Friends of the Box-Ironbark Forests

Some people think the world is a messed-up place. I prefer to stay optimistic but, I have to admit, when it comes to the rocks beneath our feet, it’s a pretty accurate view.

These anticlines are just a selection of the crumpled rock strata that make up Castlemaine’s foundations. At least twenty anticlines lie between Castlemaine and Golden Point Road and some have been traced downwards by diamond drilling for at least 500 metres. Anticlines, and their companion synclines, are everywhere.

The strata that wrap around these beautiful folds were originally laid down on the deep ocean floor. Back then, some 440 million years ago, mind-bogglingly powerful tectonic forces were messing with eastern Australia by deforming the strata then raising them to form land. It started with a tectonic plate pushing from the east creating folds that run in a north-south direction. Try this with a sheet of paper: anchor one end and push the other towards it and you will see a fold forming at right angles. Most Castlemaine anticlines have sharp, narrow hinges like ‘The Anticlinal Fold’ (Photo 1) in Lyttleton Street but some can be quite broad, especially if there are a lot of thick sandstone beds in the area (Photo 2).

Photo 1: The Anticlinal Fold is fantastic because it shows so clearly the way sandstone layers are bent. Of course, they would have been quite plastic at the time of folding.

Photo 2: Broad anticline south side of Forest Creek near the Leanganook Track. Just north of Montgomery Street.

A lesser-known anticline is on the Kalimna Tourist Road (Photo 3) and a forgotten one is in Bull Street (Photo 4).

Photo 3: This anticline on the Kalimna Tourist Road is about 700 metres north of the top end of Lyttleton Street. It’s best to look at it from the north side.

Photo 4: A very nice anticline on the south side of Bull Street about 70 m east of Kennedy Street – it was once a quarry and now is a Council depot. Memoir 2, Geological Survey of Victoria 1903.

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

Geology creeps into everyday life in the sneakiest ways. Take Castlemaine’s gutters and buildings; they exhibit a gallery of local rocks. At Stonemans Bookroom corner, deep gutters are paved with sandstone blocks and a dash of Harcourt granite along the road edge (Photo 1). By adding a sprinkling of basalt around the corner we have three rocks representing the key events in our region’s geological history; sandstone from a vast and ancient ocean floor, a later intrusion of granite (granodiorite to be technical) and lava flows from almost yesterday.

Photo 1: Gutter outside Stonemans Book Room with local sandstone on the inner edge and Harcourt Granodiorite forming the main gutter.

Posting a letter at the Post Office alcove in Lyttleton St gives a glimpse into two successive local metamorphic events (Photo 2). Look down at the magnificent slate flagstones that pave the Lyttleton Street alcove (also the Market building steps). They began as mudstone on the seafloor and later were weakly metamorphosed by heat and pressure, which converted clays to mica minerals. But it took a second dose of heat from the Harcourt granite to finish the job and harden the slate. Everything around the granite was baked hard so that the only decent and durable paving slate is found within 1.3km of its perimeter.

Photo 2: Slate paving in the Post Office alcove. The inset magnifier shows the typical spotted appearance of slates baked by the Harcourt granite. Note the granite has been used as a foundation stone which is common to other major Castlemaine buildings.

In the mid 19^th century sandstone quarries were dotted all around the town. The Telegraph Station in Barkers Street, like many of our earliest buildings, is built of local sandstone (Photo 3). It has a beautiful warm colour with fascinating textures derived from its seafloor origins but the rock can become flaky where the clay content is high.

Photo 3: Castlemaine sandstone in the old Telegraph Station, Barkers Street.

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