The following information was kindly supplied by local expert, Warwick Wilmott.
The southern side of Brisbane has seen dramatic changes over geological time. Originally it wasn’t even land, but a deep oceanic trench off the eastern edge of the old continent. This was back in the Devonian and Carboniferous periods, about 370 to 310 million years ago. Sediments were eroded off the continent, to slump down into the trench as overlapping muds and dirty sands. Later these were compressed by movements of crustal plates, hardened, deformed and uplifted into mountain terrain. These old ‘meta-sediments’ were resistant to erosion and now form the high hills of Mount Gravatt and Mount Cotton.
Somewhat later, in the late Triassic, about 220 million years ago, small depressions formed in the mountains and sediments were deposited; initially gravels and sands, but later silt, mud and coal in coal swamps. Fossil plant leaves give an idea of the vegetation at the time. Such sediments give the subdued terrain around Ipswich and in patches further east such as around Tingalpa. Soon afterwards (210 to 190 million years ago, Triassic – Jurassic), these depressions were covered by great sheets of sand, eroded off mountains to the west, and deposited on broad river plains. These consolidated to thick beds of sandstone, which now form the hills of the Springfield – White Rock area and Toohey Forest Park. This was the time of the dinosaurs, but only a few footprints are left as evidence.
A long period of stability and erosion followed, but about 65 million years ago in the Tertiary period, several small depressions formed in the landscape, and were filled with silts and muds. Where these remain, they form flat undulating country, such as around Redbank Plains and Oxley. In places small volcanoes erupted basalt lavas, which have weathered to fertile red soils, such as around Sunnybank and Redland Bay.
Larger volcanoes erupted in a semi-circle to the west and south of Brisbane about 25 to 23 million years ago, but the only evidence near us are the plugs of lava pushed into subsidiary vents now giving the resistant peaks of the Mount Flinders group.
Since those days, modern streams such as Oxley and Bulimba Creeks have carved valleys in the old landscape, and alluvial sediments have accumulated on their flats in major floods. Oxley Creek in particular has very sandy alluvium as its headwaters have eroded the old sandstone terrain.