marginal Archives - Geology North

Bellyside Crag and Upper Goldscleugh Sike

Stephen — Thu, 22 Sep 2016 19:32:09 +0000

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.

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

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
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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Shielcleugh Edge

Stephen — Wed, 06 Jul 2016 09:13:01 +0000

SHIEL CLEUGH 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 parent Breamish burn. 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
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

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

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Return visit to Harthope Linn

Stephen — Sat, 16 Apr 2016 19:40:22 +0000

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.

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