Dunmoor Hill

Dunmoor Hill with Cunyon Crags from Knock Hill

Dunmoor Hill with Cunyon Crags from Knock Hill

This excursion provides opportunities to investigate the rock at the granite/lava border. At best, we might find an exposure where the junction is visible but, given the degree to which the Cheviot bedrock is cloaked in peat, we think this is unlikely. It also gives us he chance to compare the rock here with that on nearby Hedgehope Hill. Al-Hafdh identified them as different types, intruded at different times but, so far in our investigations, we are not persuaded. A third point of interest is an inlier of andesite mapped close to Long Crag here on Dunmoor Hill – we are interested in the degree to which it has been altered by the surrounding plutonic rock.

Map showing the route of the Dunmoor Hill excursion with locations and igneous rock types

Key

Key for the Dunmoor Hill excursion map

Location 1. Hornfelsed andesite at NT984179

Having parked at Hartside and walked west to Linhope, our route follows the bridleway from the settlement at the foot of Dunmoor Hill along the andesite lava/granite pluton junction towards the east end of Dunmoor Hill. The pluton is indicated by the lie of the land to our left as it climbs steeply uphill.
Just north of the site of the medieval village, a footpath leads north from the bridleway towards the corner of Threestoneburn Wood. The bedrock just to the left of the path is andesite with its plagioclase phenocrysts set in a matrix that is rich in mafic materials including magnetite and pyroxene. We are very close to the andesite/granite boundary and the rock in this vicinity (NT 98494 1792) is particularly interesting.


Andesite hornfels outcropping at Location 1 (NT984179). Prepared hand specimen viewed in reflected light.

Andesite hornfels outcropping at Location 1 (NT984179)
Prepared hand specimen viewed in reflected light measuring 32mm across.

Andesite hornfels (S.G. 2.84) Dunmoor Hill. NT984179. Thin section viewed in plane polarised light (Section measures 20 mm across).

Thin section from the same sample viewed in plane polarised light.This section measures 20 mm across.

Andesite hornfels (S.G. 2.84) Dunmoor Hill. Thin section viewed with crossed polarising filters (Section measures 20 mm across)

The same thin section viewed with crossed polarising filters


Remnants of mafic phenocrysts in chloritic groundmass of andesite hornfels. Viewed in plane polarised light (FoV 5 x 3.3mm).

Remnants of mafic phenocrysts in the chloritic groundmass of andesite hornfels at Location 1
Viewed in plane polarised light (FoV 5 x 3.3mm).

Remnants of mafic phenocrysts in chloritic groundmass of andesite hornfels. Viewed with crossed polarising filters (FoV 5 x 3.3mm)

The same area viewed with crossed polarising filters

Tourmaline in chloritic groundmass of andesite hornfels. Section viewed in plane polarised light ((FoV 1.3 x 0.8mm)

Tourmaline in the chloritic groundmass of the same sample.
The crystals Section viewed in plane polarised light ((FoV 1.3 x 0.8mm)

Tourmaline in chloritic groundmass of andesite hornfels. Section viewed with crossed polarising filters ((FoV 1.3 x 0.8mm).

The same crystals viewed with crossed polarising filters

Tourmaline in chloritic groundmass of andesite hornfels. Section viewed in plane polarised light (FoV 0.6 x 0.4mm)

Tourmaline in chloritic groundmass of andesite hornfels
The tourmaline crystals that appear in our samples of the hornfels at location 1 are all this brown – grey in colour. This crystal’s zoning is clearly seen.
Section viewed in plane polarised light (FoV 0.6 x 0.4mm)


Andesite hornfels with quartz veining (SG 2.85 ) outcropping on Dunmoor Hill at NT983179. Prepared hand sample measuring 40mm across viewed in reflected light.

Andesite hornfels with quartz veining (SG 2.85 ) outcropping on Dunmoor Hill at NT983179
Prepared hand specimen viewed in reflected light measuring 40mm across.

Andesite hornfels with some quartz veining XP Dunmoor Hill NT983179. Viewed in plane polarised light (Section measures 40mm across)

Thin section from the same sample viewed in plane polarised light.This section measures 40 mm across.

Andesite hornfels with some quartz veining XP Dunmoor Hill NT983179. Viewed with crossed polarising filters (Section measures 40mm

The same thin section viewed with crossed polarising filters

Chlorite in the andesite hornfels shown above viewed in lane polarised light (FoV 5 x 3.3 mm).

Chlorite in the andesite hornfels shown above
Viewed in plane polarised light (FoV 5 x 3.3mm).

Chlorite in andesite hornfels viewed with crossed polarising filters. Chlorite with blue and brown interference colours and chlorite with grey to yellow interference colours are visible in this rock. (FoV 5 x 3.3 mm)

The same area of chlorite viewed with crossed polarising filters
Chlorite with blue and brown interference colours and chlorite with grey to yellow interference colours are visible in this rock.

