Harthope Valley

The higher waterfall at Harthope Linn

The higher waterfall on the Harthope Burn

This excursion takes us up the beautiful Harthope Valley to the waterfalls at Harthope Linn. We walk along one of the major geological faults in the Cheviot complex where, according to Robson, andesitic lava may have poured out of vents or fissures in Devonian times. There are no signs of vents now, but we will see evidence for movements along the fault caused by tectonic forces in the Late Carboniferous as well as plentiful evidence for the hydrothermal activity that accompanied these movements.
Geological guide books identify the location of visible contact between two types of granitic rock here, the granophyric, medium-grained type and the coarser, non-granophyric ‘Standrop’ type. A sharp contact would show one type chilled against the other, it’s margin of smaller crystals resulting from more rapid cooling against the already consolidated older rock. This fact would indicate a sequence for the assembly of the Cheviot pluton. The contact is said to be in outcrops near the first of the valley’s two waterfalls and we hope to be able to confirm and record this.

Map showing the route of the excursion and igneous rock types

Hathope Valley Excursion Map

Key

Key to the Harthope Valley excursion map

After parking on the grassy area just before the bridge over the Hawsen burn, we walk along the road past Langleford to our first location – a spot suggested by Colin Scuton in his very useful field guide, ‘Northumberland Rocks and Landscape’.

Location 1. The first outcrop at NT9450 2168

Colin Scruton describes the exposure here as a true granite of the ‘Granophyric’ variety being pink, medium-grained with pink feldspar, transluscent quartz and black shiny flakes of biotite. He draws attention to the 1cm thick veins of aplite that run through the rock and also the2-3mm thick veinlets of black tourmaline.
The pink, medium-grained granite that we found here doesn’t have a granophyric texture but sure enough, aplite is all around, seemingly in larger areas than just 1cm veins, and tourmaline veinlets are common.

Outcrop of pink medium-grained granite and tourmaline veined aplite at location 1 on the Harthope Burn, NT 9450 2168

Outcrop of pink medium-grained granite and tourmaline veined aplite at location 1 on the Harthope Burn, NT 9450 2168

Prepared hand specimen of medium-grained granite at location 1, Harthope Burn NT 9450 2168

Prepared hand specimen of medium-grained granite at location 1, Harthope Burn NT 9450 2168

Whole slide of felsic granite at location 1, Harthope Linn viewed in plane polarised light.

Thin section from the same sample viewed in plane polarised light
The sample measures 42mm across

Whole slide of felsic granite at location 1, Harthope Linn viewed with crossed polarising filters.

The same thin section viewed with crossed polarising filters.

Amphibole and muscovite in medium-grained granite at location 1, Harthope Burn viewed in plane polarised light

Amphibole and muscovite in medium-grained granite at location 1, Harthope Burn viewed in plane polarised light

Amphibole and muscovite in altered granitic rock at location 1, Harthope Burn viewed with crossed polarising filters

The same area of the sample viewed with crossed polarising filters

Prepared hand specimen of quartz-rich, tourmaline veined rock at location 1, Harthope Linn viewed in reflected light.

Prepared hand specimen of quartz-rich, tourmaline veined rock at location 1, Harthope Linn viewed in reflected light

Quartz-rich rock with tourmaline veins at location 1, Harthope Linn viewed in plane polarised light

Thin section from the same sample viewed in plane polarised light
The sample measures 45mm across.

Thin section of quartz-rich rock with tourmaline veins at location 1, Harthope Linn viewed with crossed polarising filters

The same thin section viewed with crossed polarising filters

Tourmaline veinlet in quartz-rich rock at location 1, Harthope Linn viewed in plane polarised light

Tourmaline veinlet in quartz-rich rock at location 1, Harthope Linn viewed in plane polarised light

Tourmaline veinlet in quartz-rich rock at location 1, Harthope Linn viewed with crossed polarising filters

