We return to the collar of boulders and outcrops below Great Standrop to confirm whether the fine-grained rock is indeed a chilled margin. It turns out to be yet another aplite dyke.
This trip confirms that the coarse-grained porphyritic ‘Standrop’ variety of granite. changes to the pink, medium-grained porphyritic variety (Al-Hafdh’s ‘Hedgehope granodiorite’) at the base of the rocky collar.
Tag Archives: medium-grained porphyritic
Great Standrop
I visit Little and Great Standrop to confirm that these fine tors consist of coarse-grained porphyritic ‘granite’.
The rock here is the classic grey type with large white phenocrysts of andesine feldspar. At Great Standrop aplite dykes are apparent cutting through the coarser rock which extends down to the rocky collar ringing the slopes above the Linhope Burn at around NT 944 177. At the foot of the collar, the rock appears to change to the medium-grained pink ‘granite’.
At NT 94590 17775, I find a fine-grained pink rock which I take to be a chilled margin of the pink medium-grained rock against the coarser material of the main Standrop ridge.
The characteristic coarse-grained ‘Standrop’ rock proves difficult to classify. Quartz content is roughly 20%, and in places plagioclase exceeds K-feldspar. The rock lies on the granite/syenite/monzonite/granodiorite boundary.
Rigg Cairn, High Cantle to High Bleakhope
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.
Dunmoor Hill
We make a return visit to Dunmoor Hill. The summit tors of the hill present a medium-grained pink granitic rock that look very similar to that of the summit of Hedgehope Hill.
Long Crag (not the same as Long Crags, near Housey Crag in the Harthope valley) reveals a very similar rock. Al-Hafdh classifies the former as ‘Hedgehope granodiorite’ and the latter as ‘Dunmoor granodiorite’. Actually, both in hand specimen and thin section, they are very similar in appearance and mineral content – an observation born out by Al-Hafdh’s own chemical analysis of the types.
We remain unconvinced that they can be separated into two types belonging to separate intrusions.
Al-Hafdh argues that the two are separated by a belt of coarse porphyritic granodiorite (‘Standrop’ variety). A tor containing the coarse rock certainly appears at NT 96890 17969 about half way between the summit and Long Crag, but the extent of peat bog over the hill precludes any way of establishing whether this is part of a wider belt separating the summit of Dunmoor Hill from Long Crag.
A further descent down the south side of the hill reveals again the junction between the pink medium grained porphyritic rock ‘(Dunmoor’/’Hedgehope’) and the dioritic ‘Marginal’ variety.
Cat Crag is definitely composed of the ‘Marginal’ variety.
We look for the hornfelsed andesite that geological map locate close to Long Crag, but we don’t find it.
Dunmoor Hill
We go to Dunmoor Hill in search of Al-Hafdh’s hitherto elusive chilled margins. Dunmoor Hill is a particularly good site as exposure, unlike in many areas of the Cheviot hills, is plentiful. We are finding Al-Hafdh’s thesis on the Cheviot pluton immensely stimulating. It has provided clear direction to our own research, although we are beginning to doubt his proposal that the Cheviot pluton consisted of a series of ring dykes.
The area below Cunyon Crags has plenty of small outcrops where we find a bewildering mixture of felsic and mafic fine-grained material. At first, we think that the mafic rock is altered andesite. However, thin sections show very well-developed granophyric texture in the felsic rock which imply late intrusion into already established plutonic bodies. The mafic rock has a very high biotite/opaque iron oxide content, and appears to be restite.
Subsequently, we are able to trace the junction between the pink rock (Al-Hafdh’s Dunmoor type) and the more mafic Marginal rocks right across the south slope of Dunmoor Hill. The junction doesn’t show a clear cut boundary but it does present frequent inter-penetration of the two types. The implication is that neither type was fully consolidated when the intrusion took place.
We are still unable to find any chilled margins to confirm an intrusion sequence. Finer rocks always turn out to be aplite veins or small dykes.
Knock Hill and Upper Linhope Burn
Knock Hill (NT 99584 16499) with its attendant gorge is an impressive feature on the road towards Linhope. It is composed of ignimbrite, and gives evidence that the Cheviot volcanic system was explosive, and produced abundant pyroclastic surges and ash fall, as well as andesite lava flows.
The 1” OS geological map notes a ‘hypersthene-porphyrite’ dyke which we locate at NT 99584 16499 on the south side of the Breamish burn, just beyond the road bridge.
From here, a walk to the upper reaches of the Linhope Burn (around NT 94094 17459) and on to the Standrop Burn. (NT 93794 17839) There is a dacite dyke showing in the streambed just above the two burns’ confluence.
The streambed reveals a variety of rock types. The coarse-grained (Standrop) and finer-grained pink (Dunmoor) varieties in evidence. There is also a much finer grained pale rock similar to the ‘evolved’ granophyre from the Cheviot.
According to Al-Hafdh, this is ‘Woolhope’ granite, part of a ring dyke that circles from the upper Standrop Burn area through to the upper Dunmoor Burn area.
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.