These are mainly made of a type of sandstone, known as Torridonian sandstone (see Geology of Great Britain), which over time has become eroded to produce the unique characteristics of the Torridon Hills. In geology, Torridonian describes a series of proterozoic arenaceous sedimentary rocks of Precambrian age. They are amongst the oldest rocks in Britain, and sit on yet older rocks, Lewisian gneiss. Some of the highest peaks, such as Beinn Eighe are crowned by white Cambrian quartzite, which gives those peaks a distinctive appearance. Some of the quartzite contains fossilized worm burrows and known as pipe rock. It is circa 500 million years old. The strata are largely horizontal, and have weathered into terraces on the mountains.