June 2017 Summary

After several years of field work in the Cheviot Hills, it is appropriate to offer some conclusions from our studies. We lack access to, and expertise in many of the contemporary chemical and isotope procedures, so we accept that our conclusions must remain tentative. We would welcome informed suggestions and corrections.

We have noticed the following striking and somewhat unusual aspects to the Cheviot pluton.

  • There was a huge outpouring of lavas, mainly of andesite. The extent of this is now believed to reach up to 600 sq km and, before erosion, at least 2000m deep. Observed layering of tephra deposits in relation to lava confirms the traditional understanding that the outpouring of lava was preceded by an explosive phase. The depth of ignimbrite in the Knock and Brough Hill areas, even after 400 million years of erosion, suggests that the Cheviot volcanic system may have achieved a pretty high VEI rating.
  • There is a very marked absence of volcanic necks or vent agglomerate. The brecciated rocks which we have found in the upper Harthope and Hawsen Burn valleys cannot be safely identified as vent material. The abundant veining of quartz, tourmaline and haematite could equally point to a hydrothermal origin.
  • There is an absence of evidence for compression except at exposures close to the North side of the summit plateau of the Cheviot, and at the junction of two types of granitic rock on Shiel Cleugh Edge above High Bleakhope. There is, however, widespread evidence of thermal and metasomatic action (but not pressure) on the lavas adjacent to the pluton.
  • There are some distinct boundaries within the pluton. These are especially noticeable on Dunmoor Hill. Elsewhere, exposures of bedrock are too scanty to trace boundaries.
  • We have only recorded one chilled margin within the pluton, at Shiel Cleugh edge. There is no obvious chilling at the junction of granitic rock types (Marginal and Central Belt types) on Dunmoor Hill. The Quartz-monzonite of Dunmoor Hill close to the junction with the lavas does show an increasing fineness of grain consistent with chilling of the pluton against the earlier lavas.
  • Recent research suggests that the pluton is at least 4km deep and 20km wide at that depth.
  • Most of the plutonic rock is medium-grained. The Upper Cheviot evolved granite borders on the finest and is therefore almost a felsite. Much of the granitic rock has larger phenocrysts mainly of andesine feldspar, and sometimes of augite and biotite, in a fine-grained groundmass of alkali feldspar and quartz.
  • Current dating by the BGS suggests a slightly earlier date of 395-400Ma which would make the Cheviot more or less contemporary with the Skiddaw, Shap and Criffel plutons.
  • Other contemporary plutons of the early Devonian period do not have an associated volcanic phase. The Cheviot is unusual in exhibiting both plutonic intrusion and volcanic extrusion.
  • There is widespread pyroxene in the ‘Marginal’ quartz-monzonite. This is especially prevalent in the Northern Marginal area.
  • Evidence for cauldron subsidence which is an obvious characteristic of the almost contemporary Ben Nevis volcano, is not apparent in Cheviot. Surviving outcrops of lava above the pluton show bedding parallel to the slope of the pluton. This does not support evidence for either stoping or cauldron subsidence.

In light of these observations, we would offer the following suggestions for interpretation of the Cheviot system.

The extent of the andesite lava flows is rather remarkable. Andesite is a relatively viscous lava which normally does not flow far from source; hence the steep-sidedness of andesite composite volcanoes. To achieve the extent of the Cheviot lava flows, multiple vents scattered over a wide area, must have been present. However, the lack of any evidence for vents is problematic. Even allowing for late Devonian erosion, it is surprising that any such evidence is missing. Looking across the Southern Uplands of Scotland from the upper slopes of Cheviot, the numerous volcanic necks of Southern Scotland’s Carboniferous volcanics are clearly visible. Yet these necks survive from only about 50 million years later. Perhaps the answer is that there were no necks in the Cheviot system, and that the massive volumes of andesite were expelled from a series of rifts valleys. Could it possibly be that the network of faults which mark the main river valleys in the Cheviot Hills, are the remnants of these rifts?

The impressive volume of the Cheviot pluton rules out the traditional view that it is a laccolith. There is a real possibility that it is the remnants of the magma chamber or one of the chambers which fuelled the Cheviot volcanic system. Presumably the less silica-rich fractions of the chamber being more fluid, were expelled first as andesite, leaving the more silica rich melt behind to crystallise out into varying forms of granitic rock. Whether this theory is correct or not will depend on evidence gathered from modern chemical and isotope analysis which is beyond our own resources.

