Purple Mountain is composed of sandstone particles of various sizes which are 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 the mountain group. The composition of Old Red Sandstone is variable and contains quartz stones, mudstones, siltstones, and sandstone particles (boulders of conglomerate rock containing quartz pebbles are visible).