A passing conversation with local artist, Carol Phayer, saw me head down to the beach at Doctor’s Gully one day in mid June. Carol had shown me some chunks of black “rock” she sometimes picked up at the beach and makes into jewellery, rings mainly. I had exclaimed “OMG, that’s coal, what the hell is that doing on the beach here?”
So, I just had to investigate it for myself…
Unfortunately, I didn’t find much coal, only small bits lying around the place, presumably from historic coal unloading facilities in the area. Apparently one such wharf was bombed by the Japanese during WWII. What I did find, however, was a rainbow beach of porcellanite boulders – SIMPLY BEAUTIFUL.
Porcellanite is a siliceous duricrust which has developed within altered Cretaceous mudrocks in the Darwin city area and is well exposed in coastal cliffs to the north. Its distinctive geotechnical properties include low bulk density, variable strength that is highly dependent on moisture content, and relatively high (but inconsistent) durability. The porcellanite rock mass is inhomogeneous and anisotropic; it is un-rippable at the surface, but becomes weaker with depth. It is also highly permeable in places, even karst-like, due to solution cavities.
From: AJES Vol 47 2010, Porcellanite and the urban geology of Darwin, NT, McNally, Clarke & Weber, 2010.
I have pretty much been captivated by this rock since I came to Darwin. It is used everywhere as a building stone and the textures and colours that are in it just amaze me.
So, my morning expedition to find coal ended up becoming a phone photo-fest. Luckily it was low tide and I made sure I stayed well out of the mangrovey bits so I did not find any salty “friends.”
Enjoy the pics.