It is also clear from previous research and from the BGS past surveys that the pluton is also an intrusive body. The sharp contact between the pluton and the lavas, and the obvious chilling of the quartz-monzonite at Cunyon Crags on Dunmoor Hill, suggest that the lavas had already been consolidated before the intrusion of the granitic rock. The exception is in the North near Goldscleugh where tongues of granite penetrate the andesite, suggesting a perhaps more fluid environment there. The lava shows evidence of thermal and chemical alteration but not pressure from the granites. This suggests that the intrusion took place in circumstances where the lavas were relatively easily pushed aside. The relatively fine-grained structure of the plutonic rocks suggests that consolidation took place close to the surface. There is, however, evidence that, close to the summit plateau of Cheviot, there was some compression of granite and andesite. This evidence taken with the composition of the Upper Cheviot rock as the only unequivocally true granite in terms of Kspar and SiO2 content, together with its relatively fine grain, suggests that it may have formed an incipient but failed rhyolitic lava dome which nevertheless failed to break through the andesite crust. It should be remembered that we can only record data from what is apparently the topmost layer of this large pluton. Different but unobserved conditions may well prevail at greater depth.

The overwhelming presence of glacial drift and blanket bog obscures most of the Cheviot pluton’s bedrock. However, in the few places where contacts can be seen between different types of plutonic rock, they appear clearly defined. Just above Linhope Spout there is a sharp contact between the darker Marginal rock (Quartz-monzonite), and the much pinker Central Belt rock (borderline monzonite/granite). The boundary between the two types can be traced across much of Dunmoor Hill although it is somewhat more fuzzy there with tongues of either type penetrating the other for several metres suggesting that neither were fully consolidated when contact took place. On Shiel Cleugh Edge, there is clear evidence that one phase of Central Belt rock has been chilled against another. All this evidence indicates that multiple intrusions of granitic magma into the top reaches of the pluton must have occurred.

Our final concern is with the abundant presence of pyroxene, both clino- and ortho-, in some parts of the pluton, especially in the Marginal rocks. We have not been able to find any satisfactory explanation how pyroxenes can form in a subductional melt without the injection of mantle material. Pyroxenes form at high temperatures, whereas subduction hydrous melts achieve lower temperatures and are likely to produce the lower temperature hydrous minerals such as amphibole and biotite. Biotite is a universal constituent of the Cheviot pluton with primary amphibole (rather than as an alteration product) more scarce. This is as would be expected in a calc-alkaline subduction hydrous melt. So where did the high temperature pyroxenes come from? The only reasonable explanation that we can find is that much hotter and more basic mantle melt became available in the Cheviot pluton possibly as a result of contemporary transtensional tectonic activity. This could also offer an explanation why the Cheviot pluton out of all its contemporary colleagues in Northern England and Southern Scotland, produced an extensive volcanic outpouring.

Upper Harthope Valley

On Tuesday 29th November we set out to make a more detailed study of the rocks in the upper Harthope Valley above Harthope Linn. We believe from preliminary samples that these may differ significantly from the granitic rocks of the Cheviot pluton. The day is bright and cold with a hard frost promising to make boggy ground easier to walk over. The furthest permitted parking is at the Hawsen Burn just below Langleeford. From there it is a long but easy farm track to Langleefordhope, and then about a quarter of a mile to the lower Harthope Linn close to the stell (circular sheepfold). After that, the going becomes much rougher with several potentially difficult burn crossings.

The rocks outcropping above the stell prove to be granitic Central Belt group although rather more mafic than usual. Similarly the dyke-like outcrop beside the path about 200 yards above the upper Harthope Linn, is also granitic. This rock seems to give rise to a fairly even rolling hillsides.

Looking down the Harthope Valley from near Harthope Linn.
The granitic rocks of granite/quartz-monzonite give smooth rounded slopes.

From this point onwards, the type of rock changes consistently. We checked this most of the way to the watershed. The rock appears to be a form of breccia. In many places it has obviously been severely shattered. In other places it seems more stratified and less disturbed. Because of the shattered nature of the rock, it erodes into gullies more readily giving a more irregular pattern to the slopes of the hills.

A view looking up the Harthope Valley from above Harthope Linn.
The erosion gullies formed from the breccia rocks can be clearly seen.

An outcrop of typical breccia.

An outcrop of stratified breccia.

The breccia has been cemented together by silica and red haematite. The silica occasionally has space to crystallise out into attractive quartz crystals, mostly rock crystal but a few specimens show a hint of mauve. Occasionally, veinlets of black tourmaline appear with the quartz.