The same area tourmaline veinlet viewed with crossed polarising filters

Location 2. Highly altered blotchy rock at NT9282 2033

Altered quartz monzonite, Hathope Linn

Continuing on the track past Langleford Hope we arrive at a point where the track crosses a small eastwards-flowing stream that runs into the Harthope Burn (NT 9280 2038). We go down the level of the Harthope Burn and scramble a short distance alongside the burn under its steep northern bank to our second location at NT 9282 2033.
There’s an interesting section of rock here that is characterised by large-scale vertical fissuring and a blotchy appearance. Colin Scruton mentions that some of the rock in these parts feature ‘areas of blotchy porphyry that have been described as volcanic xenoliths.’ In the hand specimen, it has lighter blotches on a reddish ground but the blotches themselves are not obviously xenolithic. We suppose them to be a product of the high degree of alteration that this rock has undergone – evidenced in the presence of tourmaline and abundant sericite. In thin section, the rock appears to be very low in quartz, most of it appearing in association with tourmaline, and it is without biotite and pyroxene. There are plagioclase phenocrysts, many of them appearing in clumps of broken crystals that maybe xenolithic. The small crystals of iron-titanium opaque minerals are widely scattered throughout the rock.

Highly altered rock at Location 2 (NT 9282 2033 ) that has a blotchy appearance. The very dark areas are tourmaline. Prepared hand specimen in reflected light.

Prepared hand specimen of highly altered rock at Location 2 showing a blotchy appearance with patches of black tourmaline.

Highly altered rock at Location 2 (NT 9282 2033) showing blotchy appearance and very dark patches of tourmaline. Thin section viewed in plane polarised light.

Thin section from the same sample viewed with crossed polarising filters
The sample measures 45mm across.

Highly altered rock at Location 2 showing blotchy appearance and very dark patches of tourmaline. Thin section viewed with crossed polarising filters.

The same thin section viewed with crossed polarising filters

Zoned tourmaline in altered rock at Location 2 viewed with crossed polarising

Zoned tourmaline in altered rock at Location 2 viewed with crossed polarising

Sericite (white mica) in the highly altered rock at Location 2 viewed in plane polarised light.

Muscovite (sericite) in the highly altered rock at Location 2 viewed in plane polarised light

Sericite (white mica) in highly altered rock at Location 2 viewed with crossed polarising filters.

The same section viewed with crossed polarising filters

Location 3. Low quartz, alkali-rich, high mafic content rock below the first waterfall at NT927202

The pool at location 3

The closer we get to the first waterfall, the darker the rock appears, suggesting an increase in the proportion of mafic minerals like biotite and pyroxene. At location 3, above the cliff overlooking the pool, the rock appears to be very low in quartz (less than 20%) with roughly equal proportions of potassium feldspar and plagioclase and with a mafic content of about 14%. The accuracy of these figures is limited by our ‘pixel count’ method of determining mineral proportions, but that said, the rock here appears to be a syeno-granite.
The quantity of sericite in the rock bears witness to the high degree of hydrothermal alteration that occured in this area.

Prepared hand specimen of syeno-granite at location 3 viewed in reflected light.

Prepared hand specimen of syeno-granite at location 3 viewed in reflected light.

Thin section of syeno-granite at location 3 viewed in plane polarised light

Thin section from the same sample viewed in plane polarised light
The sample measures 42mm across

Thin section of syeno-granite at location 3 viewed with crossed polarising filters

The same thin section viewed with crossed polarising filters

Plagioclase, pyroxene, quartz and associated alteration products in syeno-granite at location 3. Thin section viewed in plane polarised light.

Plagioclase and pyroxene with alteration products in syeno-granite at location 3.
Thin section viewed in plane polarised light.

Plagioclase, pyroxene, quartz and associated alteration products in   syeno-granite  at location 3. Thin section viewed with crossed polarising filters.

