How Rivers Became Mountains: The Grampians

#grampians #victoria #australia
Discover the astonishing geological story behind the Grampians — one of Victoria’s most iconic and breathtaking mountain ranges. In this in-depth video, we uncover how a vast network of ancient rivers, flowing over 400 million years ago during the Silurian and Early Devonian periods, laid down immense layers of sand and mud across what was then a low-lying inland basin on the edge of the Gondwanan supercontinent. These layers, now known as the Grampians Group, were originally flat and horizontal, deposited in environments ranging from braided rivers and floodplains to wind-blown deserts. Over time, these sediments hardened into sandstone, siltstone, and shale, preserving ripple marks, mudcracks, and even fossil burrows from an ancient world long lost to time.

But the story doesn’t end with quiet sedimentation. The real transformation began when powerful tectonic forces took hold. As an oceanic plate subducted beneath the eastern edge of Gondwana — somewhere far east of present-day Woods Point — immense compressional forces pushed inland. These forces, driven by subduction and crustal shortening during the Lachlan Orogeny, crumpled the rigid crust like a slowly closing vice. The Grampians region, situated hundreds of kilometers inland, was not immune to these effects. As stress travelled through the lithosphere, the once-horizontal riverbeds were compressed, tilted, folded, and thrust upward, forming a rugged series of ridges and escarpments. It was here that rivers quite literally became mountains — not through volcanic upheaval, but through the patient, unrelenting pressure of Earth’s tectonic plates.

At the heart of this mountain-building episode lies the Moora Moora Fault, a major thrust fault running beneath Halls Gap. It marks a dramatic geological boundary, where entire slabs of rock have been pushed up and over one another. This fault helped carve out the iconic topography of the region, where vertical sandstone beds tower over valleys like ancient stone books standing on their spines. The Grampians became a thrust-and-fold belt — a magnificent, exposed example of how deeply buried sedimentary layers can be folded into anticlines, synclines, and near-vertical strata by the slow yet unstoppable forces of tectonic compression.

Following the main phase of deformation, pulses of magma rose from deep within the Earth, a legacy of the subduction process. These intruded into the crust beneath the Grampians, forming granitic bodies such as the Mafeking Granodiorite and the Victoria Valley Granite. Though much of this granite remains hidden, its presence influences the surrounding landforms, adding another layer to the region’s complex geological evolution. These granitic intrusions are part of a wider Devonian magmatic arc stretching across eastern Australia, further cementing the connection between the Grampians and the ancient tectonic activity along Gondwana’s eastern margin.

Today, the Grampians stand as a striking monument to deep time. Their bold escarpments, serrated ridgelines, and tilted sandstone beds are not just visually stunning — they are physical records of a story that spans hundreds of millions of years. This video brings that story to life through cinematic visuals, clear scientific explanation, and a passion for revealing the hidden forces that shape our world. We’ll take you across the ranges, from Mount William — the highest peak at 1,167 metres — to Halls Gap, exploring how faulting, folding, and erosion continue to shape the landscape today.

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