A 400-million-year-old riverbed
The dramatic red and white banded cliffs of Kalbarri National Park are layers of an ancient geological formation called the Tumblagooda Sandstone. This thick sequence of sedimentary rock, measuring over 1,200 meters at its outcrop, was deposited between 400 and 500 million years ago, during the Ordovician and Silurian periods. At that time, this part of Australia was a wide coastal plain near the equator, characterized by a network of constantly shifting, sand-choked rivers.
The environment was not a placid sea; it was a dynamic environment of braided river channels and tidal sand flats. There was very little vegetation on land to hold the banks, so large volumes of sand were constantly eroded and redeposited. The characteristic red and white banding seen today comes from iron oxides staining the rock, a process that likely occurred soon after deposition in an oxygen-rich environment. The sandstone layers are almost entirely composed of medium-to-coarse grains, with less than 1% being mud or conglomerate, attesting to the high-energy river system that laid them down. These layers, now lithified into hard rock, preserve a detailed record of a world before complex life had fully conquered the continents.
The first steps
The Tumblagooda Sandstone contains virtually no body fossils, but it is full of trace fossils—the preserved tracks, trails, and burrows of animals. These ichnofossils provide the record of one of the most significant events in the history of life: the moment animals first moved from water to land. The most famous traces are large trackways identified as Diplichnites, believed to have been made by eurypterids. These large arthropods, sometimes called sea scorpions, were some of the first animals to venture out of the water.
The tracks in Kalbarri are considered some of the earliest evidence of animals walking on land anywhere on Earth. The creatures that made them were substantial; trackway dimensions suggest arthropods at least a meter in length. The sandstone preserves walking trails and a variety of behaviors. Vertical burrows known as Skolithos, some a meter long, indicate animals digging into the sand for protection or food in a marine setting. Other traces show complex feeding and crawling patterns. The variety of trackways indicates a diverse fauna of arthropods, including eurypterids but possibly early scorpionids and myriapod-like animals, all navigating the interface between a large river system and a tidal sea.