Metamorphic rocks

Metamorphic rocks have formed through the alteration of other rocks by heat or pressure, or both. The original constituents of the rock may have been recrystallised to produce an assemblage of new minerals, textures and grain size. Metamorphism is the term applied to these processes. During mountain building, rocks buried deep within the Earth’s crust may be affected by both intense heat and pressure, or by pressure alone, and hence undergo regional metamorphism.

By contrast, when igneous rocks are emplaced the adjacent rocks are heated by the intrusion and experience thermal, or contact metamorphism; the zone of affected rocks surrounding an intrusion is known as a metamorphic aureole.

Influence on the landscape

Several of the tors in the higher parts of the Cheviot massif are developed on contact metamorphosed volcanic rocks. In addition to those mentioned above, tors in hornfels are present to the west of the granite on The Schil [NT 870 223], West Hill [NT 894 213], Auchope Cairn [NT 892 198], and Hanging Stone [NT 892 190]; to the east are Middleton Crags [NT 977 215].

Because of their very limited extent, contact metamorphic rocks associated with the Whin Sills have a little or no impact upon landscape in the district.

Influence on biodiversity

Because of their very limited surface extent, contact metamorphic rocks within the district have very limited impact on biodiversity although some species may persist on ledges away from grazing animals. The metamorphic rocks in the Cheviots often have a rich lichen flora with good mosaics of crusts competing for space.

Economic use

None of the metamorphic rocks of the district have been specifically exploited. However, some of the harder calc-silicate-rich rocks, formed by thermal metamorphism of the Oxford Limestone within the contact zone of the Whin Sill at Barrasford Quarry, are included within certain crushed whinstone products supplied by this quarry. Apart from such use it is extremely unlikely that any of the district’s metamorphic rocks will ever attract commercial interest.

Conservation issues

Natural exposures of metamorphic rocks within the district are mainly closely associated with exposures of intrusive igneous rocks and, like them, may be assumed to be generally robust. Comments made on the conservation of intrusive igneous rocks in working quarries, applies equally to metamorphic rocks.

Wider significance

Despite the great importance of the Whin Sills in the development of understanding of such rocks, as evidenced by the voluminous technical literature on their form, composition, age and mode of origin, little attention has been directed towards the metamorphic effects associated with them. Thus, although of limited extent, the metamorphic rocks associated with the Whin Sills, in particular, offer considerable potential for future research on metamorphic processes in such geological environments.

Although the volume of metamorphic rocks within the district is small, they are extremely important in giving clues to the district’s geological evolution.