The 'deep south' of the Blue Mountains is a different and more complicated landscape.

Here, the Sydney Basin sedimentary rocks are represented only by a few thin remnants of Permian strata on plateau tops, as in the Wild Dog Mountains, at Kanangra Walls and on the Bindook Highlands.

These caps are made of sandstone and conglomerate of the Snapper Point Formation (formerly called Megalong Conglomerate).

The rest of the south-west is dominated by Lachlan Fold Belt rocks, which are mostly meta-sedimentary rocks (sedimentary rocks altered by heat and pressure), but with associated granite intrusions and volcanics.

Quartzite (metamorphosed sandstone) is prominent in the complex ridge-and-gorge terrain because of its resistance to erosion.

The Lachlan Fold Belt includes some areas of limestone, most famously at Jenolan Caves, but also at Colong, Tuglow and Wombeyan caves.

A large granite batholith sprawls across part of the Boyd Plateau and upper gorge of the Kowmung River, creating a distinctive landform with tors on the plateau and slabs in the gorge.

Another special geological feature of the southern Blue Mountains resulted from extended volcanic activity in the Devonian.

The Bindook Volcanic Complex is a huge sequence of volcanic sedimentary rocks that makes up much of the Bindook Highlands and extends well to the south.

Multiple pyroclastic ash flows piled up to a depth of 500 metres, with a total volume of at least 1000 cubic kilometres!

This Barrallier Ignimbrite features columnar outcrops, as in Bindook Gorge.

The Yerranderie volcano, with its neck below Yerranderie Peak, was part of the last phase of Devonian vulcanism.