Woolhope Crag

Woolhope Crag lies on the north east flank of the Cheviot just above Goldscleugh. It is one of the few convincing outcrops on the Cheviot itself. When we first visited it in 2013, we were looking for, and thought we had found, evidence that the outcrops occur because of the presence of harder, more mafic rock.
Subsequently, we read Al-Hafdh’s thesis which uses Woolhope Crag as the type location for his ‘Woolhope’ variety which is the least mafic, and therefore the most felsic and silica-rich, of his classifications.

Woolhope Crag from the East

Woolhope Crag from the East
The break on the right is where the felsic rock (left) becomes more mafic.

We returned here on19 August 2016 to investigate this apparent contradiction.
The answer is relatively simple. The southern end (higher end) of the crag is felsic; the northern (lower) end is more mafic in appearance. The join occurs at the broken mid-point of the crag. It is not an abrupt contact but a gradual change over about 20 cm and appears at first sight to be an example of fractionation – the first we have discovered in the Cheviot pluton.

Thin section of the upper (more felsic) rock at Woolhope Crag viewed with crossed polars at X40
There are some large phenocrysts of alkali feldspar showing Carlsbad twinning.

This theory breaks down when the two types are compared in thin section.
There seems to be very little difference in content between the two. Thin section analysis suggests that the difference between them is more apparent to the eye in hand specimen than actual. There is a slight increase in plagioclase with the more mafic rock but no increase in mafic minerals (both around 7%). The darker colour is probably due to the smaller grain size of the lower darker rock. Identifiable feldspars in both types show a ratio of about 1:3, plagioclase to alkali. Both types are rich enough in quartz (25-30%) and alkali feldspar to be classified as granite, albeit a relatively fine-grained variety.
An interesting feature of the upper more felsic rock is the presence of frequent chlorite.

Thin section of the lower (‘mafic’) rock at Woolhope Crag viewed with crossed polars at X40
There is no increase in mafic content compared with the more felsic rock. However, the grain size of the matrix is much smaller which must account for the darker appearance.