Geology along the Munda Biddi

Perth is pretty flat. The only hills we have on the Swan Coastal plain are the undulating hills that parallel the coastline which are actually million year old sand dunes. Many of Perth's swamps and lakes have formed within the dune system.

The Darling Scarp, also referred to as the Darling Range, is a low escarpment running north-south to the east of the Swan Coastal Plain and Perth, Western Australia. It extends generally north of Bindoon, to the south of Pemberton, and easterly to include Mount Bakewell near York and Mount Saddleback near Boddington.  It is the visible signs of the Darling Fault, a 1000km long geological structure visible from space. The fault, which was formed early in the Earth's history, has been moving regularly, although the last major activity was 135 million years ago when Australia broke away from Gondwana, the super continent. The sandy Swan Coastal Plain is separated from the Yilgarn Craton (a part of the Earth's surface) by the Darling Fault.

The Darling Scarp started to form about 570 million years ago and is made of rock that is up to 3700 million years old, which is geologically very old (The oldest part of Australia is the south west of Western Australia). The western edge is called the Darling Scarp, and it rises steeply 200m+ from the Swan Coastal plain (It was a challenge for the early settlers to find a suitable route for getting trains up and down, and it took them 3 attempts to find the right route!). The top is called the Darling Plateau and it features vast, gently undulating jarrah covered hills and valleys that may rise to nearly 600m. The highest peaks are Mount Cooke (582m), Solus (574m) and Dale (546m). In the past 100 years, the Darling Scarp has been exploited for stone quarries, forestry (jarrah trees) and bauxite (rock containing metal aluminium) mining.

Granite makes up most of the rocks of the Darling Range, which was once molten rock below the Earth's surface.

Laterite ( a red clay material that is hard when dry) covers much off the Darling Plateau. It consists of mottled soils overlain by duricrust, which is a hard red/brown cap that looks like cemented gravel. When it weathers it forms "pea gravel" , small ball like rocks that are very hard to ride through. Where laterite is aluminium rich it is called bauxite, and often mined.

The annual rainfall of the scarp varies from north to south from about 750 mm to 1400 mm. Most of it falls in the winter and soaks through the gravelly ground surface to the clays below. The Darling Scarp has been weathered by rain for many millions of years, so essential nutrients that are most likely to promote plant growth, have been leached out and are in short supply. Jarrah seedlings frequently germinate and grow after the occasional fire, on the ash-bed where the nitrates and phosphates are more readily available and competing understorey plants have been killed.

The soil in which karri (found south of Nannup) grow is often poor, and the tree tends to flower after fire to take advantage of the nutrients released by the combustion of forest litter. The soil is classified as Karri Loam. Though low in some minor nutrients it is admired for its depth and pasture-growing properties. The depth of the soil is several metres and thought to be created primarily from the bark shed by the tree, which collects at the trunk base to a depth upwards of six metres in mature trees. The karri supports an extensive ecosystem which is connected to the granite outcrops of the lower south-west and the many subsequent creeks and rivers created from runoff. Karri generally dominate in the deep valleys between granite outcrops surrounding the creeks and rivers.

Often the Bureau of Meteorology identifies different weather for "the hills" in comparison to that of the Swan Coastal Plain. Also, in traditionally hot summers, strong easterly winds travelling across the scarp have presented serious issues for planes using the Perth Airport because of the alignment of the runways. A documented accident in 1999 involving wind shear from the scarp is recorded. In addition, orographic uplift is produced when rain clouds move over the hills, giving higher rainfalls in settlements in the ranges compared with their coastal neighbours.

Monadnocks is a term used by geologists to describe large flat isolated hills that rise out of generally flat plains. So this name was perfect for a group of large "humpback", round, bald hills of granite behind Gleneagles. The Monadnocks Conservation Park contains one of the highest point in the Darling Ranges, Mount Cooke (582m), as well as several others such as Eagle Hill (on the Munda Biddi maps). These hills form the divide between the Serpentine and Canning drainage systems. They were formed 2,650 million years ago when magma from 14,000 metres below the earth's surface came to the surface and crystallised. Large slabs of granite are commonly scattered over the Darling Range and they often feature "onion peeling" caused by continual cooling & heating causing the rock's surface to split in layers. Mount Lindesay on Map 8 is also a monadnock.

William Bay (Map 8) lies within the geological Nornalup Complex, which is part of an eroded mountain chain. It features massive granite out crops and boulder strewn headlands amongst the steep sandy slopes and dunes.

This page is the property of Follow My Ride, a website detailing off road cycle tracks near Perth and in Western Australia.