Cheviot and Woolhope Crag

Cheviot from Hedgehope Hill in winter

Cheviot from Hedgehope Hill in winter

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

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

There are no exposures of bedrock on this well-worn walk up Cheviot other than that at Woolhope Crag that we will take in on the return leg. However, there are many loose rocks lying on and around the footpath and there is a good chance that the rock types we find amongst them represent the rocks types that underlie the peat. We are aiming to continue our investigation of the fine to medium-grained sub-equigranular, ‘sugar textured’, felsic granite that Al-Hafdh called the ‘Woolhope’ type. We believe it is the predominant type in the stones, cobbles and boulders that litter the top of Cheviot and we know that it outcrops at Woolhope Crag – but how constant is its composition and are we justified in following Al-Hafdh in recognising it as one distinct type?

Location 1. Scald Hil

Loose granitic rock, much of it with quartz veins, on the footpath up to Cheviot. Hedgehope is visible to the south-east.

Loose granitic rock, much of it with quartz veins, litter the footpath up to Cheviot
Hedgehope is visible to the south-east.

From here at location 1 on Scald Hill to location 2 at the large boulder to the left of the path up to Cheviot, erosion along the footpath has exposed many stones and cobbles. Amongst them, we find many with tourmaline and quartz that is often stained orange-red by haematite. Sometimes the tourmaline is in lumps, sometimes in stringers criss-crossing the rock but it is always in association with quartz.
The presence of silica veining with haematite in the fractures and faults in the Cheviot complex has been associated with tectonic processes in the Hercynian, occurring between 325and 300 Ma. These physical and chemical processes, constituting the last phase of igneous activity, are thought to have enabled the fluid transport and deposition of silica through the lava, granite and dyke rocks causing widespread mineralogical changes including in the low-temperature oxidation of their iron-titanium oxides to haematite and their re-magnetization.


Specimen 1

Tourmaline with quartz and haematite, Scald Hill NT929219. Hand sample.

Tourmaline with quartz and haematite, Scald Hill NT929219
Hand sample

Tourmaline in fine-grained granite. Scald Hill NT929219. Prepared hand specimen in ordinary reflected light

A prepared (semi-polished) hand specimen from the same rock

Tourmaline in quartz vein. Scald Hill NT929219. Thin section viewed with crossed polarising filters (22mm across)

A thin section from the same specimen viewed with crossed polarising filters(22mm across)

Tourmaline with quartz. Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Tourmaline with quartz
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Tourmaline with quartz. Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

The same area viewed with crossed polarising filters

Acicular tourmaline in quartz viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Acicular tourmaline in quartz viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Specimen 2

Quartz vein with rock fragments in granite, Cheviot NT921210. Hand specimen

Quartz vein with rock fragments in granite, Cheviot NT921210
Prepared hand specimen

Quartz vein with rock fragments in granite, Cheviot NT921210. Thin section viewed with crossed polarising filters (40mm across)

Quartz vein with rock fragments in granite, Cheviot NT921210
Thin section viewed with crossed polarising filters (40mm across)

Fine-grained quartz around fragments of larger-grained quartz and sericitised feldspar. Cheviot NT921210. Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

Fine-grained quartz around fragments of larger-grained quartz and sericitised feldspar, Cheviot NT921210
Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

Along with the quartz and tourmaline dominated rocks there are many pieces of red to grey granitic rock amongst the debris on the path.
Thin sections reveal that some of it is the sub-equigranular, alkali feldspar-rich ‘Woolhope’ type of Cheviot granite. Other samples are different, being more porphyritic in character with plagioclase phenocrysts in a fine-grained groundmass. A third type of rock is much coarser grained and more mafic – so much so that we thought it might be the ‘Marginal’ type appearing here because of this location’s proximity to the roof of the magma chamber. In thin section, this rock is more akin to the ‘Central Belt’ rocks we see in the Harthope Valley below.

Specimen 3

Fine-grained porphyritic granitic rock, Cheviot NT919210. Prepared hand specimen in ordinary reflected light (45mm across) Note alignment of biotite needles

Fine-grained porphyritic granitic rock, Cheviot NT919210
There is an interesting alignment of biotite needles in this specimen.
Prepared hand specimen in ordinary reflected light (45mm across)

Fine-grained porphyritic granitic rock, Cheviot NT919210. Thin section viewed in plane polarised light (30mm across)

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

Fine-grained porphyritic granitic rock, Cheviot NT919210. Thin section viewed with crossed polarising filters (30mm across)

The same thin section viewed with crossed polarising filters

Fine-grained porphyritic granitic rock, Cheviot NT919210. Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Fine-grained porphyritic granitic rock, Cheviot NT919210
Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Specimen 4

