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.
Tag Archives: aplite
Great Standrop again
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.
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.
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.