Tag Archives: Harthope Linn

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

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.

A hypothesis and an excursion to Harthope Linn

 Harthope Linn (NT 92734 20229)

We are pursuing the idea that the rocky features in the mostly rounded Cheviot Hills were caused by a more mafic content making the rocks harder. This would account for waterfalls such as Harthope Linn and Linhope Spout. Field work has shown that the andesite at the Carey Burn Linn and at Davidson’s Linn are of the dark harder variety.

There seems to be considerable variety in the rocks of the Cheviot pluton. The 1” geological survey shows the whole area as ‘granite’ but it is also apparent that the visible mineral content make this classification doubtful. This has led me to begin working with Ian to investigate and map the area in an effort to find out what is there and to better understand how it came to be so .

Today’s work at Harthope Linn did reveal rocks with a high colour index and mafic content but there were also other paler varieties and some showing substantial alteration.