Tag Archives: granite

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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.

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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.