Pyroxene inclusions in plagioclase viewed in plane polarised light. The clinopyroxene has a pale yellow colour in plane polarised light. (FoV 1.3 x 0.8 mm)

Clinoyroxene (augite) inclusions in plagioclase in the same sample
The clinopyroxene has a pale yellow colour in plane polarised light (FoV 1.3 x 0.8 mm).

Pyroxene inclusions in plagioclase viewed with crossed polarising filters (FoV 1.3 x 0.8 mm)

Pyroxene inclusions in plagioclase
Section viewed with crossed polarising filters (FoV 1.3 x 0.8 mm)


Pyroxene inclusions in plagioclase viewed with crossed polarising filters. The largest clinopyroxene crystal that is partially included in the plagioclase shows twinning and is close to extinction. (FoV 1.3 x 0.8 mm)

Pyroxene inclusions in plagioclase viewed with crossed polarising filters
The largest clinopyroxene crystal that is partially included in the plagioclase shows twinning and is close to extinction. (FoV 1.3 x 0.8 mm)

Location 2. ‘Marginal’ quartz monzonite at NT981179


Thin-sections of the outcropping rock from this location prove it to be granitic and therefore part of the pluton. Grain-sizes of the quartz, feldspars, biotite, pyroxenes and opaque iron/titanium ores vary enormously. Often individual crystals and small pieces of medium-grained rock similar to that found higher up the path at Cunyon Crags, are seen suspended in the fine-grained material suggesting the fine-grained component of the rock was injected later than the medium-grained component causing disruption before cooling quickly.

Fine-grained quartz monzonite outcropping at Location 2. prepared hand specimen viewed in reflected light

Fine-grained ‘Marginal’ quartz monzonite outcropping at Location 2
Prepared hand specimen viewed in reflected light

Fine-grained Marginal quartz  monzonite at Location 2, Dunmoor Hill viewed with crossed polarising filters.

A thin-section from the same sample viewed with crossed polarising filters

Pyroxene and biotite in the groundmass of the same fine-grained Marginal quartz  monzonite. Section viewed in plane polarised light. (FoV 1.3 x 0.8 mm)

Pyroxene and biotite in the groundmass of the same fine-grained ‘Marginal’ quartz monzonite
The high relief, clear to pale-green crystals scattered throughout the groundmass are most probably pyroxene. There is also an abundance of fine-grained magnetite. Section viewed in plane polarised light. (FoV 1.3 x 0.8 mm)

Pyroxene and biotite in the groundmass of the same sample viewed with crossed polarising filters. (FoV 1.3 x 0.8 mm)

The same area of the thin section viewed with crossed polarising filters
The mid-first to second order interference colours suggest clinopyroxene rather than orthopyroxene. Section viewed in with crossed polarising filters (FoV 1.3 x 0.8 mm)

Altered clinopyroxene in the same fine-grained quartz monzonite viewed in plane polarised light (FoV 2.5 x 1.7 mm)

Altered clinopyroxene in the same fine-grained ‘Marginal’ quartz monzonite
There is no sign of pleochroism in these crystals.
The section is viewed in plane polarised light (FoV 2.5 x 1.7 mm)

Clinopyroxene crystals in  the same fine-grained quartz-monzonite viewed with crossed polarising filters (FoV 2.5 x 1.7 mm)

The same clinopyroxene crystals viewed with crossed polarising filters
The twinning that these crystals exhibit along with roughly 90o cleavage, inclined extinction and higher interference colours are all typical of clinopyroxene.
(FoV 2.5 x 1.7 mm)

Orthopyroxene at location 2, Dunmoor Hill viewed in plane polarised light. The non-pleochroic crystal, much altered and darkened by opaques, is viewed in plane polarised light.(FoV 2.5 x 1.7 mm)

Orthopyroxene in fine-grained quartz monzonite at location 2
The non-pleochroic crystal, much altered and darkened by opaques, is viewed in plane polarised light. (FoV 2.5 x 1.7 mm)

Orthopyroxene at location 2, Dunmoor Hill viewed with crossed polarising filters. The crystal exhibits fine lamella, low interference colours and straight extinction, all characteristics of orthopyroxene (FoV 2.5 x 1.7 mm).

The same orthopyroxene crystal viewed with crossed polarising filter (FoV 2.5 x 1.7 mm)
The crystal exhibits fine lamella, low interference colours and straight extinction, all characteristics of orthopyroxene (FoV 2.5 x 1.7 mm).

