Idiosoma nigrum, also called Black rugose trapdoor spider, occurs only in south-western Australia, in dry woodlands east of the Darling Scarp and north to Moore River.
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Greenmount Hill, either by Old York Road, or Great Eastern Highway is the main ascent to, or descent from the Darling Scarp, and has been a regular site of vehicle breakdowns or accidents due to the slope.
It is found along the Darling Scarp, Northam, and is well distributed in the woodlands and heath surrounding Perth.
The class were used extensively on WAGR lines with light rails and sharp curves, as a consequence many of the smaller older branch lines on the Darling Scarp; as well as those with steep inclines such as those on the Mundaring Weir Branch Railway, the Nannup Branch, and the Flinders Bay Branch Railway railways.
The river valley is the centre of the catchment of the Helena River and extends from the edge of the Darling Scarp where Boya, Gooseberry Hill and Helena Valley define the 'mouth' of the valley that opens to the Swan Coastal Plain, east and south east past Mundaring Weir and Lake C.Y. O'Connor to its origins in the region of Mount Dale.