The Harthope valley marks the line of a SW-NE fault almost bisecting the pluton. Lateral displacement can be detected from the plutonic/lava margins on either side of the fault, and can be measured to about a quarter of a mile. Vertical movement, if any, is unknown. The faulting begs the question whether the breccia is a product of a crush zone, or evidence for volcanic vent activity. We remain uncertain about this. However, certain conclusions can be drawn. The breccia must have been cemented together under high temperatures for crystalline silica and tourmaline to have been deposited. The flatter layers imply more stable conditions for at least some of the hydrothermal activity. The presence of haematite (ferric oxide) indicates oxidising conditions but whether these were the result of iron reacting with high temperature water vapour inside the magma chamber, or exposure to the air at a vent, is uncertain.

Severely shattered breccia outcrop

Bellyside Crag and Upper Goldscleugh Sike

We buy a 10.00 GBP day permit from Savills, Glendale Road, Wooler and drive up the College Valley to Dunsdale where we start our walk up Bellyside Hill. The purpose of the expedition is to examine the granitic rock at the northern end of the pluton at Bellyside Crag which is classified by Al-Hafdh and by previous writers as ‘Marginal’ like the rock of Cunyon Crag and Dunmoor Hill on the south east side of the pluton. We also hope to find and examine the andesite outlier mapped at the head of Goldscleugh to see if it gives any clues as to its relationship with the pluton.

Bellyside Hill

View from Bellyside Hill looking up towards Goldscleugh Sike
The ground behind is the summit plateau of the Cheviot.

Bellyside Hill is Cheviot’s closest approximation to a classic ridge walk. Until about 600m there is a good path passing a series of granitic tors on the way. These are composed of a granitic but in appearance mafic rock with occasional felsic intrusions of pink rock.
At about 600m the path disappears and the only way forward is through tiresome blanket bog vegetation.

View looking north down Bellyside Hill showing the outcropping tors

View looking north down Bellyside Hill showing the outcropping tors

View looking north down Bellyside Hill showing the outcropping tors

A felsic phase or dyke in one of the tors on Bellyside Hill

Upper Goldscleugh reveals some rather heavily altered rock which may possibly be andesite, but it occurs as scattered blocks and pebbles, so it is impossible to determine any relationship with the pluton. If it is indeed andesite, its altered state indicates that it must have originated from close to the pluton contact.

We now tramp over to Bellyside Crag. This turns out to be composed of a mafic granitic rock similar to that found on the tors of Bellyside Hill.
The screes around the crag have a large colony of clubmosses.

Thin section of rock from Bellyside Crag viewed with crossed polars at X40

Thin section of rock from Bellyside Crag viewed with crossed polars at X40
Granophyric texture can be seen in several places. There is a large crystal of clinopyroxene at the bottom of the picture.

Subsequent thin section analysis reveals some interesting facts. The rocks of Bellyside Hill and Bellyside Crag are very similar to each other although the Bellyside Hill specimens may have a slightly higher proportion of alkali feldspar. However, neither are anything like the ‘Marginal’ rock of Dunmoor Hill and Cunyon Crag.
The Bellyside Hill specimens have a lower proportion of plagioclase to alkali feldspar (identifiable feldspars show a ratio of about 1:2, plagioclase to alkali feldspar); there is very little biotite, the main mafic mineral being pyroxene (10-15%); quartz content is similar at about 20%. It also has a more marked fine matrix than the southern Marginal with frequent granophyric texture.
It therefore seems rather doubtful that it has the same origin as the southern Marginal rock.

As regards the “andesite” of upper Goldscleugh Sike, if we are correct in identifying this as a granite/andesite contact, both the andesite and granite show clear signs of banding which may indicate contact compression.
Apart from the contact on Shiel Cleugh Edge, this is the only evidence for kinematic action which we have found on Cheviot. Where it exists it appears to be very localised. However, we remain rather uncertain whether the rock from upper Goldscleugh Sike was andesite or just an altered granitic rock.

Woolhope Crag

Woolhope Crag lies on the north east flank of the Cheviot just above Goldscleugh. It is one of the few convincing outcrops on the Cheviot itself. When we first visited it in 2013, we were looking for, and thought we had found, evidence that the outcrops occur because of the presence of harder, more mafic rock.
Subsequently, we read Al-Hafdh’s thesis which uses Woolhope Crag as the type location for his ‘Woolhope’ variety which is the least mafic, and therefore the most felsic and silica-rich, of his classifications.

Woolhope Crag from the East

Woolhope Crag from the East
The break on the right is where the felsic rock (left) becomes more mafic.