Location 4. Similar rock on the lip of the falls at NT9273 2021

Location 4 Harthope Linn Lower Falls (NT 9273 2021)

The lip of the lower waterfall is at NT 9273 2021 where the mafic content of the rock is approximately the same as the quartz content: around 20%. The plagioclase (or what was plagioclase and is now sericite) makes up only about 40% of the feldspar content so again, this appears to be a syeno-granite rock with a sub-equigranular texture with grains ranging between 1 and 5mm. The rock appears very dark on account of the plentiful mafic minerals that include, in order of quantity, biotite, clinopyroxene and orthopyroxene. More of the plagioclase has retained its polysynthetic twinning here, while the small amount of quartz in the rock appears in interstitial anhedral crystals and, in places, in a coarse micrographic texture with the potassium feldspar and perthite.

  Syeno-granite   at location 4. Prepared hand specimen in reflected light.

Prepared hand specimen of syeno-granite at location 4, Harthope Burn.

Thin section of   syeno-granite  at location 4 viewed in plane polarised light.

Thin section from the same sample viewed in plane polarised light

Syeno-granite   at location 4, Harthope Linn. Thin section viewed with crossed polarising filters

The same thin section viewed with crossed polarising filters

Partly altered twinned clinopyroxene in  syeno-granite   at Location 4 Harthope Linn. Section viewed with crossed polarising filters.

Partly altered twinned clinopyroxene in syeno-granite at location 4, Harthope Linn. Section viewed with crossed polarising filters.

Altered orthopyroxene in   syeno-granite   at Location 4 Harthope Linn viewed with crossed polarising filters.

Altered orthopyroxene in syeno-granite at Location 4 Harthope Linn viewed with crossed polarising filters.

 Primary biotite with zircon inclusion in syeno-granite at Location 4 Harthope Linn. Thin section viewed with crossed polarising filters.

Primary biotite with zircon inclusion in syeno-granite at Location 4 Harthope Linn. Thin section viewed with crossed polarising filters.

Coarse micrographic texture in   syeno-granite at Location 4 Harthope Linn. Section viewed with crossed polarising filters.

Coarse micrographic texture in syeno-granite at Location 4 Harthope Linn. Section viewed with crossed polarising filters

Location 5. Similar rock above the waterfall at NT9269 2019

Location 5, Harthope Linn (NT 9269 2019)

A little further upstream at NT 9269 2019, the rock continues to be a sub-equigranular syeno-granite although it is slightly less mafic here and contains a little more quartz than at the lip of the waterfall. Here too, the grain sizes vary between 1 to 5mm with potassium feldspar sometimes forming a course micrographic texture with the quartz.
We searched for the contact between the coarse-grained ‘Standrop’ type and the medium-grained ‘that is said to exist

  Syeno-granite   at location 5 Harthope Linn. Prepared hand specimen in reflected light.

Prepared hand specimen of syeno-granite at location 5 viewed in reflected light

 Syeno-granite  at location 5 Harthope Linn. Thin section viewed in plane polarised light.

Thin section from the same sample viewed in plane polarised light

Syeno-granite   at Location 5 Harthope Linn. Thin section viewed viewed with crossed polarising filters.

The same thin section viewed with crossed polarising filters

Zircon in biotite in   quartz syeno-granite  at Location 5 Harthope Linn. Section viewed with crossed polarising filters

Zircon in biotite in syeno-granite at Location 5 Harthope Linn. Thin section viewed with crossed polarising filters

Coarse Micrographic texture in   syeno-granite   at Location 5 Harthope Linn. Section viewed with crossed polarising filters.

Coarse micrographic texture in syeno-granite at Location 5 Harthope Linn. Thin section viewed with crossed polarising filters.

Location 6. Felsite intrusion, altered rock and andesite xenolith at NT9268 2017

Location 6, Harthope Linn (NT 9268 2017)

At NT 9268 2017 there is a vein of red, fine-grained felsic rock intruded into the syeno-granite and also more of the blotchy altered rock that we saw downstream at location 2 – although here it appears to have a higher quartz content. Here too we find tourmaline and muscovite as alteration products of the original rock. It’s interesting that we haven’t located the contact that is reported to exist between the granophyric and ‘Standrop’ types of cheviot granitic rock, nor seen a change in the general rock type.

