It protects the Porongurup Range, an extremely ancient and largely levelled mountain range formed in the Precambrian over 1200 million years ago. The present range is no more than fifteen km (10 miles) from east to west and consists of granite peaks levelled into domes. The highest point in the Porongurup Range is Devils Slide at 670 metres (2,200 ft) whilst there are several other peaks above 600 metres (1,970 ft), which is about 400 metres above the surrounding plain. Indeed, for much of the Cretaceous and Paleogene the Porongurup Range was an island surrounded by the sea, with the Stirling Range forming the southern coastline.