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: hornfels
The Hawsen Burn
We find plenty of fine-grained granophyric rock but we are not convinced that it really is of the same type as the ‘Evolved’ rock from the summit and north slopes of the Cheviot. Subsequent thin section analysis suggests that it and samples from the Standrop Burn are rather different from the ‘Evolved’ type. However, the Hawsen Burn samples do share the high quartz and low plagioclase content of the ‘Evolved’ rock of the Upper Cheviot area. At any rate, we now feel that there is insufficient evidence to argue that the fine-grained granophyric rocks are all part of a common ring dyke system. They are quite likely to be separate intrusions, some of them representing very late magmatic activity.
We locate a medium-grained but rather altered, pink porphyritic variety at NT 94299 23027 which seems to correspond with varieties found on Dunmoor and Hedgehope Hills.
There is plenty of evidence of altered andesite, hornfelsed by contact with the pluton as we would expect here as the Hawsen Burn runs along the boundary between the two in some places.
Altered yellowish andesite at NT 94504 23011 is stated in some guides to contain massive epidote but there was none in the samples that we thinned.
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.
Housey and Long Crags
A fine day and we go in search of these magnificent outliers of hornfelsed andesite.
At both crags, careful searching reveals a fine-grained pink granitic rock at the base which we interpret as granite chilled against the andesite.
The stratification of the altered lava crags follow exactly the same contour as the base terrain which is made up of plutonic rock. This supports the idea that these crags were part of the final roof of the magma chamber that remains pretty much in its original position rather than a portion that has fallen into the magma.
(While by no means conclusive, this evidence points away from cauldron subsidence or stoping.) (Couldn’t the stoping process have occurred at a lower level than these portions of roof, leaving them unaffected?)
The space problem in relation to plutons has reared its head.
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.