Prepared hand specimen of   syeno-granite   with a fine-grained intrusion at location 6, Harthope Linn (NT 9268 2017)

Prepared hand specimen of syeno-granite with a fine-grained intrusion at location 6, Harthope Linn (NT 9268 2017)

Syeno-granite   with a fine-grained intrusion at location 6, Harthope Linn. Thin section viewed in plane polarised light.

Thin section from the same sample viewed in plane polarised light

Syeno-granite   with a fine-grained intrusion at Location 6 Harthope Linn. Thin section viewed viewed with crossed polarising filters.

The same thin section viewed with crossed polarising filters

Prepared hand specimen of highly altered rock at location 6, Harthope Linn (NT 9268 2017)

Prepared hand specimen of highly altered rock at location 6, Harthope Linn (NT 9268 2017)

Highly altered rock at location 6, Harthope Linn. Thin section viewed in plane polarised light.

Thin section from the same sample viewed in plane polarised light

Highly altered rock at location 6, Harthope Linn. Thin section viewed viewed with crossed polarising filters.

The same thin section viewed with crossed polarising filters

Light and dark patches in altered rock at location 6, Harthope Linn. Section viewed in plane polarised light.

Light and dark patches in highly altered rock at location 6, Harthope Linn. Section viewed in plane polarised light

Light and dark patches in the highly altered rock at Location 6, Harthope Linn. Section viewed with crossed polarising filters.

The same thin section viewed with crossed polarising filters

Muscovite in highly altered rock at location 6, Harthope Linn viewed in plane polarised light.

Muscovite in highly altered rock at location 6, Harthope Linn viewed in plane polarised light

Tourmaline in highly altered rock at location 6, Harthope Linn viewed in plane polarised light..

Tourmaline in highly altered rock at location 6, Harthope Linn viewed in plane polarised light

In the bank of the burn at the same location there is an area of rock that is very dark and fine grained. Thin sections show the darkest rock to be andesitic – quite probably a xenolith that had it’s origins in the roof of the magma chamber. The surrounding syeno-granite appears to be contaminated by the andesite whilst the andesite has undergone alteration to hornfels.

Location of andesite xenolith in altered   quartz syeno-granite   in the bank of Harthope Linn at location 6

What is probably the location of an andesite xenolith in the syeno-granite in the bank of the burn at location 6

Prepared hand specimen of andesite at location 6, Harthope Linn viewed in reflected light

Prepared hand specimen of andesite at location 6, Harthope Linn viewed in reflected light

Whole slide sample of andesite at location 6, Harthope Linn viewed in plane polarised light

A thin section from the same sample viewed in plane polarised light
The section measures 48mm across,

Thin section of andesite at location 6, Harthope Linn viewed with crossed polarising filters

The same thin section viewed with crossed polarising filters

Plagioclase phenocryst in opaque-rich feldspar matrix at location 6, Harthope Linn viewed in plane polarised light.

Plagioclase phenocryst in opaque-rich feldspar matrix at location 6 viewed in plane polarised light

Plagioclase phenocryst in opaque-rich feldspar matrix at location 6, Harthope Linn viewed with crossed polarising filters.

The same portion of the thin sectionviewed with crossed polarising filters

Prepared hand specimen of   syeno-granite   (red) in contact with what is probably an andesitic xenolith (black) viewed in reflected light. The sample measures 45 mm across.

Prepared hand specimen of syeno-granite (red) in contact with what may be an andesitic xenolith (black).
The sample measures 45mm across and is viewed in reflected light.

 Syeno-granite   (right) in contact with an andesitic xenolith. Sample measures 40mm across and is viewed in plane polarised light..

