Hedgehope Hill

Hedgehope Hill from the edge of Threestoneburn Wood

Looking up towards Hedgehope Hill from the edge of Threestoneburn Wood.

This excursion enables us to put to the test Al-Hafdh’s proposals regarding the distribution of rock types as evidence for a Cheviot ring dyke system. We will investigate the Dunmoor Burn lookIng for his ‘Woolhope’ type evolved granite, search for the contact between his medium-grained ‘Hedgehope’ type and the coarse-grained ‘the ‘Standrop’ type that he reports on the north side of Hedgehope Hill, sample Great and Little Standrop rocks, and finally look for some sign of contact between the different granites on the south side of the hill below Standrop Rigg that might support Al Hafdh’s rock distribution pattern.

Map showing excursion route, igneous rock types and sample locations

Hedehope Hill excursion,  map showing footpaths and sample locations

Key showing igneous bedrock types

Key to Hedgehope Excursion Map

Location 1. Felsite dyke at NT958180

Having parked at Hartside and walked the road to Linhope, followed the track below Ritto Hill and then N.E. to the bridge over the Linhope Burn, we walk the short distance to the sign-posted track through the gate northwards up the slope of Hedgehope Hill. We leave the path and head east across rough ground, down the steep slope towards the Dunmoor Burn and our first location at NT95894. In this area, there are exposures of a very fine-grained, pink to red granitic rock that Al Hafdh identifies as the ‘Woolhope’ granite type. He suggests these exposures represent part of the ring dyke system shown on his map that is important to an understanding of the origin and structure of the Cheviot complex.
Thin sections and comparison with the fine-grained granitic rock at Woolhope (click here for a visual analysis of a sample of this rock) suggest to us that although hand specimens of the two rocks do have aspects in common, they are unlikely to be of the same type intruded in the same mass.
Perhaps the exposures at this location are more closely related to the felsite dykes that the BGS map shows cutting across the Dunmoor Burn downstream.

Prepared hand sample of felsic intrusion, Dunmoor Burn, NT958180. Sample viewed in reflected light.

Prepared hand sample of felsic intrusion, Dunmoor Burn NT958180Sample viewed in reflected light.

Part of the thin section from the same sample viewed in plane polarised light. Note xenoliths and flow structures in the fine grained matrix.

Part of a thin section from the same sample viewed in plane polarised light Flow structures are visible in the fine grained rock. The grey area to the right is a hole in the section.

Fine-grained felsic intrusion, Dunmoor Burn viewed in plane polarised light. This is a photomicrograph of the xenolith on the left side of the sample shown above.

Fine-grained felsic intrusion, Dunmoor Burn viewed in plane polarised light
Glomerocryst or microlith visible left of centre in the thin section shown above.

Fine-grained felsic intrusion, Dunmoor Burn viewed with crossed polarising filters. Glomerocryst or microlith  on the left side of the sample shown above.

The same sample viewed with crossed polarising filters

 A plagioclase phenocryst or xenocryst rimmed with iron oxide in the same felsic intrusion, Dunmoor Burn viewed with crossed polarising filters. Flow structures are evident in the fine-grained feldspar laths.

A plagioclase phenocryst or xenocryst rimmed with iron oxide in the same felsic intrusion, Dunmoor BurnViewed with crossed polarising filters. Flow structures are evident in the fine-grained feldspar laths.

Plagioclase and potassium feldspar phenocrysts or xenocrysts in a felsic intrusion, Dunmoor Burn. Sample viewed with crossed polarising filters.

Plagioclase and potassium feldspar phenocrysts or xenocrysts in the same felsic intrusion, Dunmoor Burn. Again, the view through crossed polarising filters reveals the flow of feldspar laths around the larger crystals.

