These are the eroded cores of central volcanoes of Cenozoic age, formed 25 to 27 million years ago, which rise abruptly from the coastal region about 60 kms north of Brisbane, Queensland. They were sighted and named by Captain James Cook in 1770 from the way sunlight was reflected from the very fine-grained to glassy volcanic rocks - mainly alkali rhyolite (comendite), and trachyte. Many of the dome-shaped hills and conical peaks have Aboriginal names. Mt. Beerwah, made of trachyte, is the highest peak at 556 m. Mt. Beerburrum (276 m) and Mt. Miketeebumulgrai form dome-like intrusions of trachyte, and Mt. Coonowrin is a steep spire of rhyolite. Mt. Ngun Ngun and Mt. Tibrogargan are made of coarser rhyolite. Mt. Tunbubudla, Mt. Tibberoowuccum, Mt. Coochin and Wildhorse Mountain are rhyolite intrusions. There were at least ten centres of eruption, each represented by a separate hill. They have intruded through almost horizontally-bedded Jurassic sandstones and shales of the Landsborough Formation. Many of the volcanics have well-developed columnar jointing. Originally, each of the eruptive centres would have had volcanic cones of softer pyroclastic material, now eroded away.
Copyright © Australian Museum, 2004