We returned here on19 August 2016 to investigate this apparent contradiction.
The answer is relatively simple. The southern end (higher end) of the crag is felsic; the northern (lower) end is more mafic in appearance. The join occurs at the broken mid-point of the crag. It is not an abrupt contact but a gradual change over about 20 cm and appears at first sight to be an example of fractionation – the first we have discovered in the Cheviot pluton.

Thin section of the upper (more felsic) rock at Woolhope Crag viewed with crossed polars at X40
There are some large phenocrysts of alkali feldspar showing Carlsbad twinning.

This theory breaks down when the two types are compared in thin section.
There seems to be very little difference in content between the two. Thin section analysis suggests that the difference between them is more apparent to the eye in hand specimen than actual. There is a slight increase in plagioclase with the more mafic rock but no increase in mafic minerals (both around 7%). The darker colour is probably due to the smaller grain size of the lower darker rock. Identifiable feldspars in both types show a ratio of about 1:3, plagioclase to alkali. Both types are rich enough in quartz (25-30%) and alkali feldspar to be classified as granite, albeit a relatively fine-grained variety.
An interesting feature of the upper more felsic rock is the presence of frequent chlorite.

Thin section of the lower (‘mafic’) rock at Woolhope Crag viewed with crossed polars at X40
There is no increase in mafic content compared with the more felsic rock. However, the grain size of the matrix is much smaller which must account for the darker appearance.

Scald Hill – The Cheviot – Cairn Hill – Upper Harthope Valley

The main purpose of this expedition is to confirm the presence of a distinct type of plutonic rock on the upper reaches of the Cheviot itself. This is what we have called the ‘Evolved’ type, and which Al-Hafdh called the ‘Woolhope’ type. Chemical analysis done by Al-Hafdh, and our own thin section work suggests that this rock has a lower mafic, higher alkali feldspar and higher quartz content than the other plutonic rocks of the Cheviot pluton. Chemically and mieralogically it appears to be a true granite but with its finer grain size (often under 0.25mm) it is nearer to an intermediate rock such as a felsite. The consensus is that this sort of rock being a more acid type, is a later differentiation and intrusion. It lies at the current top of the pluton, and may have almost broken through the andesite covering. This would account for its fine grain size which must be the result of more rapid cooling.
We start from the foot of the Hawsen Burn and ascend the footpath via Scald Hill. On the traverse of Scald Hill we find a mixture of pink porphyritic ‘granite’ as well as the Evolved type. This mixture continues on the ascent of the Cheviot itself, but the Evolved type rapidly begins to predominate. There is no discernible clear boundary between the types. At NT 92103 21115 we find plenty of quartz and hematite veining.

Quartz and haematite veining

Quartz and haematite veining

At NT 91826 20984 the rock looks brecciated. The situation is confused at NT 92020 21066 and NT 91789 20966 by the appearance of a much more mafic fine-grained rock with the Evolved type. We are uncertain whether these represent a more mafic differentiation within the Evolved type or are xenoliths of andesite which have collapsed into the roof of the pluton. Thin section analysis should answer this problem.

Andesite xenolith or fine-grained plutonic rock

Andesite xenolith or fine-grained plutonic rock?

The summit plateau of Cheviot is magnificent with a large extents of blanket bog containing cotton-grass, clubmosses and cloudberry some of which was fruiting quite prolifically. We abandon any plans to cross over the bog to Bellyside Crag because there is no path and we do not wish to damage the fragile ecology. We continue along the paved footpath to Cairn Hill and descend to the head of the Harthope Burn. What exposures exist indicate that the rock type is Evolved over this whole area.
We begin to see a change back to the typical coarser pink porphyritic ‘granite’ in the upper reaches of the Harthope Burn at NT 90671 19035. From then onwards down to Harthope Linn there are some exposures of very weathered rock. We lacked time to make a detailed examination of this, and plan another expedition specifically to this area.

