The geology of the area is mainly Pilliga sandstone. This is a coarse red to yellow Jurassic sandstone containing about 75% quartz, 15% plagioclase and 10% iron oxide.In the south are sandstone outcrops with basalt-capped ridges. In the north there are plains of alluvium from flooding creeks. There are small, shallow, usually dry lakes, called gilgais, in some areas. In the west there are dry sandy creeks, called "sand monkeys". In the east is a heavily eroded sandstone mountain range, which can be seen as sandstone outcrops such as those around Gin's Leap between Baan Baa and Boggabri.