Ancient story in the rocks
Once upon an ancient time, water gently lapped: Tidbinbilla was beach side; Canberra was submerged, covered by a vast ocean. There was no need to drive three hours to the coast.
It was, however, the Ordovician period. 480 million years ago.
Beyond Tidbinbilla, volcanoes erupted and lava flowed. Volcanic ash settled across a landscape indiscernible through the prism of present day. Connected with Antarctica, Australia was somewhat tropical. It was humid. There were vast reefs, shallow seas. Animal life was marine based.
Over millenniums, sediment was deposited. This shale became the predominant rock type of today.
Over time this primordial landscape was crafted by incredible, powerful geological forces. Tectonic plate movements folded the rock, creating fault lines, passages where meandering rivers now run.
To gain a glimpse into this ancient landscape, take a trip around State Circle. Listed on the Commonwealth Heritage Register, this remarkable geological site represents an amazing time capsule to our distant past. Beautifully layered, rich in texture, deposited over millennia, sandstone from shallow seas sits above deep ocean shale.
Dynamic in its geomorphic nature, the sea retreated and the region became dry. The potent forces of nature continued to erode and mould the landscape. Some 420 million years ago the sequence of forces pushed granite incursions upwards to form a dry, barren landscape reminiscent of what we witness today.
With elements of sandstone, Black Mountain was once a beach. Volcanic rock from Mt Ainslie formed the cornerstone of St Johns Church. The red in Red Hill originated from rich iron deposits. Formed in shallow seas, limestone became a sought after raw commodity.
This key cement ingredient laid the foundations for a future city on what is colloquially dubbed the ‘limestone plains’.
The region’s stunning array of limestone infused fossils are extraordinary. From Burrinjuck coral shells and armored plated fish, to Woolshed Creek seashells discovered in the 1840s. At the time, these ancient fossils rewrote geological text books as the oldest known fossils in Australia.
Local geologist Doug Finlayson will explore this remarkable story as part of the Friends of Black Mountain mid-winter talks, on Tuesday 18 June. Book via their website page.
Brett McNamara is with ACT Parks & Conservation Service
Article also appeared in The Chronicle