Shielcleugh Edge

Ian made an expedition to Shiel Cleugh in 2015 and noted the rather complex variety of granitic rocks here. We return to check this out, and also to see if we can find where Al-Hafdh (1985) reckoned he had found the ‘Standrop’ type chilled against the ‘Dunmoor’ type (both names Al-Hafdh’s classification). If correct, this would indicate that the coarser ‘Standrop’ was intruded later than the finer ‘Dunmoor’, both of which Al-Hafdh interpreted as part of a series of ring intrusions.
In the course of the day we climb to the top of Shiel Cleugh Edge, and notice wistfully in the distance the prominent tor of Coldlaw Cairn. This is within the plutonic area and needs a visit but it is very remote and only reached over trackless blanket bog.
We find a rather bewildering confusion of rock types. There is no clear cut boundary between finer (Dunmoor) and coarser (Standrop) types. Both are found widely over the Southern side of Shiel Cleugh. The position is complicated by the limited exposures, many of which cannot be accepted with certainty as bedrock. However, a pattern does emerge as the day progresses. The coarser ‘granite’ predominates at a higher level, while the finer ‘granite’ predominates lower down. At the very bottom close to the River Breamish the rock becomes much more mafic and dioritic. This appears to be a continuation of the Marginal type which is found extensively on the southern slopes of High Cantle.

From left to right: the succession of rock=types down the Shiel Cleugh burn, Increasing mafic content as the burn is descended to the River Breamish.

From left to right: the succession of rock=types down the Shiel Cleugh Burn
We see increasing mafic content as the burn is descended to the River Breamish.

However, there is little sign of clear boundaries. The rocks which are fairly well exposed in the Shiel Cleugh burn which flows off the southern slope of the Edge into the Breamish, show a gradual change from coarse to finer pink ‘granite’ and then a gradual increase in colour index towards Marginal rock.
There however some rather exciting dicoveries. At NT 91922 16767 we do find a distinct boundary between coarser and finer rock.

Contact between the coarser and finer-grained rock

Contact between the coarser and finer-grained rock

At NT 91847 16893 and NT 91825 16916 we find further junctions between the two types. At these locations, there is evidence for compression of one against the other together with some flow structure. Significantly there appears to be a chilled margin, and later thin section analysis confirms this. However it is the finer (‘Dunmoor’) which is chilled against the coarser (‘Standrop’). This suggests that the coarser rock cannot have been intruded into the finer.

Thin section with crossed polars showing the chilling of the finer-grained rock against the coarser-grained.

Thin section with crossed polars showing the chilling of the finer-grained rock against the coarser-grained

Upper Coquetdale

This is a mainly car-based expedition to examine various features in this beautiful dale. We drive straight to Fulhope to sample the limited exposures of Cheviot rhyolite. The geological survey classifies the rock as ‘mica-felsite’. It is a purplish rock with white feldspar phenocrysts. There is little evidence of mica to the naked eye. In the dry stone walls it has a rather more crinkly texture than the commoner andesite.
We then drive to Blindburn to view the excellent exposures of blocky andesite resting on a bed of tuff and ignimbrite on the opposite (South) side of the Coquet. We drive past the traditional hay meadows of Barrowburn, and notice that the Wood Cranesbill is in flower beside the road.
We stop at Bygate to try to locate the Acklington Dyke in the river bed. We find it in a rather inaccessible spot in the river. The Dyke has nothing to do with either the Devonian Cheviot volcanics or the Permian Whin Sill. It is one of a series of tholeiitic dolerite dykes radiating from the Eocene Mull volcano, and reaching right on to the Northumberland coastal plain.
Our final port of call is Kateshaw Crag at the roadside between Bygate and Shillmoor. The andesite here has weathered to produce beautiful blue-green phenocrysts of chlorite. We were trying to find the quartz-porphyry dyke which has been mapped in the river bed here. We could see a bed of rock on the opposite bank which looked more granitic than the surrounding andesite but the river was too deep to wade so confirmation eluded us.

Up the Hawsen Burn to Goldscleugh

Our objective is to examine the different types of ‘granite’ on the uplands above the Hawsen Burn and in the upper reaches of the College Valley around Goldscleugh where the northern ‘granite’ meets the andesite. On the climb up past Hawsen Crags we have an excellent view of a pair of stonechats. The torrential rain of November and December 2015 has gouged out a ditch by the path to a depth of at least a metre (NT93932 23084). In this gulley we find plenty of breccia which may perhaps be bedrock. One piece in particular consists of rather fine banded agate which has obviously been formed before brecciation and then cemented together again with more silica. This may be evidence for the location of one of the elusive volcanic vents.
The summit plateau before descending to Goldscleugh offers splendid views of the upper College Valley. The terrain is peat bog, and what few rocks appear are of the pink porphyritic type, some coarser, some finer grained. We descend to the Lambden Burn. Ian explores along the streambed in an easterly direction, and confirms the geological survey that tongues of ‘granite’ penetrate the andesite on the north side of the burn.
We stop to examine the rocks at the southerly branch of the head of the Lambden Burn. Ian discovers an outcrop (NT 92655 22523) which closely resembles the Evolved granular granite on the upper slopes of Cheviot. We contemplate the nearby Woolhope Crag but decide to leave that for another expedition.
As we return down the Hawsen Burn, Ian branches off to examine the Hawsen Crag (NT 94797 23216).

