The Diamantina Crossing is a stone causeway across the bed of the Diamantina River, on the route of the old road south from Birdsville township. It was surfaced with concrete in the 1960s. The crossing is not trafficable anymore. You can see why in this image.
The significance and grazing potential of the Diamantina River were recognised by John McKinlay in 1861. When grazing settlement began in the Channel Country, the strategic significance of the crossing became apparent, as it was the only place in hundreds of kilometres where the river narrowed down to two channels and was relatively easy to cross in wet conditions. By 1879 there was a store near the crossing, which within a few years grew into the town of Birdsville. The crossing defined the route from the Queensland
Channel Country to the South Australian markets, which would have implications in shaping the Birdsville Track and Sidney Kidman’s landholdings for a century to come. The old crossing place remained in use until superseded by the bridge on the Birdsville Developmental Road in the 1970s.
You can learn more about the heritage of the Birdsville and Strezlecki Tracks here https://data.environment.sa.gov.au/Content/heritage-surveys/2-Birdsville-and-Strzelecki-Tracks-Heritage-Survey-2002.pdf