Friends of the Box-Ironbark Forests

We don’t often see mesa-like hills in Victoria but the Guildford Plateau is a wonderful example. The story starts around 40 million years ago when the ancient Loddon River carved its way from the Glenlyon headwaters. This was a vigorous stream in a high rainfall period. The deep valley was full of rainforest species, ferns and maybe the odd freshwater crocodile. Over time the Loddon valley filled with clay, sand, gold and gravel forming a stream bed up to 50 m thick.

But in one catastrophic event, within the last 4.5 million years, the Glenlyon volcanoes sent a rush of lava northwards. Lava spread like honey seeking any valley it could find and instantly buried the ancient gravels and their contained gold.

Since then, erosion has lowered the entire surrounding landscape – but not the hard basalt. The basalt was carved away in some places but mostly it was left high and dry as a series of isolated mesas, like our beautiful Guildford Plateau.

Looking north from the old Guildford railway station. Hard basalt forms the top of the plateau. The basalt covered ancient gravels which are now visible in places along the lower plateau slopes, and at the old railway station.

This is the second post in our geology series written by Clive Willman. 

Ordinary looking rocks can tell some amazing stories: that’s the magic of geology. We’ve asked geologist Clive Willman to write an occasional series of posts on what the stones of our region are telling us. This is his first:

The Kennedy Street River Gravels – Remnants of the ancient Barkers Creek

An ancient river bed is beautifully exposed in Kennedy Street Castlemaine, along the west flank of Agitation Hill. This was an energetic stream in a period of high rainfall that was able to move sizable pebbles.

The mainly quartz pebbles were eroded from nearby reefs and their incessant bouncing along the stream bed sculpted many into rounded shapes. Similar deposits of sand, gravel and gold are preserved on nearby hills.

On Kennedy Street: what looks like an untidy  embankment tells an epic story.  These river gravels are the remnants of the ancient bed of Barkers Creek.

They are all remnants of the ancient valleys of Forest and Barkers creeks. Over time the streams meandered away, cutting deeper into the landscape, but the old deposits were left high and dry as hard hills. Fossil pollens at Guildford tell us these were cool rain-forested valleys, full of Nothofagus (the southern beeches).

Barkers and Forest creeks have a truly ancient history starting perhaps 40 million years ago, and remarkably, as the map below shows, the modern streams are still close to the original valleys of their ancestors.

This is the first post in our geology series written by Clive Willman.