Shielcleugh 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 River Breamish 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