Numerous Jurassic volcanic diatremes (or 'necks') and mountain-tops of basalt increase the complexity of the sandstone country, especially in the east and north.
Diatremes are often made up of breccia, a jumbled mess of basalt and sandstone resulting from sudden explosive force, and tend to weather into roughly circular basins surrounded by sandstone bluffs.
Most of the basalt lava (much younger than the diatremes) has been eroded away, but remnants survive on some of the higher tops such as:
◊ Mount Hay
◊ Mount Banks
◊ Mount Wilson
◊ Mount Coricudgy and
◊ Nullo Mountain.
These were probably low points when the lava erupted and naturally flowed along the valleys.
Remains of river gravels can be found under some of these basalt 'caps'. The basalts cooled into semi-hexagonal columns, which can be seen in outcrops such as a cutting near Mount Bell on Bells Line of Road.
A few basalt exposures occur in some deep valleys (e.g. Capertee and Wolgan gorges), and these are probably eruptive lava vents rather than explosive diatremes.