Syeno-granite (right) in contact with an andesitic xenolith viewed in plane polarised light

Syeno-granite   (right) in contact with an andesitic xenolith (left)

Syeno-granite (right) in contact with an andesitic xenolith (left) viewed with crossed polarising filters

Location 7. Head of the stream at NT926203

Exposed brecciated rock at the head of the stream location 7, NT926203

Exposed brecciated rock at the head of the stream location 7, NT926203

From the sheep stell near location 6, we head north over rough ground up to the exposed rock we see by the stream at NT926203. This location is on a fault marked on the online BGS map of the area and shown on our map above.
The rock at this location is brecciated and cemented with quartz and haematite. The presence of silica veining with haematite in the fracture and fault zones of the Cheviots is associated with earth movements in the Variscan orogeny (also known as the Amorican or Hercynian).
This period of mountain building occurred over a period of about 100 million years with the greatest effect in these parts about 300 million years ago in late Carboniferous and early Permian times.
The Iapetus Ocean had closed in the latter stages of the preceding Caledonian orogeny as Avalonia and Baltica converged with Laurentia giving rise to the Laurasian continent. At the same time, Gondwana had continued to move northwards so that in the Variscan, the Laurasia and Gondwana converged and formed the Pangean supercontinent. These tectonic processes gave rise to the Variscan mountain belt which includes the mountains of Portugal and western Spain, southwest Ireland, Cornwall, Devon, Pembrokeshire, the Gower Peninsula and the Vale of Glamorgan, all of which have fold axes that trend east to west suggesting a general north to south direction of collision.
In our region, old faults were reactivated, the Great Whin Sill and related dykes were intruded and, what is relevant here along the Harthope fault, silica-rich hydrothermal fluids circulated through fractures and faults in the lava, granite and dyke rocks causing relatively low-temperature oxidation of their iron-titanium oxides to haematite.
We find haematite in just about every Cheviot rock, but it is particularly abundant and massive here and in similar zones in the complex. The combined occurrence of breccia, quartz and haematite that we see here and will see.also at locations further up the Harthope Valley on this excursion, is a consequence of, and evidence for, events in the Varsican.

Brecciated rock at location 7 above the Harthope Burn at NT926203

Brecciated rock at location 7 above the Harthope Burn at NT926203

Prepared hand specimen of   quartz-cemented brecciated rock   at location 7 viewed in reflected light.

Prepared hand specimen of quartz-cemented brecciated rock at location 7 viewed in reflected light.

Thin section of   quartz-cemented brecciated rock   at location 7 viewed in plane polarised light

Thin section from the same sample viewed in plane polarised light

Thin section of  quartz-cemented brecciated rock   at location 7 viewed with crossed polarising filters

The same thin section viewed with crossed polarising filters

Quartz and alkali feldspar with some magnetite with alteration to haematite and limonite. Section viewed in plane polarised light (FoV 3.2 x 2mm)

Quartz and alkali feldspar with some magnetite with alteration to haematite and limonite.
Section viewed in plane polarised light (FoV 3.2 x 2mm)

Quartz and alkali feldspar with some magnetite with alteration to haematite and limonite. Section viewed with crossed polarising filters.

The same area viewed with crossed polarising filters

Haematite with quartz and predominantly potassium feldspar in brecciated rock. Section viewed in plane polarised light.

Haematite with quartz and predominantly potassium feldspar in brecciated rock
Section viewed in plane polarised light.

Haematite with quartz and predominantly potassium feldspar in brecciated rock viewed with crossed polarising filters.

The same area viewed with crossed polarising filters

Location 8. Higher Harthope Linn at NT923199

The higher falls at Location 8, Harthope Linn

The rock at location 8, slightly downstream from the higher falls at Harthope Linn continues to approximate a syeno-granite but with less mafic content than the rock further downstream. We measured the mafic content at 11% and noted the absence of pyroxene but an increase in the amount of chlorite in the rock.

Prepared hand specimen of   syeno-granite   at location 8 viewed in reflected light.

Prepared hand specimen of syeno-granite at location 8 viewed in reflected light.

Thin section of   syeno-granite   at location 8 viewed in plane polarised light

Thin section from the same sample viewed in plane polarised light

Thin section of  syeno-granite   at location 8 viewed with crossed polarising filters

The same thin section viewed with crossed polarising filters

Alteration products of plagioclase in   syeno-granite   at location 8, Harthope Linn viewed in plane polarised light.

