We park the car with others in the space by the road at NT 97674 16229 just before Hartside Farm. Looking north-east from here, there is a good view towards Dunmoor Hill with Cunyon Crags standing out at the east end and Long Crag towards its west end on the skyline. Cat Crag is visible below Long Crag. Looking south-west, there is a view of the deep cleft of the Breamish valley as it follows the line of the main fault at the southern edge of the Cheviot pluton. We walk the road from Hartside Farm to Linhope and admire the bridge over the burn just before the settlement at NT 96474 16249. There is an outcrop of altered (hornfelsed) andesite here, above the east side of the Linhope Burn. The Burn follows the junction of the lavas and the pluton.
Boulder below Ritto Hill
With the steep slopes of Ritto Hill on the left and a plantation of conifers on the right, there are frequent boulders on the left-hand side of the path, around NT960163, which show a mixture of pink intrusive rock and dark dioritic rock. These boulders may not allow an accurate determination of the bedrock but they do imply the proximity of the junction between the quartz-monzodiorite or ‘Marginal’ variety of granite) that forms the perimeter of the Cheviot pluton and the ‘Central Belt’ granitic rock that lies within it. The story goes that the dark ‘Marginal’ granite was the first to crystallise when the pluton was formed, but in thin section we can see that, in this case at least, the very fine-grained, iron-titanium ore bespeckled mafic material that we might take to be ‘Marginal’ contains clots of the ‘Central Belt’ rock suggesting that it was injected into the already consolidated ‘Central Belt’ rock. Of course, this may have nothing to do with the ‘Marginal’ granite and may be mafic dyke material.
‘Central Belt’ rock in contact with fine-grained mafic material
Prepared hand specimen in ordinary reflected light.
A thin section from the same sample viewed in plane polarised light
The same thin section viewed with crossed polarising filters
Felsite dyke at Linhope Spout
This view, at close range from above and looking directly down, shows the tension jointing that is typical for these dykes.
Linhope Spout at NT 95834 17099 is a small (approx. 10m high) but impressive waterfall.
At its very top, there is a distinctive intrusion of a very fine-grained felsic rock. This is one of the frequent fine-grained granitic dykes classified as aplite or, more commonly, felsite dykes. This one may have hardened the rock locally, increasing its resistance to destructive forces and thereby contributing to the formation of this beautiful waterfall.
Hand specimen from felsite dyke, Linhope Spout.
Prepared hand specimen from felsite dyke, Linhope Spout.
Thin section of felsite dyke, Linhope Spout viewed in plane polarised light
The green area is chlorite.
Thin section of felsite dyke, Linhope Spout viewed with crossed polarising filters
Thin section of felsite dyke with perthitic and micrographic textures, Linhope Spout viewed in plane polarised light
The same section viewed with crossed polarising filters
Junction of ‘Central Belt’ and ‘Marginal’ granitic rocks at Linhope Spout
A little bit further back from the top of the Spout some boulders reveal the junction between the ‘Marginal’ (dioritic) and pinker, less mafic ‘Central Belt’ granite.
Location 5. Quartz & haematite veining
Location 6. Mica-porphyry dyke
Below the Spout along the path beside the burn in an easterly direction, there is one of the many mica-porphyry dykes in the Cheviots visible if the bracken isn’t tall and thick. The mica-porphyry forms a substantial laccolith near Biddlestone where it is extensively quarried to provide decorative stone and road material. It occurs as dykes frequently in the Cheviot pluton, and forms a more extensive clump on Cold Law.
References
Al-Hafdh N.M. 1985. The Alteration Petrology of the Cheviot Granite. Thesis submitted for PhD. at Newcastle University.
British Geological Survey Online geology map http://mapapps.bgs.ac.uk/geologyofbritain/home.html