Location 2. Track up Hedgehope Hill at NT952187

Returning to the track, we continue walking up the flank of Hedgehope Hill. Alongside the track, half buried in the peat and heather, the many large boulders have been transported here by water and gravity making them unreliable guides to bedrock concealed below. The rock is highly altered, much of it to a dull red colour with black and green blotches of biotite and chlorite and occasionally with black tourmaline. Many of the plagioclase phenocrysts have biotite inclusions and are sericitised to a greater or lesser extent.

Altered porphyritic granitic rock at Location 2 Hedgehope Hill NT952187. Prepared hand specimen viewed in reflected light.

Altered porphyritic granitic rock at location 2, Hedgehope Hill NT952187. Prepared hand sample viewed in reflected light.

Andesite at Location 2 Hedgehope Hill. Thin section viewed in plane polarised light

Thin section from the same sample viewed in plane polarised light

Altered porphyritic granitic rock at Location 2 Hedgehope Hill. Thin section viewed with crossed polarising filters.

The same thin section viewed with crossed polarising filters.

Altered porphyritic granitic rock at location 2, Hedgehope Hill. Plagioclase with inclusions viewed with crossed polarising filters.

Altered porphyritic granitic rock at location 2, Hedgehope Hill. Plagioclase with inclusions viewed with crossed polarising filters.

Altered porphyritic granitic rock at Location 2, Hedgehope Hill. A zircon alongside biotite that has been partly altered to chlorite.

Altered porphyritic granitic rock at Location 2, Hedgehope Hill. A zircon alongside biotite that has been partly altered to chlorite.

Altered porphyritic granitic rock at Location 2, Hedgehope Hill. Pyroxene crystal interpenetrating another crystal

Altered porphyritic granitic rock at Location 2, Hedgehope Hill. Pyroxene crystal interpenetrating another crystal.

Altered porphyritic granitic rock at location 2, Hedgehope Hill.  A potassium feldspar phenocryst with feldspar and biotite inclusions viewed with crossed polarising filters.

Altered porphyritic granitic rock at location 2, Hedgehope Hill. A potassium feldspar phenocryst with feldspar and biotite inclusions viewed with crossed polarising filters.

Some of the rock here is less porphyritic than that at the previous location. The following pictures are of an altered subequigranular granitic rock with no pyroxine or biotite, the latter having been altered to chlorite and some larger patches of white mica, or sericite, and possibly also the tourmaline that appears here. Most of the plagioclase has lost its polysynthetic twinning having been changed to albite or orthoclase and sericite and some of the quartz and potassium feldspar has formed micrographic inter-growths. We are unsure whether the micrographic texture arose at the time of the first crystallisation or later as a result of alteration.

Equigranular altered granitic rock at Location 2, Hedgehope Hill. Prepared hand specimen viewed in reflected light.

Subequigranular altered granitic rock, Location 2, Hedgehope Hill. Prepared hand specimen viewed in reflected light.

Subequigranular altered granitic rock at  Location 2, Hedgehope Hill. Sample viewed in plane polarised light.

Thin section from the same sample viewed in plane polarised light

Subequigranular altered granitic rock at Location 2, Hedgehope Hill. Sample viewed with crossed polarising filters

The same thin section viewed with crossed polarising filters

Subequigranular granitic rock from location 2, Hedgehope Hill. Most of the plagioclase in this altered rock is heavily sericitised.

Subequigranular granitic rock from location 2, Hedgehope Hill.
Most of the plagioclase in this altered rock has lost its polysynthetic twinning and is heavily sericitised.

Subequigranular granitic rock at location 2, Hedgehope Hill showing micrographic inter-growth of quartz and feldspar together with chlorite and secondary white mica

Micrographic inter-growth of quartz and feldspar together with chlorite and secondary white mica in the same thin section

Subequigranular granitic rock at tocation 2, Hedgehope Hill. Tourmaline together with chlorite, quartz and potassium feldspar. Sample viewed with crossed polarising filters.

