pyroxene crystals. Crystal size vary widely.
‘Marginal’ syenitic rock, Bellyside Crag, NT905211
Prepared hand specimen in ordinary reflected light (48mm across)
A thin section from the same sample viewed in plane polarised light
The same thin section viewed with crossed polarising filters
Pyroxenes, plagioclase with coarsely micrographic quartz and alkali feldspar with opaques in Marginal rock, Bellyside Crag
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)
The same area viewed with crossed polarising filters
Secondary biotite fringing altered orthopyroxene
Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)
The same area viewed with crossed polarising filters
Partially albitised plagioclase crystals, Bellyside Crag
Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)
The same area viewed with crossed polarising filters
Orthopyroxene and clinopyroxene with micrographic quartz and alkali feldspar with opaques, Bellyside Crag
Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)
References
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
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
Jhingran, A.G .1943. The Cheviot Granite. Reprint from the Quarterly Journal of the Geological Society of London, Vol. 98, pp, 240-254.
Robson, D.A.. 1976. A Guide to the Geology of the Cheviot Hills. Natural History Society of Northumbria, The Hancock Museum, Newcastle Upon Tyne, Vol. 43, 1.