A Grand Geological Unroofing
The Oxley Wild Rivers National Park is one of Australia's largest and most dramatic gorge systems. Rivers like the Macleay and Apsley have sliced deep into the New England Tablelands, creating chasms that reach depths of up to 600 meters. This process measures the slow, relentless erosion of the Great Dividing Range, the continental spine of Eastern Australia. The range itself was formed over 300 million years ago during the Carboniferous period when the Australian continent collided with parts of what are now South America and New Zealand.
The gorges owe their impressive depth and steepness to the underlying geology. The rivers carve through Paleozoic-era meta-sedimentary rocks, primarily slate, which were deposited on an ancient seafloor around 400 million years ago. These rocks were later buckled, folded, and tilted to a near-vertical orientation during a major mountain-building event 250 million years ago. This vertical alignment of the rock layers makes them more susceptible to erosion by the rivers, allowing for rapid down-cutting and the formation of sheer gorge walls. The process is river incision, where the vertical erosion rate outpaces the weathering and widening of the valley sides.
A Landscape in Motion
The landscape here is not static; it is an active geological laboratory. The entire New England region has been subject to uplift since the Late Cretaceous period, a process that may have continued into the Quaternary. This uplift provides the potential energy for the rivers to carve so deeply. While precise, localized erosion rates for the gorges are difficult to pinpoint, studies of similar bedrock-dominated landscapes in Australia show rates that can be as low as 1.5 to 7 meters per million years. This slow dismantling of a mountain range occurs over immense timescales.
Volcanic activity has also shaped the region. Millions of years ago, basaltic lava flowed across the landscape, filling ancient river valleys. Over time, the softer, surrounding sedimentary rocks of the old hills eroded away more quickly than the hard basalt. This created a phenomenon known as "relief inversion," where the basalt that once filled the valleys now forms the tops of the hills. The rivers had to re-establish their courses, cutting down through the much older meta-sedimentary rocks below to create the gorges we see today. The entire system records the deep-time processes that shape continents.