Tourmaline with chlorite, quartz and potassium feldspar in the same sample viewed with crossed polarising filters

Location 3. Boulders around the summit at NT94554 19489

There is a very distinctive rocky collar of boulders around the summit of Hedgehope. At NT 94554 19489, most of the rock is a medium to coarse-grained porphyritic type that has been subject to alteration. In the example shown below a vein of now much redder, fine grained felsic material has been intruded into the previously consolidated rock tearing clots of crystals from the host rock. There is some coarse granophyric texture in the quartz and feldspar, much chlorite replacement of the original biotite and epidote and sericite in the plagioclase but no pyroxene.

Altered medium to course-grained porphyritic at Location 3, Hedgehope Hill. Prepared hand sample viewed in reflected light.

Altered medium to course-grained porphyritic at Location 3, Hedgehope Hill. Prepared hand sample viewed in reflected light.

Altered medium to course-grained porphyritic at Location 3, Hedgehope Hill. Thin section viewed in plane polarised light.

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

Altered medium to course-grained porphyritic at Location 3, Hedgehope Hill. Thin section viewed with crossed polarising filters.

The same thin section viewed with crossed polarising filters

Patches of epidote in altered feldspars in a medium to course-grained rock with some plagioclase phenocrysts at location 3, Hedgehope Hill. Thin section viewed with crossed polarising filters

Patches of epidote in altered feldspars in a medium to course-grained rock with some plagioclase phenocrysts at location 3, Hedgehope Hill. Thin section viewed with crossed polarising filters

Epidote and sericite in altered feldspars in a medium to course-grained rock with some plagioclase phenocrysts at location 3, Hedgehope Hill. Thin section viewed with crossed polarising filters

Epidote and sericite in altered feldspars in a medium to course-grained rock with some plagioclase phenocrysts at location 3, Hedgehope Hill. Thin section viewed with crossed polarising filters

Location 4. Hedgehope Hill summit, NT943198

The summit of Hedgehope Hill is scattered with the boulders of medium-grained porphyritic rock similar to that found on the summit of Dunmoor Hill and at Long Crag on Dunmoor Hill. There are many rocks here that show evidence of severe hydrothermal alteration resulting in the disappearance of pyroxenes and biotite, the conversion of plagioclase to albite and even potassium feldspar along with the appearance of secondary minerals that include sericite, chlorite, tourmaline, quartz, haematite and limonite.

Relatively unaltered mid- to coarse-grained porphyritic rock from the summit of Hedgehope Hill, NT943198. Prepared hand sample viewed in reflected light

Relatively unaltered mid- to coarse-grained porphyritic rock from the summit of Hedgehope Hill NT943198
Prepared hand sample viewed in reflected light

Relatively unaltered mid- to coarse-grained porphyritic rock from the summit of Hedgehope Hill. Thin section from the same sample viewed in plane polarised light.

Thin section from the same sample viewed in plane polarised light.

Relatively unaltered mid- to coarse-grained porphyritic rock from the summit of Hedgehope Hill. Thin section from the same sample viewed with crossed polarising filters.

The same thin section viewed with crossed polarising filters

Clinopyroxene crystals in a relatively unaltered rock on the summit of Hedgehope Hill viewed in plane polarised light.

Clinopyroxene crystals in a relatively unaltered rock at the summit of Hedgehope Hill viewed in plane polarised light

Carlsbad and polysynthetic twinning in a plagioclase phenocryst in a relatively unaltered rock from the summit of Hedgehope Hill. Sample viewed with crossed polarising filters.

Carlsbad and polysynthetic twinning in a plagioclase phenocryst in a relatively unaltered rock from the summit of Hedgehope Hill. Sample viewed with crossed polarising filters.

Clinopyroxene crystals in a relatively unaltered rock on the summit of Hedgehope Hill viewed with crossed polarising filters.

Clinopyroxene crystals in a relatively unaltered rock on the summit of Hedgehope Hill viewed with crossed polarising filters

Complex inter-growth of plagioclase, potassium feldspar, quartz and biotite in a relatively unaltered rock from the summit of Hedgehope Hill viewed with crossed polarising filters.