Glomerocryst of plagioclase and augite in the fine-grained matrix viewed at location 2. Section viewed in plane polarised light (FoV 5 x 3.3 mm)

Glomerocryst of plagioclase and augite in the fine-grained matrix viewed at location 2.
Section viewed in plane polarised light (FoV 5 x 3.3 mm).

Glomerocryst of plagioclase and augite in the fine-grained matrix viewed with crossed polarising filters (FoV 5 x 3.3 mm)

The same crystals viewed with crossed polarising filters (FoV 5 x 3.3 mm)

Chlorite vein with actinolite and possibly epidote (with very high relief in upper right of vein) in the same thin section viewed in plane polarised light (FoV1.3 x 0.8 mm)

Chlorite vein with actinolite and possibly epidote (with very high relief in upper right of vein) in the same thin section
Section viewed in plane polarised light (FoV 1.3 x 0.8 mm).

Chlorite vein with actinolite and possibly epidote viewed with crossed polarising filters (FoV1.3 x 0.8 mm)

The same area viewed with crossed polarising filters (FoV1.3 x 0.8 mm)


Intrusion into granitic rock at location 2, Dunmoor Hill. Prepared hand sample viewed in reflected light. The intrusion is the dark material to the right.

Intrusion into granitic rock at location 2, Dunmoor Hill
Prepared hand sample viewed in reflected light. The intrusion is the dark material to the right.

Intrusion into granitic rock viewed with crossed polarising filters.

A thin-section from the same sample viewed with crossed polarising filters

Contact between the more mafic fine-grained intrusion and the red, medium-grained  granitic rock at location 2. Section viewed in plane polarised light. (FoV 5 x 3.3 mm)

Contact between the more mafic fine-grained intrusion and the red, medium-grained, granitic rock at location 2
Section viewed in plane polarised light. (FoV 5 x 3.3 mm)

Contact between the more mafic fine-grained intrusion and the medium-grained, red granitic rock at location 2. Section viewed with crossed polarising filters. (FoV 5 x 3.3 mm)

The same area viewed with crossed polarising filters (FoV 5 x 3.3 mm)


Alteration of mafic material in the red, medium-grained granitic rock giving rise to two forms of biotite along with actinolite and magnetite. One form of biotite, perhaps primary, is a dark brown while the other, most likely secondary, is much lighter in colour. The actinolite is the greenish mineral near the centre of the cluster. Section viewed in plane polarised light (FoV 5 x 3.3 mm)

Alteration of mafic material in the medium-grained granitic rock giving rise to two forms of biotite, actinolite and magnetite
One form of biotite, perhaps primary, isa darkbrown while the other, most likely secondary, is much lighter in colour. Section viewed in plane polarised light (FoV 5 x 3.3 mm)

Alteration of mafic material in the red, medium-grained granitic rock giving rise to two forms of biotite along with actinolite and magnetite at location 2.  Section viewed with crossed polarising filters(FoV 5 x 3.3 mm)

The same group of crystals viewed with a slight rotation of the stage and with crossed polarising filters (FoV 5 x 3.3 mm)

Location 3. Cunyon Crags NT977180

Cunyon Crags NT 97704 18099 presents some of the best exposures of the Cheviot’s dark grey, medium-grained ‘Marginal’ quartz monzonite rock.

Marginal quartz monzonite at location 3, Cunyon Crags. Prepared hand specimen viewed in reflected light.

‘Marginal’ quartz monzonite at Cunyon Crags
Prepared hand sample viewed in reflected light

Marginal quartz monzonite at location 3, Cunyon Crags viewed with crossed polarising filters

A thin-section from the same sample viewed with crossed polarising filters

Location 4. Marginal quartz monzonite at NT970177


Next, there’s a long boggy stretch that crosses, according to Robson’s map, the boundary between the outer ‘Marginal’ quartz monzonite and the ‘Central Belt’ medium-grained, porphyritic granites that lie closer to the centre of the area occupied by the pluton. Sadly, we are unable to find any exposures where we can see this junction.
We are heading for an outcrop at NT970178 where Al-Hafdh records his ‘Hedgehope’granodiorite chilled against the ‘Marginal’ quartz monzonite and therefore, evidence for sequencing the intrusions. We spend time carefully observing the outcrops in the vicinity but we do not find find a chilled margin.
However, our search does reveal a wonderful variety in the ‘Marginal’ rocks in quite a small area at this locality, evidencing a seemingly chaotic interaction of differently constituted magmas. Over a few square metres, we find; rock that is wholey medium-grained ‘Marginal’ quartz monzonite, ‘Marginal’ rock that has been invaded by a fine-grained relatively mafic-rich material, some strongly granophyric rock, and also rocks intruded by fine-grained mafic-poor material.

