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 15 kilometres (9.3 mi) 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),:1 whilst there are several other peaks above 600 metres (2,000 ft), which is about 400 metres (1,300 ft) 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.
It is believed that the Porongurup Range is a remnant of the Precambrian collision that joined Australia and Antarctica until they separated in the Paleocene. There remain major granite intrusions in what is left of what must once have been a large mountain range.
Climate and vegetation
The plains surrounding the Porongurup Range have an annual rainfall of around 800 millimetres (31 in) to the south and around 600 millimetres (24 in) on the northern side. Most of this rain falls between April and October: although light showers are common in the summer months, the average rainfall between November and March totals only around 110 millimetres (4.3 in) in the southern plains and less than 75 millimetres (3.0 in) to the north. Summer temperatures on the plains average a very warm 26 °C (79 °F) in the daytime and decline to around 18 °C (64 °F) at night. In winter, although it can be rainy, temperatures average a very pleasant 16 °C (61 °F) during the day and a cool 8 °C (46 °F) in the morning.
Temperatures on the peaks are around 3 °C (37 °F) lower than on the plains, and snow has occasionally fallen on the ranges (heavily in October 1992 and June 1956).
This high rainfall explains the survival of karri forests quite a distance from their main stronghold between Manjimup and Walpole. The karri forests are one of the major attractions of the Porongurups and occur chiefly on the upper slopes of the range on deep red soils known as “karri loam”. On lateritic soils downslope, the predominant type of vegetation is a mixed forest of jarrah and marri, whilst on the highly exposed and frequently waterlogged summits, an open mossy herbland prevails.
Though not nearly as rich biologically as the more northerly Stirling Range, there exist ten endemic species of plant in the Porongurup Range, the best known being the mountain villarsia (Villarsia caltbifolia).