Complex inter-growth of plagioclase, potassium feldspar, quartz and biotite in a relatively unaltered rock from the summit of Hedgehope Hill viewed with crossed polarising filters.

Altered porphyritic rock from the summit of Hedgehope Hill. Prepared hand sample viewed in reflected light.The dark material is mostly tourmaline.

Altered porphyritic rock from the summit of Hedgehope Hill. Prepared hand sample viewed in reflected light. The dark material is mostly tourmaline.

Altered porphyritic rock from the summit of Hedgehope Hill. Thin section from the same sample viewed in plane polarised light.

Altered porphyritic rock from the summit of Hedgehope Hill. Thin section from the same sample viewed in plane polarised light.

Altered porphyritic rock from the summit of Hedgehope Hill. Thin section viewed with crossed polarising filters.

Altered porphyritic rock from the summit of Hedgehope Hill. Thin section viewed with crossed polarising filters.

Platy and acicular tourmaline in an altered granitic rock from the summit of Hedgehope Hill. Sample viewed with crossed polarising filters.There are feint traces of remaining polysynthetic twinning in the plagioclase crystal at top left.

Platy and acicular tourmaline in an altered granitic rock from the summit of Hedgehope Hill. Sample viewed with crossed polarising filters. There are feint traces of remaining polysynthetic twinning in the plagioclase crystal at top left.

Zoned tourmaline and quartz in altered granitic rock Hedgehope Hill, Location 4, from the summit of Hedgehope Hill. Sample viewed in plain polarised light.

Zoned tourmaline and quartz in altered granitic rock Hedgehope Hill at location 4, the summit of Hedgehope Hill. Sample viewed in plain polarised light.

Location 5. Quartz-porphyry dyke at NT944202

A dyke curves across the summit of Hedgehope Hill that the British Geological Survey identifies as a quartz-porphyry type.
This type of dyke is characterised by the presence of phenocrysts of quartz and usually albite and biotite in an iron-stained, devitrified quartz-orthoclase groundmass. The groundmass can be sub- or micro-crystalline and if the latter, then often with some granophyric texture. Generally, the quartz phenocrysts are rounded and free from inclusions or cavities and the plagioclase phenocrysts are albite or have a composition between albite and andesine. The plagioclase is frequently turbid and sericitised. Feldspar may also be present as perthite and anorthoclase.
Fresh or altered biotite is common but hypersthene and augite, or a calcite pseudomorph after augite, are seen more rarely. Occasionally, large crystals of apatite can be found. Iron-titanium ores are sparse occurring mostly around relict biotite crystals.
Carruthers identifies the Hedgehope summit dyke as being of interest, ‘…in that it contains nests of a highly birefringent yellow mica, which is either zinnwaldite or polylithionite.’
Our sample is from a point of contact between the dyke and the granite at the north end of the dyke. The contact is sharp with some chilling of the dyke rock against the granite that appears to be only slightly changed by the intrusion. Fragments of the granite have been torn off becoming xenoliths in the dyke and the outermost crystals in contact with the intrusion have cracked in line with the boundary, allowing strings of tiny blend of quartz and feldspar to form within.

Contact between quartz-porphyry dyke and granite at location 5, Hedgehope Hill NT944202. Prepared hand specimen viewed in reflected light.

Contact between quartz-porphyry dyke and granite at location 5, Hedgehope Hill NT944202
Prepared hand specimen viewed in reflected light.

Contact between quartz-porphyry dyke and granite at location 5, Hedgehope Hill. Thin section from the same sample viewed in plane polarised light.

Contact between quartz-porphyry dyke and granite at location 5, Hedgehope Hill. Thin section from the same sample viewed in plane polarised light.

Contact between quartz-porphyry dyke and granite at location 5, Hedgehope Hill. Thin section from the same sample viewed with crossed polarising filters.

