Some of the most spectacular mountains occur in the northwest highlands, especially on Skye, the largest island off the coast. On the mainland nearby lie some great ranges based on the Torridonian sandstone, a Precambrian rock which overlies yet older rocks such as the Lewisian gneiss. Some of the highest peaks, such as Beinn Eighe, are crowned by white quartzite, which gives those peaks a distinctive appearance. The trend continues to the north, with larger caps of the white rock at Foinaven and Arkle. Some of the quartzite contains fossilised worm burrows. It is known as pipe rock and is around 500 million years old.