Tag Archives: fine-grained

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

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

The main purpose of this expedition was 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.

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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.

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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.

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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.