Return visit to Harthope Linn

This is a splendid day of warm sunshine. Non-geological highlights include 2 rather torpid adders, one nearly two feet long, crossing our path. There is a fairly long walk through delightful upland birch-alder woodland before crossing a boulder field deposited by floodwaters from the Harthope Burn. We find some rather fine tourmalinised ‘granite’ but, being in the boulder field, it is impossible to tell its origin.
We stop for refreshments at the upper Harthope Linn (waterfall) climbing down to the burn margin. We find a boulder of brecciated rock cemented together by silica. This must have been formed at high temperature, and we would like to believe that it represents part of a volcanic vent. For a 100 square kilometres of andesite to have been poured out, andesite being a fairly viscous lava, there must have been numerous vents during the cycle of active vulcanicity, but their location remains unknown after eons of erosion have destroyed the evidence for them. The problem with this piece of breccia is that it is not clear that it is bedrock and so may have been transported to its present position by ice or water. It also may have been formed by the pressures generated by the Harthope Fault on the line of which it lies.
We walk back down to the lower Harthope Linn just by the stell (circular dry stone sheepfold). There are some interesting rocks in the stream bed. The usual pink granite can be seen adjoining the darker Marginal dioritic variety. There is also some hornfelsed andesite. We would like to believe that this might be evidence for stoping from the andesite roof 400 metres above, but, again, it lies on the Harthope Fault and may have been brought down by earth movements after vulcanicity ceased.

Great Standrop again

We return to the collar of boulders and outcrops below Great Standrop to confirm whether the fine-grained rock is indeed a chilled margin. It turns out to be yet another aplite dyke.
This trip confirms that the coarse-grained porphyritic ‘Standrop’ variety of granite. changes to the pink, medium-grained porphyritic variety (Al-Hafdh’s ‘Hedgehope granodiorite’) at the base of the rocky collar.

Great Standrop

I visit Little and Great Standrop to confirm that these fine tors consist of coarse-grained porphyritic ‘granite’.
The rock here is the classic grey type with large white phenocrysts of andesine feldspar. At Great Standrop aplite dykes are apparent cutting through the coarser rock which extends down to the rocky collar ringing the slopes above the Linhope Burn at around NT 944 177. At the foot of the collar, the rock appears to change to the medium-grained pink ‘granite’.
At NT 94590 17775, I find a fine-grained pink rock which I take to be a chilled margin of the pink medium-grained rock against the coarser material of the main Standrop ridge.
The characteristic coarse-grained ‘Standrop’ rock proves difficult to classify. Quartz content is roughly 20%, and in places plagioclase exceeds K-feldspar. The rock lies on the granite/syenite/monzonite/granodiorite boundary.

The Hawsen Burn

We find plenty of fine-grained granophyric rock but we are not convinced that it really is of the same type as the ‘Evolved’ rock from the summit and north slopes of the Cheviot. Subsequent thin section analysis suggests that it and samples from the Standrop Burn are rather different from the ‘Evolved’ type. However, the Hawsen Burn samples do share the high quartz and low plagioclase content of the ‘Evolved’ rock of the Upper Cheviot area. At any rate, we now feel that there is insufficient evidence to argue that the fine-grained granophyric rocks are all part of a common ring dyke system. They are quite likely to be separate intrusions, some of them representing very late magmatic activity.
We locate a medium-grained but rather altered, pink porphyritic variety at NT 94299 23027 which seems to correspond with varieties found on Dunmoor and Hedgehope Hills.
There is plenty of evidence of altered andesite, hornfelsed by contact with the pluton as we would expect here as the Hawsen Burn runs along the boundary between the two in some places.

Altered yellowish andesite at NT 94504 23011 is stated in some guides to contain massive epidote but there was none in the samples that we thinned.

Dunmoor Hill

I trace samples of dioritic ‘Marginal’ rock between NT 97475 17206 and NT 96914 17281 to see whether there is any significant change in mafic content approaching the junction with the andesite.
I found no evidence, at least on Dunmoor Hill, for increasing mafic content towards the perimeter of the pluton. There are however local variations which are consistent with the principle of local fractionation bands within the Marginal.