Fine-grained porphyritic granitic rock, Cheviot NT919210. Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

The same crystals viewed with crossed polarising filters

Fine-grained porphyritic granitic rock, Cheviot NT919210. Prepared hand specimen in ordinary reflected light (50mm across)

Fine-grained porphyritic granitic rock, Cheviot NT919210
Prepared hand specimen in ordinary reflected light (50mm across)

Fine-grained porphyritic granitic rock, Cheviot NT919210. Thin section viewed in plane polarised light (48mm across)

Thin section from the same sample viewed in plane polarised light

Fine-grained porphyritic granitic rock, Cheviot NT919210. Thin section viewed with crossed polarising filters (48mm across)

The same thin section viewed with crossed polarising filters

Altered biotite with feldspars in quartz-rich groundmass, Cheviot. Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Altered biotite with feldspars in quartz-rich groundmass, Cheviot
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Altered biotite with feldspars in quartz-rich groundmass, Cheviot. Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

The same area viewed with crossed polarising filters

Yellow, pleochroic, chloritic mineral in altered biotite. Section viewed in plane polarised light (FoV 1.2 x 0.8 mm)

Yellow, pleochroic, chloritic mineral in altered biotite
Section viewed in plane polarised light (FoV 1.2 x 0.8 mm)

Yellow, pleochroic, chloritic mineral in altered biotite. Section rotated 45 degrees viewed in plane polarised light (FoV 1.2 x 0.8 mm)

The same section rotated 45o

Yellow, pleochroic, chloritic mineral in altered biotite. Section rotated 90 degrees viewed in plane polarised light (FoV 1.2 x 0.8 mm)

Section rotated 90o showing more saturated colour

Yellow, pleochroic, chloritic mineral with calcite in altered biotite. Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Yellow, pleochroic, chloritic mineral with calcite in altered biotite
Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Yellow, pleochroic, chloritic mineral with calcite in altered biotite. Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

The same area viewed with crossed polarising filters

Zircon with inclusions. Section viewed with crossed polarising filters (FoV 0.5 x 0.3 mm)

Zircon with inclusions in the same sample
Section viewed with crossed polarising filters (FoV 0.5 x 0.3 mm)

Altered Central Belt granitic rock, NT917209. Prepared hand specimen in ordinary reflected light (45mm across)

Altered Central Belt granitic rock, NT917209
Prepared hand specimen in ordinary reflected light (45mm across)

Altered Central Belt granitic rock. Thin section viewed in plane polarised light (20mm across)

Thin section from the same sample viewed in plane polarised light

Altered Central Belt granitic rock. Thin section viewed with crossed polarising filters (20mm across)

The same thin section viewed with crossed polarising filters

Plagioclase, perthitic feldspar, altered pyroxens and biotite and micrographic quartz and feldspar. Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Plagioclase, perthitic feldspar, altered pyroxene and biotite and micrographic quartz and feldspar
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm))

Plagioclase, perthitic feldspar, altered pyroxene and biotite and micrographic quartz and feldspar. Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

The same area viewed with crossed polarising filters

Micrographic texture. Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

Fine micrographic texture in the same sample
Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

Chlorite with a high relief mineral that is possibly, epidote with opaque iron titanium oxides. Section viewed in plane polarised light (FoV 1.2 x 0.8 mm)

Chlorite with a high relief mineral that is possibly, epidote with opaque iron titanium oxides
Section viewed in plane polarised light (FoV 1.2 x 0.8 mm)

Chlorite with a mineral showing moderately high birefringence that is possibly, epidote with opaque iron titanium oxides. Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

The same area viewed with crossed polarising filters
The mineral with high relief shows moderately high birefringence.

Biotite, magnetite and chlorite with quartz and feldspar in micrographic texture. Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Biotite, magnetite and chlorite with quartz and feldspar in micrographic texture
Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Biotite, magnetite and chlorite with quartz and feldspar in micrographic texture. Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

The same area viewed with crossed polarising filters

Location 2. Large boulder by the path up to Cheviot

Location 2. Large boulder of sub-equigranular granite, Cheviot NT915206

Large boulder of sub-equigranular granite, Cheviot NT915206
The path runs alongside the fence in the background.

This conspicuous large boulder lies beside a field of smaller boulders and cobbles that are a mix of the more equigranular, very felsic rock and the more porphyritic, more mafic rocks that we have seen on the way up. The boulder is of the first type and fromhere to location 3 on Cairn Hill, any loose rock we find along the way is more likely to be the same.
The second specimen shown here is of the second type.

