Biocrusts: Flinders Ranges and GrampiansSouth Australia's Flinders Ranges are renown for their spectacular red mountainsides, while in the Grampians (Victoria, Australia), a range composed of similar sandstones, the rock-faces are a more mundane grey. This seems to be because of the common presence of biocrusts on rocks in the Grampians and their less common presence in the Flinders Ranges.
It seems that the higher temperatures reached by exposed rocks in the Flinders Ranges, especially those facing the north or north-west, does not permit the establishment of a biocrust. In both places the rocks, if quite bare, are red or will develop a red surface coating of iron oxide, but when conditions are more amenable to life, a greyish biocrust will establish itself on rock surfaces.
The Grampians are around 37°S, the Flinders Ranges is a little closer to the equator at around 32°S. The climate of the Grampians is cooler and probably damper than that of the Flinders Ranges.
The Flinders Ranges stretch for over 430 km, this page deals mainly with the most popular section for tourism, around Wilmington, Quorn, Hawker, Wilpena and Blinman. Reliable and meaningful figures on the climate in the Flinders Ranges and Grampians are difficult to obtain.
Contact: David K. Clarke – ©
Boroka Lookout, 37°07'S. Note that the near and distant rocks all appear to be predominently grey.
As shown in another photo from the Grampians, below, when freshly broken at least some of the Grampians sandstone is red.
Generally red colour in rocks indicates the presence of the iron oxide haematite. The haematite can be in the rock, or it can be a thin stain that has washed over the surface.
When the surface of the rock is covered with biocrust such as lichen it is the colour of the coating biocrust that is seen, rather than the rock itself or any mineral staining on the rock.
Alligator Gorge, 32°45'. These rocks, exposed fully to the Sun, are quite red.
A piece of Grampians sandstone. The section on the left has been freshly broken, the part on the lower right has been exposed to the weather for a lengthy period and is covered with biocrust.
A view from The Balconies, Ried's Lookout area of the Grampians, showing the typical grey colour of the rock surface.
This view of Alligator Gorge in the Flinders Ranges shows both red bare rock and grey, biocrust covered, rock. Being deep in the gorge, partly shaded by trees, and facing east, these rocks do not get so hot as those that are more exposed to sunlight.
Rainbow Waterfall near Rawnsley Park
The colour of the rock itself and the presence or absence of a biocrust are
not the only variables in determining the colour of rock surfaces.
Mineral solutions can stain a rock surface: iron oxides commonly cause red
staining while manganese oxides (or possibly stains of vegetable origin), as
here, cause black staining.
ReferenceBiological soil crust, Wikipedia; does not deal specifically with biocrusts on bare rock surfaces