Contact between quartz-porphyry dyke and granite at location 5, Hedgehope Hill. The same thin section viewed with crossed polarising filters

Altered biotite in quartz-porphyry dyke, location 5 Hedgehope Hill PPL

Altered biotite in quartz-porphyry dyke at location 5 Hedgehope Hill. Thin section viewed in plane polarised light

Altered biotite in quartz-porphyry dyke, location 5 Hedgehope Hill viewed with crossed polarising filters

Altered biotite in quartz-porphyry dyke, location 5 Hedgehope Hill. Thin section viewed with crossed polarising filters.

A polysynthetically twinned plagioclase crystal in the quartz-porphyry dyke at location 5 Hedgehope Hill. Thin section viewed in plane polarised light.

A polysynthetically twinned plagioclase crystal in th quartz-porphyry from location 5 Hedgehope Hill. Thin section viewed in plane polarised light.

Polysynthetically twinned plagioclase in quartz-porphyry dyke, location 5 Hedgehope Hill. Thin section viewed with crossed polarising filters.

The same sample viewed with crossed polarising filters

Tourmaline and chlorite in quartz-porphyry dyke, location 5 Hedgehope Hill. Thin section viewed in plane polarised light.

Tourmaline and chlorite in quartz-porphyry dyke, location 5 Hedgehope Hill. Sample viewed in plane polarised light

Biotite and amphibole in the granophyric host rock to the dyke at location 5 Hedgehope Hill. Sample viewed with crossed polarising filters.

Biotite and amphibole in the granophyric host rock to the dyke at location 5 Hedgehope Hill. Sample viewed with crossed polarising filters.

Quartz-porphyry dyke in contact with granitic host rock at location 5, Hedgehope Hill. Sample viewed with crossed polarising filters. Thin strings of quartz and feldspar blebs have formed along the cracks in the host rock

Quartz-porphyry dyke in contact with granitic host rock at location 5, Hedgehope Hill. Sample viewed with crossed polarising filters.
Thin strings of quartz and feldspar blebs have formed along the cracks in the host rock

Location 8. Boulders at NT944177

Boulders of coarse-grained Central Belt rock on Hedgehope Hill at NT944177

Boulders of coarse-grained Central Belt rock on Hedgehope Hill at NT944177

We head down hill across rough ground to a boulder-strewn area with some outcropping rock. It is coarse, ‘Central Belt’ rock again, similar to that on Standrop Rigg. In thin section we find a patch of amphibole displaying typical amphibole 60/120 cleavage. Some primary pyroxene and biotite survives but much has been altered.

Coarse-grained Central Belt, Hedgehope Hill NT944177. Prepared hand specimen in ordinary reflected light.

Coarse-grained Central Belt, Hedgehope Hill NT944177
Prepared hand specimen in ordinary reflected light.

Coarse-grained Central Belt, Hedgehope Hill NT944177. Thin section viewed in plane polarised light.

Coarse-grained Central Belt, Hedgehope Hill NT944177
Thin section from the same sample viewed in plane polarised light.

Coarse-grained Central Belt, Hedgehope Hill NT944177. Thin section viewed with crossed polarising filters.

Contact between quartz-porphyry dyke and granite at location 5, Hedgehope Hill. The same thin section viewed with crossed polarising filters

Amphibole, biotite and chlorite in Central Belt granitic rock. Section viewed in plane polarised light.

Amphibole, biotite and chlorite in Central Belt granitic rock.
Section viewed in plane polarised light

Amphibole, biotite and chlorite in Central Belt granitic rock. Section viewed with crossed polarising filters.

Amphibole, biotite and chlorite in Central Belt granitic rock.
Section viewed with crossed polarising filters.

Location 9. Felsite dyke-‘Central Belt’ contact at NT948177

Felsite dyke rock in contact with coarse-grained Central Belt rock, Hedgehope Hill NT948177

Felsite dyke rock in contact with coarse-grained Central Belt rock, Hedgehope Hill NT948177

We contour around to location 9 where there is a boulder bearing the contact between, what looks like, felsite and ‘Central Belt’ rock. A felsite dyke doesn’t appear on the map here and there is no supporting evidence in the field for one nearby, so this boulder may well have been dumped here from further afield by ice, water and gravity. This serves to remind us to question the degree to which the character of loose surface material can inform us about the character of concealed underlying bedrock.