Rigg Cairn, High Cantle to High Bleakhope

We find a coarse-grained granitic rock at NT 94016 16737. This is pinker with fewer large white plagioclase phenocrysts than the rocks of Hedgehope Hill and Great Standrop but it has a similar grain size. Most of the terrain is trackless peat bog lacking any exposure of bedrock.
Descending towards High Bleakhope, the pink medium-grained rock outcrops at NT 92469 16365. The outcrops around NT 92643 16262 are all of high colour index and correspond with the dioritic ‘marginal’ variety found on Dunmoor Hill and at Linhope Spout.
In places there are xenoliths of what appear to be hornfelsed andesite.

Housey and Long Crags

A fine day and we go in search of these magnificent outliers of hornfelsed andesite.
At both crags, careful searching reveals a fine-grained pink granitic rock at the base which we interpret as granite chilled against the andesite.
The stratification of the altered lava crags follow exactly the same contour as the base terrain which is made up of plutonic rock. This supports the idea that these crags were part of the final roof of the magma chamber that remains pretty much in its original position rather than a portion that has fallen into the magma.
(While by no means conclusive, this evidence points away from cauldron subsidence or stoping.) (Couldn’t the stoping process have occurred at a lower level than these portions of roof, leaving them unaffected?)
The space problem in relation to plutons has reared its head.

Dunmoor Hill

We make a return visit to Dunmoor Hill. The summit tors of the hill present a medium-grained pink granitic rock that look very similar to that of the summit of Hedgehope Hill.
Long Crag (not the same as Long Crags, near Housey Crag in the Harthope valley) reveals a very similar rock. Al-Hafdh classifies the former as ‘Hedgehope granodiorite’ and the latter as ‘Dunmoor granodiorite’. Actually, both in hand specimen and thin section, they are very similar in appearance and mineral content – an observation born out by Al-Hafdh’s own chemical analysis of the types.
We remain unconvinced that they can be separated into two types belonging to separate intrusions.
Al-Hafdh argues that the two are separated by a belt of coarse porphyritic granodiorite (‘Standrop’ variety). A tor containing the coarse rock certainly appears at NT 96890 17969 about half way between the summit and Long Crag, but the extent of peat bog over the hill precludes any way of establishing whether this is part of a wider belt separating the summit of Dunmoor Hill from Long Crag.
A further descent down the south side of the hill reveals again the junction between the pink medium grained porphyritic rock ‘(Dunmoor’/’Hedgehope’) and the dioritic ‘Marginal’ variety.
Cat Crag is definitely composed of the ‘Marginal’ variety.
We look for the hornfelsed andesite that geological map locate close to Long Crag, but we don’t find it.

Hedgehope Hill

A visit to Hedgehope Hill confirms that the upper Dunmoor Burn contains a very fine-grained granophyric rock. We have provisionally classified this as ‘Evolved’ granite but have since become increasingly doubtful whether it is really part of the same intrusion as the evolved granophyre on the upper slopes of the Cheviot, and whether it really does form part of a ring dyke as Al-Hafdh suggested. Exposure is too limited in the upper Dunmoor Burn to draw definite conclusions.
About 100m below the summit of Hedgehope Hill, the track crosses a large boulder field which is probably the result of periglacial activity. The great majority of these boulders belong to the coarse porphyritic type which Al-Hafdh named ‘Standrop granodiorite’. The summit of Hedgehope has a medium to fine-grained pink rock which is similar to that of Dunmoor Hill. Many samples from the summit show evidence of significant hydrothermal alteration.
Returning via the north side of Hedgehope Hill towards the Harthope valley, we find more of the coarse-grained ‘Standrop’ rock but, after much searching, fail to find the chilled margin between the finer and coarser varieties that Al-Hafdh says is visible there. The one example of really fine-grained rock chilled against the courser rock turns out to be another felsite or aplite dyke.

Dunmoor Hill

We go to Dunmoor Hill in search of Al-Hafdh’s hitherto elusive chilled margins. Dunmoor Hill is a particularly good site as exposure, unlike in many areas of the Cheviot hills, is plentiful. We are finding Al-Hafdh’s thesis on the Cheviot pluton immensely stimulating. It has provided clear direction to our own research, although we are beginning to doubt his proposal that the Cheviot pluton consisted of a series of ring dykes.
The area below Cunyon Crags has plenty of small outcrops where we find a bewildering mixture of felsic and mafic fine-grained material. At first, we think that the mafic rock is altered andesite. However, thin sections show very well-developed granophyric texture in the felsic rock which imply late intrusion into already established plutonic bodies. The mafic rock has a very high biotite/opaque iron oxide content, and appears to be restite.
Subsequently, we are able to trace the junction between the pink rock (Al-Hafdh’s Dunmoor type) and the more mafic Marginal rocks right across the south slope of Dunmoor Hill. The junction doesn’t show a clear cut boundary but it does present frequent inter-penetration of the two types. The implication is that neither type was fully consolidated when the intrusion took place.
We are still unable to find any chilled margins to confirm an intrusion sequence. Finer rocks always turn out to be aplite veins or small dykes.

