We find a coarse-grained granitic rock at NT 94016 16737. This is pinker with fewer large white plagioclase phenocrysts than the rocks of Hedgehope Hill and Great Standrop but it has a similar grain size. Most of the terrain is trackless peat bog lacking any exposure of bedrock.
Descending towards High Bleakhope, the pink medium-grained rock outcrops at NT 92469 16365. The outcrops around NT 92643 16262 are all of high colour index and correspond with the dioritic ‘marginal’ variety found on Dunmoor Hill and at Linhope Spout.
In places there are xenoliths of what appear to be hornfelsed andesite.
Tag Archives: Linhope Spout
Al-Hafdh, Linhope Spout and Linhope Burn
We have discovered Al-Hafdh’s 1985 PhD thesis on the alteration petrology of the Cheviot pluton and we’ve decided to chart and verify his rock types.
He claims to have found a series of chilled margins which establish the sequence of the various intrusions.
So, we make an excursion to the Linhope area where we find no convincing evidence for chilled margins but we do find the junction between Al-Hafdh’s Marginal and Dunmoor varieties just above Linhope Spout.
The ‘Felsite’ dyke marked on the 1” OS map at the top of Linhope Spout turns out to be a wider than normal version of the aplite veins which are frequent in the Cheviot pluton. This aplite dyke probably accounts for the hardness of the rock which has caused the waterfall feature.
About 100m up stream there is a slight cliff (about 5m high) at the junction between the pink granite and the marginal. This contains significant quartz veining, lending weight to the theory that hydrothermal activity took place along lines of weakness between the two types. Thin sections from this area reveal red crystals in the quartz which we first identified as rutile but now think more likely to be hematite.
Up stream to NT 94694 17249, we find a course-grained porphyritic rock in the streambed which Al-Hafdh classified as ‘Linhope’ granodiorite. We are not convinced by his differentiation of this from ‘Standrop’ granodiorite, and can not find evidence for a chilled margin between them.
A hypothesis and an excursion to Harthope Linn
Harthope Linn (NT 92734 20229)
We are pursuing the idea that the rocky features in the mostly rounded Cheviot Hills were caused by a more mafic content making the rocks harder. This would account for waterfalls such as Harthope Linn and Linhope Spout. Field work has shown that the andesite at the Carey Burn Linn and at Davidson’s Linn are of the dark harder variety.
There seems to be considerable variety in the rocks of the Cheviot pluton. The 1” geological survey shows the whole area as ‘granite’ but it is also apparent that the visible mineral content make this classification doubtful. This has led me to begin working with Ian to investigate and map the area in an effort to find out what is there and to better understand how it came to be so .
Today’s work at Harthope Linn did reveal rocks with a high colour index and mafic content but there were also other paler varieties and some showing substantial alteration.