Tag Archives: mafic

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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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Shielcleugh Edge

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, 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