Knock Hill and Upper Linhope Burn

Knock Hill (NT 99584 16499) with its attendant gorge is an impressive feature on the road towards Linhope. It is composed of ignimbrite, and gives evidence that the Cheviot volcanic system was explosive, and produced abundant pyroclastic surges and ash fall, as well as andesite lava flows.
The 1” OS geological map notes a ‘hypersthene-porphyrite’ dyke which we locate at NT 99584 16499 on the south side of the Breamish burn, just beyond the road bridge.
From here, a walk to the upper reaches of the Linhope Burn (around NT 94094 17459) and on to the Standrop Burn. (NT 93794 17839) There is a dacite dyke showing in the streambed just above the two burns’ confluence.
The streambed reveals a variety of rock types. The coarse-grained (Standrop) and finer-grained pink (Dunmoor) varieties in evidence. There is also a much finer grained pale rock similar to the ‘evolved’ granophyre from the Cheviot.
According to Al-Hafdh, this is ‘Woolhope’ granite, part of a ring dyke that circles from the upper Standrop Burn area through to the upper Dunmoor Burn area.

Al-Hafdh, Linhope Spout and Linhope Burn

We have discovered Al-Hafdh’s 1985 PhD thesis on the alteration petrology of the Cheviot pluton and we’ve decided to chart and verify his rock types.
He claims to have found a series of chilled margins which establish the sequence of the various intrusions.
So, we make an excursion to the Linhope area where we find no convincing evidence for chilled margins but we do find the junction between Al-Hafdh’s Marginal and Dunmoor varieties just above Linhope Spout.
The ‘Felsite’ dyke marked on the 1” OS map at the top of Linhope Spout turns out to be a wider than normal version of the aplite veins which are frequent in the Cheviot pluton. This aplite dyke probably accounts for the hardness of the rock which has caused the waterfall feature.
About 100m up stream there is a slight cliff (about 5m high) at the junction between the pink granite and the marginal. This contains significant quartz veining, lending weight to the theory that hydrothermal activity took place along lines of weakness between the two types. Thin sections from this area reveal red crystals in the quartz which we first identified as rutile but now think more likely to be hematite.
Up stream to NT 94694 17249, we find a course-grained porphyritic rock in the streambed which Al-Hafdh classified as ‘Linhope’ granodiorite. We are not convinced by his differentiation of this from ‘Standrop’ granodiorite, and can not find evidence for a chilled margin between them.

Woolhope Crag and the Cheviot

We go to see if the theory of the darker rocks making tougher landscape features, would hold at Woolhope Crag (NT 92264 22169).
The answer was ambiguous. There is certainly plenty of dioritic rock there but there is also a pale fine-grained granophyric rock. We subsequently made our way up the north slopes of the Cheviot, and found plenty of scattered exposures which revealed more of the granophyric rock. We have classified this type on the map as ‘Evolved’ granite.
Descending from Cheviot summit on the main path for Langlee in the Harthope valley, we found distinct quartz veining with tourmaline content (approx. NT 918 209). There is the possibility that this quartz veining is the result of hydrothermal penetration on lines of weakness between different types of plutonic rock.
We need to pay another visit to investigate this.

A hypothesis and an excursion to Harthope Linn

 Harthope Linn (NT 92734 20229)

We are pursuing the idea that the rocky features in the mostly rounded Cheviot Hills were caused by a more mafic content making the rocks harder. This would account for waterfalls such as Harthope Linn and Linhope Spout. Field work has shown that the andesite at the Carey Burn Linn and at Davidson’s Linn are of the dark harder variety.

There seems to be considerable variety in the rocks of the Cheviot pluton. The 1” geological survey shows the whole area as ‘granite’ but it is also apparent that the visible mineral content make this classification doubtful. This has led me to begin working with Ian to investigate and map the area in an effort to find out what is there and to better understand how it came to be so .

Today’s work at Harthope Linn did reveal rocks with a high colour index and mafic content but there were also other paler varieties and some showing substantial alteration.

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