Tag Archives: Glasshouse Rocks

In search of chert

21 Monday Aug 2017

Posted by in geology, photos

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It’s a wild and windy Thursday, too wild for J to contemplate burning off, so we go in search of definitive recognition of Narooma chert.  We’ve decided to leave geological processes for now and concentrate on the naming of parts. Chert is a great choice, because there are a few large formations about 20 km from home and we know we’ll know what we’re looking at without any need for debate or any room for doubt.

First we visit Australia Rock near the Narooma breakwater. The blackish-grey rock looms above us out of an untidy rubble of huge squared granite lumps, presumably doing something breakwatery, but spoiling the majesty of the chert, which is  precisely where it ought to be. It was deposited on the Pacific Ocean floor over a period of 50 million years (from Late Cambrian to Ordovician period) and moved westward to its present location with the Pacific plate. When its carrier collided with Gondwana its journey ended and here it is. 

As we scrutinise the rock and mumble about its characteristics the gentle waves sussurrate the pebbles on patches of beach and the wind howls. The sun of course is behind the rocks, so photography is challenging, although far more familiarly so than geology.

After a half hour of scrutiny we drive to the headland cemetery and go down the track to a chert wonderland unfolding (not a word I should use in view of its geological meaning) as we stroll towards and beyond Glasshouse Rocks. The tide is low and the beach goes on, around headlands to yet another vista of islets and sand and bush-crowned cliffs, and we finally reach the pair of chert spires that has tantalised us ever since we began walking on this beach.

Maybe I have chert nailed, at least until I have to identify it somewhere else.

In our ramblings we encounter a recent acquaintance. Parading along the middle of the beach in an isolated formation, and lurking in the cliffs, is block in matrix mélange, which we now take the liberty of calling BIM.

Post-walk research – by J: I fell into a deep sun-filled sleep – suggests that we also saw mylonite. Wikipedia informs me that “as dislocations are added to subgrain boundaries, the misorientation across that subgrain boundary will increase until the boundary becomes a high-angle boundary and the subgrain effectively becomes a new grain.” Forget that, if you like, and just admire the outcome.

 Having identified chert, BIM and mylonite, we find we have a new mystery to solve. What is the origin of these swirls?

As we’re leaving the beach we encounter the chevrons we tracked down on our first geological foray, some years ago now,  glorious as the light moves and showcases their unmistakable detail. They are a comforting reminder that we have made some geological progress.