Alteration products of plagioclase in syeno-granite at location 8, Harthope Linn viewed in plane polarised light

Alteration products of plagioclase in syeno-granite at location 8, Harthope Linn viewed with crossed polarising filters.

Alteration products of plagioclase in syeno-granite at location 8, Harthope Linn viewed with crossed polarising filters.

Alteration products in   syeno-granite   at location 8, Harthope Linn viewed in plane polarised light.

Alteration products in syeno-granite at location 8, Harthope Linn viewed in plane polarised light

Alteration products of plagioclase in syeno-granite at location 8, Harthope Linn viewed with crossed polarising filters.

Alteration products in syeno-granite at location 8, Harthope Linn viewed with crossed polarising filters.

Location 9. Exposures either side of the stream above the waterfall at NT922200

Exposure at the head of the stream above the upper waterfall at NT922200

Outcrop at the head of the stream above the second waterfall

The rock exposed at the head of the stream above the upper waterfall is more of the syeno-granite. Presumably, any outcrops just north of this point would have been brecciated rock associated with the fault that continues here.

Prepared hand specimen of   syeno-granite   at location 9 viewed in reflected light.

Prepared hand specimen of syeno-granite at location 9 viewed in reflected light.

Thin section of   syeno-granite   at location 9 viewed in plane polarised light

Thin section from the same sample viewed in plane polarised light

Thin section of  syeno-granite   at location 9 viewed with crossed polarising filters

The same thin section viewed with crossed polarising filters

Chlorite and altered biotite. Section viewed in plane polarised light (FoV 2.5 x 1.7 mm)

Chlorite and altered biotite at location 9
Section viewed in plane polarised light (FoV 2.5 x 1.7 mm)

Chlorite and altered biotite. Section viewed with crossed polarising filters (FoV 2.5 x 1.7 mm)

The same area viewed with crossed polarising filters

Haematite in plagioclase. Section viewed in plane polarised light (FoV 1.3 x 0.8 mm)

Haematite in plagioclase
Section viewed in plane polarised light (FoV 1.3 x 0.8 mm)

Haematite in plagioclase. Section viewed with crossed polarising filters (FoV 1.3 x 0.8 mm)

The same plagioclase crystal viewed with crossed polarising filters

UPPER HARTHOPE VALLEY


Map showing the route of the second part of the excursion

Upper Hathope Valley Excursion Map

Location 10. Central Belt wall-like outcrop NT918196


Wall-like outcrop of Central Belt rock, Harthope Valley NT918196

Continuing up the valley, we cross a couple of streams and meet a prominent wall-like outcrop at NT918196. It is another exposure of the Central Belt rock that we have found to be an intermediate syeno-granite in these parts.

Central Belt rock, Harthope Valley NT918196. Prepared hand specimen in ordinary reflected light (43mm across)

Central Belt rock, Harthope Valley NT918196
Prepared hand specimen in ordinary reflected light (43mm across)

Central Belt rock, Harthope Valley NT918196. Prepared hand specimen in ordinary reflected light. Thin section viewed in plane polarised light (43mm across)

Thin section from the same sample viewed in plane polarised light

Thin section of  Central Belt rock  at location 10 viewed with crossed polarising filters

The same thin section viewed with crossed polarising filters

Plagioclase phenocryst in Central Belt rock. Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Plagioclase phenocryst in Central Belt rock
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Plagioclase phenocryst in Central Belt rock. Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

The same area viewed with crossed polarising filters

Chloritised biotite phenocryst with quartz and feldspar at location 10. Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Chloritised biotite phenocryst with quartz and feldspar at location 10
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Chloritised biotite phenocryst with quartz and feldspar. Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

The same area viewed with crossed polarising filters.


Location 11. Quartz cemented breccia from NT917195


Fractured and brecciated rock with quartz and haematite at NT917195.
Brecciated rock with quartz and haematite at NT917195.

