Tag Archives: evolved granite

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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.

I 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.

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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).

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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.

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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.

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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.

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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.