Geologists hold that the Adelaide Geosyncline of South Australia contains a Neoproterozoic (ca 750–630 Ma) sedimentary succession that provides an out-standing record of the two major glacial events and the intervening interglacial sequence of the Cryogenian–the name for this general period of glacial activity. The Neoproterozoic period includes the Sturtian (older) and Marinoan (younger) glacial events.

We walked past a folded Neoproterozoic barrier reef at Kingsmill Creek Gorge. This glacial history was particularly noticeable in Tillite Gorge. Massive deposits of ice-transported, boulders and mud debris are readily observed at Tillite Gorge (and Stubbs Waterhole) and record the melting of waning ice sheets that all but covered the globe during the Sturtian ice age, in the early Cryogenian Period, about 660-700 Ma.

Unfortunately, no one on the Kingsmill Creek walk had any geological knowledge of what we were walking through, or if they did, then they didn’t share the knowledge that were were walking through Arkaroola Reef in Kingsmill Creek. This reef, which existed for 5-10 million years during a period of tropical climate squeezed between two major ice age events (Sturtian and Marinoan) that constitute snowball earth, was constructed by microbial organisms around 650 million years ago. Both Arkaroola Reef and the nearby Oodnaminta Reef were some of the earliest barrier reefs on Earth and the small-scale eukaryotic fossils are among the earliest evidence for the emergence of animal life on Earth.

It is a particularly intriguing period because it also coincided with the sudden and widespread appearance of very early primitive lifeforms — ie., soft-bodied metazoan organisms during the Ediacaran (~600-544 million years ago). This makes the Neoproterozoic one of the most important and enigmatic episodes in Earth history with the Arkaroola and Oodnaminta Reefs reefs providing a rare geological window into the pre-Ediacaran world.