Specimen 1

Sub-equigranular granite, Cheviot. Prepared hand specimen in ordinary reflected light (46mm across)

Sub-equigranular granite, Cheviot
Prepared hand specimen in ordinary reflected light (46mm across)

Sub-equigranular granite, Cheviot. Thin section viewed in plane polarised light (44mm across)

Thin section from the same specimen viewed in plane polarised light

Sub-equigranular granite, Cheviot. Thin section viewed with crossed polarising filters (44mm across)

The same thin section viewed with crossed polarising filters

Mostly alkali feldspar and coarsely granophyric quartz with tourmaline in granite, Cheviot. Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Mostly alkali feldspar and coarsely granophyric quartz with tourmaline in granite, Cheviot
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Specimen 2

Mostly alkali feldspar and coarsely granophyric quartz with tourmaline in granite, Cheviot. Section viewed with crossed polarising filters(FoV 4.6 x 3.0 mm)

The same crystals viewed with crossed polarising filters

Fine-grained groundmass porphyritic Central Belt, Cheviot NT914206. Prepared hand specimen in ordinary reflected light (44mm across)

Fine-grained groundmass porphyritic Central Belt, Cheviot NT914206
Prepared hand specimen in ordinary reflected light (44mm across)

Fine-grained groundmass porphyritic Central Belt, Cheviot NT914206. Thin section viewed in plane polarised light (42mm across)

Thin section from the same sample viewed in plane polarised light

Fine-grained groundmass porphyritic Central Belt, Cheviot NT914206. Thin section viewed with crossed polarising filters (42mm across)

The same thin section viewed with crossed polarising filters

Feldspar-rich groundmass with biotite Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Feldspar-rich groundmass with biotite
Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Feldspar-rich groundmass with secondary biotite. Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

The same area viewed with crossed polarising filters

Feldspar-rich groundmass with biotite Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Flakes of secondary biotite in altered plagioclase
Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

Altered clinopyroxene enclosed in plagioclase. Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Altered clinopyroxene enclosed in plagioclase
Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Altered clinopyroxene enclosed in plagioclase. Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

The same area viewed with crossed polarising filters

Secondary biotite forming at the edge of a corroded crystal, possibly pyroxene, enclosed in plagioclase. Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

Secondary biotite forming at the edge of a corroded crystal, probably clinopyroxene, enclosed in plagioclase
Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

Hornblende. Section viewed in plane polarised light (FoV 1.2 x 0.8 mm)

Hornblende
Section viewed in plane polarised light (FoV 1.2 x 0.8 mm)

Hornblende. Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

The same area viewed with crossed polarising filters

Hornblende near inclined extinction. Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

The hornblende crystal near inclined extinction
Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

Area of quartz in the groundmass. Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Area of quartz in the groundmass
Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Area of quartz in the groundmass. Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

The same area viewed with crossed polarising filters

Location 3. Scotsman’s Cairn

As we walk over to the summit and then on to Scotsman’s Cairn on Cairn Hill, we appreciate the ecology and landscape and enjoy conversations with other walkers but there is little of petrological interest up here. Any loose pieces of rock we come across do tend to be of the felsic, non-porphyritic type.
Scotsman’s Cairn, an excellent place to shelter from the wind and eat lunch, is constructed almost entirely with this type of rock.

Sub-equigranular granite, Scotsmans Cairn, Cheviot NT903194. Thin section viewed with crossed polarising filters (47mm across)

Sub-equigranular granite, Scotsmans Cairn, Cheviot NT903194
Thin section viewed with crossed polarising filters (47mm across)

Sub-equigranular granite, Scotsmans Cairn, Cheviot. Thin section viewed in plane polarised light (47mm across)

Thin section from the same specimen viewed in plane polarised light

Sub-equigranular granite, Scotsmans Cairn, Cheviot. Thin section viewed in plane polarised light (47mm across)

The same thin section viewed with crossed polarising filters

Sericitic plagioclase with coarsely micrographic quartz and alkali feldspar, tourmaline and chlorite in sub-equigranular granite, Scotsmans Cairn, Cheviot. Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

Sericitic plagioclase with coarsely micrographic quartz and alkali feldspar, tourmaline and chlorite in sub-equigranular granite, Scotsman’s Cairn, Cheviot
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Sericitic plagioclase with coarsely micrographic quartz and alkali feldspar, tourmaline and chlorite in sub-equigranular granite, Scotsmans Cairn, Cheviot. Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

The same crystals viewed with crossed polarising filters

Biotite altered to chlorite, Scotsmans Cairn, Cheviot. Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

Biotite altered to chlorite, Scotsman’s Cairn, Cheviot
Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

Location 4. Head of the Harthope Burn

The sub-equigranular alkali-rich granite predominates in the scree on the slope from Scotsman’s Cairn down to here. It then gives way to the outcropping Central Belt type that is described in the Harthope Valley excursion.

