 The geology of the area is dominated by Pilliga sandstone, a coarse red to yellow Jurassic sandstone containing about 75% quartz, 15% plagioclase and 10% iron oxide, although local variations in soil type do occur.Sandstone outcrops with basalt-capped ridges are common in the south, while the Pilliga outwash areas in the north and west are dominated by alluvium from flooding creeks.Gilgais occur in some areas.