The Archaean Yilgam Block is represented along the northern portion of the Fitzgerald River National Park. It is composed of gneisses and migmatites, with minor infolded belts of metasediments with different grades of regional metamorphism.

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Along the southern margin of the Yilgarn Block the landscape is dominated by peaks and ridges of quartzite and phyllitic schist, which rise from 300 to 475 m above sea level. They include West, Middle and East Mount Barren, Mount Bland, Woolbemup Hill and the Whoogarup and Eyre Ranges. The coastline backed by these ranges is steep and rugged, and there is an extensive wave-cut platform about 60 m above sea level. The peaks and ridges have slopes ranging from seven degrees to 30 degrees, with the steeper slopes becoming rockier (Newbey 1979).

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The metasedimentary rocks that form this landscape were deposited approximately 1.75 bya in an oceanic basin between the West Australian and Mawson (Antarctica) Cratons. The sequence largely comprises the Kundip Quartzite and Kybulup Schist. During the Albany-Fraser Orogeny 1.3 bya this sequence of rocks was cut by a number of thrust faults that transported the units for an unknown amount of distance to the northwest. Peak high-grade metamorphism resulted in kyanite and staurolite bearing schists forming. At its base is a coarse conglomerate containing pebbles of gneissic granite. This high-grade metamorphism occurred at 1.2 bya.

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The gneissic granites contain iron rich silicate minerals such as pyroxene which weather to produce limonite – during the weathering process the iron precipitates as fine grained, poorly crystallised limonite coatings and encrustations that colour soils and stain rocks shades of red, yellow and brown.

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Examples of this weathering is seen in the rock piles:

• Near the round window, is weathered iron precipitates;

• second pile south of the door is an iron rich pebble conglomerate; and

• third pile south of the door - is weathered iron precipitates.

 

Quartzite is a non-foliated metamorphic rock composed almost entirely of quartz. It forms when a quartz-rich sandstone is altered by the heat, pressure, and chemical activity of metamorphism. These conditions recrystallize the sand grains and the silica cement that binds them together. The result is a network of interlocking quartz grains of incredible strength.

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Examples of this metamorphism is seen:

• Immediately south of the door, and

• Forth pile south of the door is iron stained quartzite

 

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Geology References for the Fitzgerald River National Park include:

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Bradby, K. (1989), A Park in Perspective: A Report on the Past, Present and Future of Fitzgerald River National Park, The Fitzgerald River National Park Association.

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Fitzsimons, I.C.W and Buchan, C (2005) Geology of the Western Albany-Fraser Orogen, Western Australia – A Field Guide. Supercontinents and Earth Evolution Symposium. Geological Survey of Western Australia. Department of Industry and Resources Western Australia. Record 2005/11.

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Newbey (1979) The vegetation of central south coastal Western Australia

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Thom, R, Lipple, SL and Sanders, CC (1977), Ravensthorpe, W.A. Geological Survey of Western Australia,1:250 000 Geological Series Explanatory Notes, 40p.

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CV Spaggiari, S Bodorkos, M Barquero-Molina, IM Tyler, and MTD Wingate (2009) Interpreted-Bedrock Geology of the South Yilgarn and Central Albany- Fraser Orogen, Western Australia, Government of Western Australia Department of Mines and Petroleum

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