Cashelard, County Donegal

This rock is described as garnetiferous psammitic gneiss and is more than 900 million years old.  The dark layers in the rock are rich in red-brown garnet. The garnet layers clearly show the ductile (flow-like) folds that are typical of the rocks of the Slishwood Division, that outcrop in Counties Donegal, Fermanagh, Sligo and Leitrim.

The Slishwood Division represents a group of metamorphosed sedimentary and igneous rocks that have been transported, during a number of mountain building (orogenic) events, to the base of the continental crust and back to the earth’s surface.

Psammite is a term often used to describe metamorphosed sandstone containing mainly quartz, feldspar and mica. The term gneiss is used for highly metamorphosed rocks that have distinct compositional banding.

This specimen was extracted from the MacSand Quarry, near Cashelard, County Donegal where the rock is normally crushed to form aggregate for the Irish construction industry.