Brecciated rock with quartz and haematite at NT917195


Between locations 11 and 12 there are numerous exposures of fractured and brecciated rock that is rich in haematite and cemented together with quartz. The explanation given for the breccia atlocation 7 applies here; reactivation of the Harthope fault in the Variscan orogeny led to fracturing of the plutonic rock and its alteration and cementation by hydrothermal fluids.

Brecciated rock with quartz and haematite at NT917195. Prepared hand specimen in ordinary reflected light (40mm across)

Brecciated rock with quartz and haematite at NT917195
Prepared hand specimen in ordinary reflected light (40mm across)

Brecciated rock with quartz and haematite at NT917195. Thin section viewed in plane polarised light (40mm across)

Thin section from the same sample viewed in plane polarised light

Brecciated rock with quartz and haematite at NT917195. Thin section viewed with crossed polarising filters (40mm across)

The same thin section viewed with crossed polarising filters

Quartz veinlet in sericitised feldspar and quartz groundmass with muscovite and tourmaline with opaques. Section viewed in plane polarised light (FoV 2.5 x 1.7 mm)

Quartz veinlet in sericitised feldspar and quartz groundmass with muscovite and tourmaline with opaques
Section viewed in plane polarised light (FoV 2.5 x 1.7 mm)

Quartz veinlet in sericitised feldspar and quartz groundmass with muscovite and tourmaline with opaques. Section viewed with crossed polarising filters (FoV 2.5 x 1.7 mm)

The same area viewed with crossed polarising filters

Anhedral quartz in sericitised feldspar and quartz groundmass with tourmaline and opaques. Section viewed in plane polarised light (FoV 1.3 x 0.8 mm)

Anhedral quartz in sericitised feldspar and quartz groundmass with tourmaline and opaques
Section viewed in plane polarised light (FoV 1.3 x 0.8 mm)

Anhedral quartz in sericitised feldspar and quartz groundmass with tourmaline and opaques. Section viewed with crossed polarising filters (FoV 1.3 x 0.8 mm)

The same area viewed with crossed polarising filters.


Location 12. Intensely altered rock weathering to red sand NT913190


Hydrothermically altered ock altered weathering to red sand, Harthope Valley, NT913190

Haematite-rich, hydrothermically altered rock weathering to sand, Harthope Valley NT913190


At location 12 we come across outcropping rock that is so altered it is reduced to a red sand. After a number of attempts we succeeded in making a thin section that revealed a very interesting arrangement of quartz and opaque oxides. Clearly, hydrothermal alteration was so intense here that the fabric and mineralogy of the rock was completely changed and weathering has decomposed it further.

Opaque-rich altered rock, Harthope Valley at NT913190. Prepared hand specimen in ordinary reflected light (42mm across)

Opaque-rich altered rock, Harthope Valley
Prepared hand specimen in ordinary reflected light (42mm across)

Opaque-rich altered rock, Harthope Valley. Thin section viewed in plane polarised light.(40mm across)

Thin section from the same sample viewed in plane polarised light

Opaque-rich altered rock, Harthope Valley. Thin section viewed with crossed polarising filters (40mm across)

The same thin section viewed with crossed polarising filters

Opaque-rich altered rock, Harthope Valley. Section viewed in plane polarised light.

Opaque-rich altered rock, Harthope Valley
Section viewed in plane polarised light

Opaque-rich altered rock, Harthope Valley. Section viewed with crossed polarising filters

The same area viewed with crossed polarising filters

Opaque-rich altered rock, Harthope Valley. Section viewed in plane polarised light.

Opaque-rich altered rock, Harthope Valley
Section viewed in plane polarised light

Opaque-rich

The same area viewed with crossed polarising filters


Location 13. Weak altered ‘Central Belt’ rock NT912190


Weak, altered rock, Harthope Valley NT912190
Weak, altered rock, Harthope Valley NT912190

Weak, altered rock, Harthope Valley NT912190


This is similar rock to that at location 12 but it isn’t so altered that the structure of the granite has been so radically changed. Because of the difficulties in preparing the thin section of this rock, it has been left slightly thick so the quartz in the central area is showing 1st order interference colours.

