Tag Archives: Harthope valley

Uncategorised

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

Uncategorised

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.

Uncategorised

Return visit to Harthope Linn

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.

Uncategorised

Hedgehope Hill

A visit to Hedgehope Hill confirms that the upper Dunmoor Burn contains a very fine-grained granophyric rock. We have provisionally classified this as ‘Evolved’ granite but have since become increasingly doubtful whether it is really part of the same intrusion as the evolved granophyre on the upper slopes of the Cheviot, and whether it really does form part of a ring dyke as Al-Hafdh suggested. Exposure is too limited in the upper Dunmoor Burn to draw definite conclusions.
About 100m below the summit of Hedgehope Hill, the track crosses a large boulder field which is probably the result of periglacial activity. The great majority of these boulders belong to the coarse porphyritic type which Al-Hafdh named ‘Standrop granodiorite’. The summit of Hedgehope has a medium to fine-grained pink rock which is similar to that of Dunmoor Hill. Many samples from the summit show evidence of significant hydrothermal alteration.
Returning via the north side of Hedgehope Hill towards the Harthope valley, we find more of the coarse-grained ‘Standrop’ rock but, after much searching, fail to find the chilled margin between the finer and coarser varieties that Al-Hafdh says is visible there. The one example of really fine-grained rock chilled against the courser rock turns out to be another felsite or aplite dyke.

Uncategorised

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.