Tag Archives: quartz

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Upper Harthope Valley

On Tuesday 29th November we set out to make a more detailed study of the rocks in the upper Harthope Valley above Harthope Linn. We believe from preliminary samples that these may differ significantly from the granitic rocks of the Cheviot pluton. The day is bright and cold with a hard frost promising to make boggy ground easier to walk over. The furthest permitted parking is at the Hawsen Burn just below Langleeford. From there it is a long but easy farm track to Langleefordhope, and then about a quarter of a mile to the lower Harthope Linn close to the stell (circular sheepfold). After that, the going becomes much rougher with several potentially difficult burn crossings.

The rocks outcropping above the stell prove to be granitic Central Belt group although rather more mafic than usual. Similarly the dyke-like outcrop beside the path about 200 yards above the upper Harthope Linn, is also granitic. This rock seems to give rise to a fairly even rolling hillsides.

Looking down the Harthope Valley from near Harthope Linn.
The granitic rocks of granite/quartz-monzonite give smooth rounded slopes.

From this point onwards, the type of rock changes consistently. We checked this most of the way to the watershed. The rock appears to be a form of breccia. In many places it has obviously been severely shattered. In other places it seems more stratified and less disturbed. Because of the shattered nature of the rock, it erodes into gullies more readily giving a more irregular pattern to the slopes of the hills.

A view looking up the Harthope Valley from above Harthope Linn.
The erosion gullies formed from the breccia rocks can be clearly seen.

An outcrop of typical breccia.

An outcrop of stratified breccia.

The breccia has been cemented together by silica and red haematite. The silica occasionally has space to crystallise out into attractive quartz crystals, mostly rock crystal but a few specimens show a hint of mauve. Occasionally, veinlets of black tourmaline appear with the quartz.

The Harthope valley marks the line of a SW-NE fault almost bisecting the pluton. Lateral displacement can be detected from the plutonic/lava margins on either side of the fault, and can be measured to about a quarter of a mile. Vertical movement, if any, is unknown. The faulting begs the question whether the breccia is a product of a crush zone, or evidence for volcanic vent activity. We remain uncertain about this. However, certain conclusions can be drawn. The breccia must have been cemented together under high temperatures for crystalline silica and tourmaline to have been deposited. The flatter layers imply more stable conditions for at least some of the hydrothermal activity. The presence of haematite (ferric oxide) indicates oxidising conditions but whether these were the result of iron reacting with high temperature water vapour inside the magma chamber, or exposure to the air at a vent, is uncertain.

Severely shattered breccia outcrop

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Scald Hill – The Cheviot – Cairn Hill – Upper Harthope Valley

Scald Hill – The Cheviot – Cairn Hill – Upper Harthope Valley

The main purpose of this expedition was to confirm the presence of a distinct type of plutonic rock on the upper reaches of the Cheviot itself. This is what we have called the ‘Evolved’ type, and which Al-Hafdh called the ‘Woolhope’ type. Chemical analysis done by Al-Hafdh, and our own thin section work suggests that this rock has a lower mafic, higher alkali feldspar and higher quartz content than the other plutonic rocks of the Cheviot pluton. Chemically and mieralogically it appears to be a true granite but with its finer grain size (often under 0.25mm) it is nearer to an intermediate rock such as a felsite. The consensus is that this sort of rock being a more acid type, is a later differentiation and intrusion. It lies at the current top of the pluton, and may have almost broken through the andesite covering. This would account for its fine grain size which must be the result of more rapid cooling.
We start from the foot of the Hawsen Burn and ascend the footpath via Scald Hill. On the traverse of Scald Hill we find a mixture of pink porphyritic ‘granite’ as well as the Evolved type. This mixture continues on the ascent of the Cheviot itself, but the Evolved type rapidly begins to predominate. There is no discernible clear boundary between the types. At NT 92103 21115 we find plenty of quartz and hematite veining.

Quartz and haematite veining

Quartz and haematite veining

At NT 91826 20984 the rock looks brecciated. The situation is confused at NT 92020 21066 and NT 91789 20966 by the appearance of a much more mafic fine-grained rock with the Evolved type. We are uncertain whether these represent a more mafic differentiation within the Evolved type or are xenoliths of andesite which have collapsed into the roof of the pluton. Thin section analysis should answer this problem.

Andesite xenolith or fine-grained plutonic rock

Andesite xenolith or fine-grained plutonic rock?

The summit plateau of Cheviot is magnificent with a large extents of blanket bog containing cotton-grass, clubmosses and cloudberry some of which was fruiting quite prolifically. We abandon any plans to cross over the bog to Bellyside Crag because there is no path and we do not wish to damage the fragile ecology. We continue along the paved footpath to Cairn Hill and descend to the head of the Harthope Burn. What exposures exist indicate that the rock type is Evolved over this whole area.
We begin to see a change back to the typical coarser pink porphyritic ‘granite’ in the upper reaches of the Harthope Burn at NT 90671 19035. From then onwards down to Harthope Linn there are some exposures of very weathered rock. We lacked time to make a detailed examination of this, and plan another expedition specifically to this area.

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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.

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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.

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Woolhope Crag and the Cheviot

We go to see if the theory of the darker rocks making tougher landscape features, would hold at Woolhope Crag (NT 92264 22169).
The answer was ambiguous. There is certainly plenty of dioritic rock there but there is also a pale fine-grained granophyric rock. We subsequently made our way up the north slopes of the Cheviot, and found plenty of scattered exposures which revealed more of the granophyric rock. We have classified this type on the map as ‘Evolved’ granite.
Descending from Cheviot summit on the main path for Langlee in the Harthope valley, we found distinct quartz veining with tourmaline content (approx. NT 918 209). There is the possibility that this quartz veining is the result of hydrothermal penetration on lines of weakness between different types of plutonic rock.
We need to pay another visit to investigate this.