The gorge is the river's masterpiece, and the medium is Tumblagooda Sandstone - layers of fluvial and coastal sediment laid down hundreds of millions of years ago and later lifted into a high, dry plateau. The Murchison did the rest, working downward grain by grain across an enormous span of time. The result is a sinuous trench banded in red and cream, deepening and narrowing as it nears the coast. Almost the whole of it lies within Kalbarri National Park, and it is considered to be in excellent condition - a rare thing for a landscape so accessible, and reason enough that it was nominated in 1991 as a geological monument for the Register of the National Estate.

Geologists travel here for what the gorge has laid bare. The exposed Tumblagooda Sandstone is famous for its trace fossils - not bones, but the marks living things left as they moved. Trackways crisscross the rock, the most striking being long paired rows of dents recording arthropods walking across what was once damp ground, among the earliest evidence anywhere of animals venturing onto land. The likely trackmakers were creatures like eurypterids - the so-called sea scorpions - and other early arthropods. Fossilised tracks of this kind turn up commonly in the vicinity of the gorge, so that the walls hold not just the history of the rock but the faint diary of its first inhabitants.

The Murchison is a creature of extremes, and the gorge records both of its faces. For much of the year the river is reduced to a chain of still pools strung along the canyon floor, fringed by river red gums and so quiet you can hear insects over the water. Then the inland rains come, and the same channel can run brown and furious, the flood that does the gorge's slow work of cutting and carrying. The river drains an enormous, sparsely peopled catchment before it reaches here, gathering the runoff of the dry interior and funnelling it through this single narrowing slot on its way to the Indian Ocean. The gorge is, in effect, the bottleneck of a whole region's water.

For all its scientific weight, the gorge is first of all a place of plain astonishment. Lookouts perch along its edge, the best known at the Z Bend, where the river kinks back on itself far below. The Loop walking trail follows a great horseshoe of the canyon and passes Nature's Window, the weathered stone arch that frames the river beyond. Travellers come for the silence and the scale - the way the heat presses down, the way an eagle riding the updraft looks small against the far wall, the way a river you could almost wade across has hollowed out something this immense.