Sydney’s sandstone and shale landscape is criss-crossed by numerous igneous intrusions. These include narrow, near-vertical sheets of basalt and dolerite (known as dykes) as well as ancient gas-driven, sub-circular vents called diatremes that blasted their way up through the strata during the Jurassic period.

Igneous dykes in the Sydney Basin range in age from about 51 to 225 million years old, spanning the Late Triassic to the Jurassic.

In geological terms, a dyke is a vertical or near-vertical sheet-like body of igneous rock that has intruded into fissures, fractures, or faults, connecting up to deeper magma sources.

The prominent cliff face at Bicentennial Park, along the eastern edge of Lofberg Oval, is an old quarry wall. It once supplied a very hard, recrystallised form of Hawkesbury Sandstone—essentially quartzite. This sandstone was baked and hardened by the heat and pressure from a north/north-east trending basalt dyke. The dyke itself is now heavily weathered but still clearly visible, nestled in a notch within the quarry face.

About 1.3 km south, at Grants Castle, the same type of molten magma intrusion into the sandstone has created beautiful columnar cooling joints, breaking the rock into striking blocky columns.

It’s a fascinating glimpse into how our local landscape was shaped over millions of years. Well worth a visit!

Click here to view location on Google maps.

Photo with Dr John Martyn, member of the GeoRegion Steering Committee