Extensively altered rock, Harthope Valley NT912190. Prepared hand specimen in ordinary reflected light.

Weak, altered rock, Harthope Valley NT912190
Prepared hand specimen in ordinary reflected light (42mm across)

Intensely altered rock, Harthope Valley NT912190. Thin section viewed in plane polarised light (35mm across)

Thin section from the same sample viewed in plane polarised light

Intensely altered rock, Harthope Valley NT912190. Thin section viewed with crossed polarising filters (40mm across)

The same thin section viewed with crossed polarising filters

Quartz and opaques in weak, altered rock, Harthope Valley NT912190. Section viewed in plane polarised light (FoV 5.0 x 3.0 mm)

Quartz and opaques in weak, altered rock, Harthope Valley NT912190
Section viewed in plane polarised light (FoV 5.0 x 3.0 mm)

Quartz and opaques in weak, altered rock, Harthope Valley NT912190. Section viewed with crossed polarising filters (FoV 5.0 x 3.0 mm)

The same area viewed with crossed polarising filters

Quartz and feldspar in intensively altered rock, Harthope Valley NT912190. Section viewed in plane polarised light.

Quartz and feldspar in intensively altered rock, Harthope Valley NT912190
Section viewed in plane polarised light (FoV 1.3 x 0.8 mm)

Quartz and feldspar in intensively altered rock, Harthope Valley NT912190. Section viewed with crossed polarising filters (FoV 1.3 x 0.8 mm)

The same area viewed with crossed polarising filters


Location 14. Last outcrop of ‘Central Belt’ rock NT909189


The last outcrop of Central Belt rock, Harthope Valley at NT909189

The final outcrop of Central Belt rock in the upper Harthope Valley at NT909189


The Harthope burn becomes a trickle up here and close by the stream at NT909189 we see the final outcrop of ‘Central Belt’ rock. It is altered but not so severely as that we have been seeing.

Altered Central Belt rock, Harthope Burn at NT909189. Prepared hand specimen in ordinary reflected light (42mm across)

Altered Central Belt rock, Harthope Burn at NT909189
Prepared hand specimen in ordinary reflected light (42mm across)

Altered Central Belt Harthope Burn NT909189. Thin section viewed in plane polarised light.(40mm across)

Thin section from the same sample viewed in plane polarised light

Altered Central Belt Harthope Burn NT909189. Thin section viewed with crossed polarising filters (40mm across)

The same thin section viewed with crossed polarising filters

Fine granophyric texture in altered Central Belt rock, Harthope Burn NT909189. Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

Fine granophyric texture in altered Central Belt rock, Harthope Burn NT909189
Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

Plagioclase phenocrysts in altered Central Belt rock. Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

Plagioclase phenocrysts in altered Central Belt rock
Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)


Twinned pyroxenes in altered Central Belt rock. Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm).

Twinned pyroxenes in altered Central Belt rock
Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)


Location 15. Last sign of ‘Central Belt’ rock


The last sign we see of the ‘Central Belt’ type of granitic rock is right at the head of the burn, in the stream bed. It might be bedrock or it could be a boulder but either way, from here on up the visible rock is all the low-mafic ‘Upper Cheviot’ evolved granite.
There is a choice of direction to be made here. We can choose to continue on up the hill to meet the path at Scotsman’s Cairn that would take us back over Cheviot and Scald Hill, or we can turn and retrace our steps. We choose the latter.


Boulder field with tourmaline-rich rocks, Harthope Valley

Boulder field with tourmaline-rich rocks, Harthope Valley with Long Crag behind.

References

N. M. Al-Hafdh 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,

C. Scruton (ed,)1995, Northumbrian Rocks and Landscape: a field guide, Yorkshire Geological Society.

Geological Society of London, Plate Tectonics at:
https://www.geolsoc.org.uk/Plate-Tectonics/Chap4-Plate-Tectonics-of-the-UK/Variscan-Orogeny

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

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

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