Location 5. Woolhope Crag

Approaching Woolhope Crag over rough ground from the west.

Approaching Woolhope Crag over rough ground from the west

We return over the summit of Cheviot to where the path splits near location 2. We take the path heading north and then cut across rough ground to Woolhope Crag.
This is one of the few outcrops of the most felsic of rock types in the Cheviot. The southern, higher end of the crag is lighter in colour than that lower down after the break at the crag – but both rocks are the much the same under the microscope.
Al-Hafdh described the rock as having a ‘saccaroidal’ character regarding its grains, and in thin section it is more equigranular than it is porphyritic. The overall crystal size is quite fine, however there are some larger, altered plagioclase crystals that measure around 2mm.
Mafic minerals make up only about 7% of the rock, biotite appearing as ragged ‘rods’ and flakes up to 1mm in length with magnetite at their rims and in cleavages. The biotite crystals have no inclusions. Tourmaline crystals are prismatic up to 0.6mm in length. There are small patches of magnetite in the rock and a dusting of haematite throughout.
There are fewplagioclase crystals – most of them heavily corroded but some still showing albite twinning. An approximate ratio of plagioclase to alkali feldspar is 1:3, some of the alkali feldspars show Carlsbad twinning.
The rock is relatively quartz-rich, much of it appearing with a micrographic texture resulting from rapid cooling in the final stages of the rock’s crystallisation or perhaps alteration processes. The inclusion of a patch of micrographic quartz in a Carlsbad twinned feldspar crystal could suggest the latter.

Granite, Woolhope Crag NT922221. Prepared hand specimen in ordinary reflected light (36mm across)

Granite, Woolhope Crag NT922221
Prepared hand specimen in ordinary reflected light (36mm across)

Granite, Woolhope Crag NT922221. Thin section viewed in plane polarised light (33 mm across)

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

Granite, Woolhope Crag NT922221. Thin section viewed with crossed polarising filters (33 mm across)

The same thin section viewed with crossed polarising filters

Coarsely micrographic quartz with feldspar, corroded plagioclase and biotite with perthitic feldspar, Woolhope Crag. Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Coarsely micrographic quartz with feldspar, corroded plagioclase and biotite with perthitic feldspar, Woolhope Crag
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Coarsely micrographic quartz with feldspar, corroded plagioclase and biotite with perthitic feldspar, Woolhope Crag. Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

The same area viewed with crossed polarising filters

Coarsely micrographic quartz included in twinned feldspar with tourmaline. Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

Coarsely micrographic quartz included in twinned feldspar with tourmaline
Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

One of a number of rods of biotite. Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

One of a number of ‘rods’ of biotite in the sample
Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

One of a number of rods of biotite. Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

The same area viewed with crossed polarising filters

Woolhope Crag NT922221. Looking north along the line of the outcrop towards Preston Hill (left) and Broadhope Hill (right)

Looking north along the line of Woolhope Crag NT922221
The line of the crag suggests its origin as a dyke but we have not seen evidence of chilling at its sides. Preston Hill is in the background to the left and Broadhope Hill to the right.

White mica, tourmaline and biotite with quartz and plagioclase and alkali feldspar, Woolhope Crag. Section viewed in plane polarised light (FoV 1.2 x 0.8 mm)

White mica, tourmaline and biotite with quartz and plagioclase and alkali feldspar, Woolhope Crag
Section viewed in plane polarised light (FoV 1.2 x 0.8 mm)mm)

White mica, tourmaline and biotite with quartz and plagioclase and alkali feldspar, Woolhope Crag. Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

The same area viewed with crossed polarising filters

Sericite, chlorite and haematite in altered plagioclase. Section viewed in plane polarised light (FoV 1.2 x 0.8 mm)

Sericite, chlorite and haematite in altered plagioclase
Section viewed in plane polarised light (FoV 1.2 x 0.8 mm)

Sericite, chlorite and haematite in altered plagioclase. Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

The same area viewed with crossed polarising filters

Altered plagioclase with alkali feldspar and quartz. Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

Altered plagioclase with alkali feldspar and quartz
Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

Return to car

From Woolhope Crag we head down over rough ground to the head of the Lambden Burn and up the other side to meet the path again on Scald Hill. From here we walk back along the path to the car park near Langleeford.

References

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.

J. G. Mitchel, K. M. Storetvedt, D. A. Robson, M. C. Abranches, and P. R. Ineson, 1993, Evidence for Carboniferous thermochemical overprinting in the Cheviot Complex, Scottish Journal of Geology 29, (1), 55-68,

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

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

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