The Boundary Zone between the Dalradian and Moine

Significance: The ‘Boundary Zone’ between the Moine and Dalradian rocks is a series of near vertical tectonic slices of kyanite grade metasediments and high grade gneisses. Narrow bands of distinctive microcline-augen-gneiss mark the upper and lower edges of the Boundary Zone. The most spectacular of these is the Valayre gneiss that delineates the base of the boundary zone.  This is a microcline-megacryst-porphyroblastic gneiss in which the megacrysts have formed in a matrix of similar composition to that of the adjacent rocks even although the composition of the adjacent rocks varies with distance. The Valayre gneiss outcrops intermittently and can be traced from near Aith, through its offset by the Nesting Fault, to Cullivoe in North Yell, a distance of 80km. This spectacular gneiss may represent a zone of material crushed by an early fault that cut different rock types and subsequently underwent burial and metamorphism. The top of the Boundary zone and the base of the Dalradian is marked by the Skelladale gneiss which is a highly tectonised granitic orthogneiss.

Locality 1. Grut Wick Quarry. HU 507 706. Valayre Gneiss. Disused Quarry.

The quarry has been cut into a wide outcrop of Valayre gneiss that can be seen to great effect in the quarry walls and on loose blocks.  Here megacrysts of microcline feldspar have grown in a matrix similar to the adjacent gneisses. A dyke of lamprophyre or spessartite, the heat from which has destroyed or melted the crystals in the country rock, cuts the quarry.