Fine-grained Central Belt granitic rock at location 4. Prepared hand sample viewed in reflected light

Fine-grained ‘Central Belt’ granitic rock at location 4
Prepared hand sample viewed in reflected light

Fine-grained Central Belt granitic rock at location 4 on Dunmoor Hill viewed with crossed polarising filters.

Thin section from the same sample viewed in transmitted light with crossed polarising filters

Medium-grained Central Belt rock at location 4 on Dunmoor Hill, Prepared hand sample viewed in reflected light

Medium-grained ‘Central Belt’ rock at location 4 on Dunmoor Hill
Prepared hand sample viewed in reflected light

Medium-grained Central Belt granitic rock at Location 4 viewed with crossed polarising filters

A thin-section from the same sample viewed with crossed polarising filters

Fine-grained felsic vein intruded into a relatively equigranular Central Belt rock. Prepared hand sample viewed in reflected light.

Fine-grained felsic vein intruded into a relatively equigranular ‘Central Belt’ rock at location 4 on Dunmoor Hill
Prepared hand sample viewed in reflected light.

Fine-grained felsic vein intruded into a relatively equigranular

Thin section from the same sample viewed with crossed polarising filters

A relatively equigranular mafic-rich granitic rock disrupted by a fine-grained intrusion that is also mafic-rich. Prepared hand sample from location 4 on Dunmoor Hill.

A relatively equigranular mafic-rich granitic rock disrupted by a fine-grained intrusion that is also mafic-rich
Prepared hand sample from location 4 on Dunmoor Hill.

A relatively equigranular mafic-rich granitic rock disrupted by a fine-grained intrusion that is also mafic-rich. Thin section viewed with crossed polarising filters.

Thin section from the same sample viewed with crossed polarising filters.


Location 5. Dunmoor hill summit NT967183


Boggy track leading to summit tors of Dunmoor Hill

The rough track leading towards the summit of Dunmoor Hill
The summit tors can be seen on the skyline.

From location 6 we take the footpath up to the summit of Dunmoor Hill where there are some small exposures. The rock here is quite different from Cunyon Crags, being a fine-grained to medium-grained ‘Central Belt’ type.

View from Long Crag of distant tors of coarser porphyritic

‘Central Belt’ tor on Long Crag.

Location 6. Long Crag at NT967178

Granite jointing at Long Crag

Outcrop at Long Crag
The rock shows typical granite tor structure and jointing.


Long Crag lies on a north-south axis from NT 967178 providing more interesting exposures of rock.
The rock is porphyritic, commonly with larger plagioclase and potassium feldspar phenocrysts and smaller biotite ones. Occasionally we find large pyroxene glomerocrysts. The matrix shows a variety of grain-sizes, from very fine to medium, and commonly contains the ragged remains of pyroxene crystals, along with opaque iron/titanium oxides.

Medium-grained Central Belt rock at location 6 at the northern end of Long Crag. Prepared hand sample viewed in reflected light.

Medium-grained ‘Central Belt’ rockat location 6 at the north end of Long Crag
Prepared hand sample viewed in reflected light

Medium-grained Central Belt rock at location 6 at the northern end of Long Crag viewed with crossed polarising filters.

A thin section from the same sample viewed with crossed polarising filters

Fine-grained Central Belt rock at the south end of Long Crag, location 6. Prepared hand specimen viewed in reflected light

Fine-grained ‘Central Belt’ rock at the south end of Long Crag, location 6
Prepared hand specimen viewed in reflected light

Fine-grained Central Belt rock at the south end of Long Crag, location 6 viewed with crossed polars.

A thin-section from the same sample viewed with crossed polarising filters

Location 7. Long Crag at NT968174

The rock here has a very fine-grained red matrix that surrounds sparser feldspar, biotite and pyroxene phenocrysts. The matrix suggests the rapid cooling that would occur if this magma was chilled by its close proximity to the already consolidated ‘Marginal’ quartz monzonite.

Rock with a fine-grained matrix with sparser phenocrysts at location 7, Long Crag. Prepared hand specimen viewed in reflected light.

Rock with a fine-grained matrix with sparser phenocrysts at location 7, Long Crag.
Prepared hand specimen viewed in reflected light.

Rock with a fine-grained matrix with sparser phenocrysts at location 7, Long Crag, viewed with crossed polarising filters.

Thin-section from the same sample viewed with crossed polarising filters

Location 8 Cat Crag NT 967172

South slope of Dunmoor Hill. The junction between diorite (right) and pink (left)

Cat Crag
At NT 96801 17553, there is a junction between ‘the Marginal’ quartz monzonite and a pink medium-grained porphyritic rock. The pink rock lies to the left in this photograph, and the quartz monzonite to the right.

Return to car

From Cat Crag we head down over rough ground to rejoin the path back to Linhope and from there, the road to Hartside Farm and the car.

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

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

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