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The Cheviot Hills  straddle the border between England and Scotland offering mile upon mile of open moorland together with the general right to roam under the the English ‘Countryside and Rights of Way Act 2000’ and the Scottish ‘Land Reform (Scotland) Act 2004’.
Things were not always so permissive. At times in the Middle Ages, it was an area of open conflict: the front-line in the battle of the Wars of Scottish Independence. At others, it was a land of simmering unrest in which Border Reivers involved themselves in armed raids designed to to intimidate, possess, and settle scores.
The Cheviot culture has been shaped in part by these north-south clashes, just as the Cheviot Hills themselves were shaped, in part, by a north-south clash of  a geological kind.

Closure and collision

Roughly 500 million years ago, an ancient ocean, the Iapetus Ocean, began to close. At its southern limit, a small continent named Avalonia, bearing what is now England, detached itself from the super-continent Gondwana and began to move northwards as the oceanic plate was subducted beneath it. One thousand kilometres to the north, subduction was also underway at the plate’s northern margin under the ancient continent of Laurentia that was bearing what is now Scotland. Thus Avalonia and Laurentia were each brought onto a slow but inevitable course that would lead to their ultimate collision.

The closure of the Iapetus Ocean and the collision of the two continents occurred roughly 100 million years later. It wasn’t a spectacular event in itself, but the tectonic processes that had brought it about had given rise to volcanic activity and mountain building in Wales, the Lake District and  the Grampian regions.  What is more, the processes that followed on from it further shaped the region, creating major faults, basins, volcanoes and plutonic activity that emplaced massive granitic  plutons on both sides of the line of contact. This line, named the Iapetus Suture, has no visible features at the surface today but geophysical studies have detected at depth Laurentia’s hanging wall over the subducted Avalonian plate along the Northumberland basin that runs from the Solway Firth in the west to the Northumberland coast between Holy Island and Tynemouth in the east.

The Cheviot volcano

The effects  of the closure of the Iapetus Ocean in the region were complicated by the effects of similar events to the east that were bringing about the the closure of the Tornquist Sea and the consequent collision of the Laurentian and Baltic plates and also the closure of the Rheic Ocean and the resulting collision of the Amorian and Avalonian plates.
In the Cheviot area, complex compressive and tensional forces promoted   the slow incremental  accumulation of magma, perhaps over a period exceeding 20 million years. This magma, a singular mix of magmas that could have been derived from the mantle and from material from the  Laurentian, Avalonian and Baltic plates, eventually exploded to the surface in violent volcanic activity that gave rise to the Cheviot Hills.

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Rock types and their locations

There are a number of rock types associated with the Cheviot Volcanic Complex. A location of each major type is shown in the map and described in the list below it.

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Ignimbrite and lava

Volcanic activity in the Cheviots began with a violently explosive phase. Beds of ash and ignimbrite exposed in the Ingram and Coquet valleys are testimony to the extent of ashfall and pyroclastic flows which were thrown out by the volcano. This was then followed by an outpouring of lava which covered almost 600  km²  to an estimated depth of 2000 m. This is particularly striking because most of the lava was andesite, trachyte or in a few areas rhyolite, all of which are fairly viscous and therefore normally do not flow very far. This was a truly devastating volcano. The site of the vent(s) cannot be placed with confidence because much of the evidence has either been eroded away or covered in drift and peat, but Robson (1976) suggests that lines of vents may have developed along the linear fractures in the Silurian basement rocks that are now represented by the Gyle-Harthope and Thirl Moor faults.

The Cheviot pluton and dykes

The volcanic sequence was concluded by the intrusion into the lava of a 50  km²  pluton of granitic magma that geophysical surveys suggest continues down to a depth of about 9 km. This intrusion didn’t reach the surface and therefore crystallised more slowly than the lava to form the coarser crystalline rock that makes up the central part of the hills. This phase was accompanied and concluded by the injection of a variety of dykes and hydrothermal veins mainly into the lava but also into the granite.
Much of the lava is believed to have been eroded away during the subsequent Carboniferous era, thus exposing the pluton.
Severe earth deformations have caused a number of major faults in the Cheviot Hills. The South-West trending Harthope valley marks the site of one such faultline, as does the Breamish valley towards Bleakhope to the South. Minor faults also occurred. These lines of structural weakness today often mark the site of valleys and drainage channels. Similarly, the boundary between pluton and lava beds provides other lines of weakness through which rivers such as the Hawsen Burn or valleys such as Goldscleugh, can channel. Land either side of the major Harthope fault was subjected to considerable lateral movement as illustrated in the photo below. It is not certain how much vertical movement was involved.

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The present-day surface of the pluton

It has been generally accepted that the present surface of the granitic rocks was very close to the andesite lava ceiling over the original pluton. The rock type on the upper slopes of the Cheviot which is the highest point in the landscape, is a very fine-grained evolved granite suggesting that it originated late on in the intrusion process and cooled more quickly because it was closer to the surface. Blocks of andesite have been found close to the summits of Dunmoor Hill and the Cheviot itself. One of the largest andesite outliers on the pluton is at Housey Crags. The photo below shows how the strata of the outcrop follows the contour of the underlying pluton which suggests that its present position is in situ.

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A puzzle

How was the space created in the bedrock and the volcanic pile for the vast volume of rock that makes up the Cheviot pluton?

Plutons must either be intruded dynamically or else find space through the collapse of the magma chamber’s roof by stoping or cauldron subsidence. However, there is no evidence for either of these occurrences.
Although the surrounding andesite lavas have been heavily metamorphosed by heat and chemical action leading to the concentration of magnetite and the alteration of andesine feldspar to saussurite, there is no sign of the shearing and contortion which would be the result of pressurised intrusion. Nor is there any sign of roof collapse – surviving lava inliers appear to remain fairly close to the roof of the pluton.

The solution to the problem has to be set against the fact that erosion, drift and peat have obscured nearly all the potential evidence. The Cheviot volcano remains something of an enigma but that of course, makes the  exploration of its geology all the more interesting and entertaining.

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Click here for a  more detailed account of the origin and structures of the Cheviot hills.

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