Brandon is composed of sandstone particles of various sizes collectively known as Old Red Sandstone. Old Red Sandstone has a purple-reddish colour, and has virtually no fossils. The colour gave its name to nearby Purple Mountain. The composition of Old Red Sandstone is variable and contains quartz stones, mudstones, siltstones, and sandstone particles (conglomerate rock boulders with quartz pebbles are visible). They are described by the Geological Survey of Ireland as the oldest Devonian rocks in Ireland.