Felsite dyke rock in contact with coarse-grained Central Belt rock, Hedgehope Hill NT948177. Prepared hand specimen in ordinary reflected light.

Felsite dyke rock in contact with coarse-grained Central Belt rock, Hedgehope Hill NT948177
Prepared hand specimen in ordinary reflected light.

Felsite dyke rock in contact with coarse-grained Central Belt rock, Hedgehope Hill NT948177. Thin section viewed in plane polarised light.

Thin section from the same sample viewed in plane polarised light.

Coarse-grained Central Belt, Hedgehope Hill NT944177. Thin section viewed with crossed polarising filters.

The same thin section viewed with crossed polarising filters

Felsite intrusion into Central Belt granitic rock with micrographic texture, quartz and K-spar. Section viewed with crossed polarising filters (FoV 5 x 3.3mm)

Felsite intrusion into Central Belt granitic rock with micrographic texture, quartz and K-spar (FoV 5 x 3.3mm)
Section viewed with crossed polarising filters

Felsite intrusion into Central Belt granitic rock with micrographic texture, quartz and K-spar. Section viewed with crossed polarising filters (FoV 2.5 x 1.7 mm)

Granophyric texture of quartz and K-spar
Section viewed with crossed polarising filters (FoV 5 x 3.3mm)

Disrupted biotite in Central Belt granitic rock subject to felsic intrusion. Section viewed with crossed polarising filters (FoV 1.3 x 0.8)

Disrupted biotite in Central Belt granitic rock subject to felsic intrusion
Section viewed with crossed polarising filters (FoV 1.3 x 0.8)

Location 10. Coarse-grained outcrop at NT949177

Outcropping coarse-grained Central Belt rock on Hedgehope Hill at NT949177.jpg

Outcropping coarse-grained Central Belt rock on Hedgehope Hill at NT949177

Slightly further east, there is a substantial outcrop of coarse-grained Central Belt rock that has some coarse granophyric texture. This rock appears mid-way on a spectrum between the coarse-grained rock we found on Standrop Rigg and the much finer-grained, ‘granophyric’ rock that we found on the track up to Hedgehope’s summit.

Coarse-grained Central Belt, Hedgehope Hill NT949177. Prepared hand specimen in ordinary reflected light.

Coarse-grained Central Belt, Hedgehope Hill NT949174. Prepared hand specimen in ordinary reflected light
Prepared hand specimen in ordinary reflected light (40mm across)

Coarse-grained Central Belt, Hedgehope Hill NT949177. Thin section viewed in plane polarised light

Thin section from the same sample viewed in plane polarised light.

Coarse-grained Central Belt, Hedgehope Hill NT949177. Thin section viewed with crossed polarising filters.

The same thin section viewed with crossed polarising filters

Location 11. Little tor at NT949174

Small tor of Central Belt rock, Hedgehope Hill, NT949174.jpg

Small tor of Central Belt rock, Hedgehope Hill, NT949174

We drop down towards the footpath to make our way back to the car and pass a shapely mini-tor of coarse-grained Central Belt rock.

Coarse-grained Central Belt rock at Hedgehope Hill, NT949174. Prepared hand specimen in ordinary reflected light

Coarse-grained Central Belt rock at Hedgehope Hill, NT949174.
Prepared hand specimen in ordinary reflected light (40mm across)

Coarse-grained Central Belt rock at Hedgehope Hill, NT949174.Thin section viewed in plane polarised light

Thin section from the same sample viewed in plane polarised light.

Coarse-grained Central Belt, Hedgehope Hill NT949177. Thin section viewed with crossed polarising filters.

Coarse-grained Central Belt rock at Hedgehope Hill, NT949174

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

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