If I didn't know better I'd say it was Horse Manure...

[reynardo]

... but it's not.



These rocks are in the sandstone at the top of a cliff at Bullimah Spur near Putty Beach, New South Wales. They're in a National Park so I couldn't bring out samples.

If you know some geology, especially sedimentary, your assistance is requested.



It's friable, and I didn't have a magnifying glass with me but I couldn't see the grain size (it was rather small). These lumps are slightly embedded in the top of a layer of the lighter-colour Narrabeen Group sandstone, in a layer of black shattered conglomerate. The two large rocks here are about 20cm across. I'd almost be tempted to say pahoehoe-style lava, possibly washed down from the New England volcanics during their formation in the Triassic.

Or do I have to hand in my Amateur Geologist's Badge?

Comments

[shiv5468.livejournal.com]

I would have said pahoehoe lava too.

[kerravonsen]

Intriguing. Of course, I have no clue, since my geology-fu is not as strong as yours. There seem to be two contradictory indicators: the fact that it is a separate mass included in sandstone implies that it's a durable type of stone, yet the fact that its friable would indicate the opposite. Unless perhaps exposure to sun and rain and wind made it more fragile?

I still don't know what it is.

[droxy.livejournal.com]

My vote is lava, but I can't actually look at it. Sedimentary rock (like sandstone)is very compact. It's clear to my that isnt sandstone.

Where you are is geologically near some of the oldest rocks exposed.

if it's not lava then some minerals have turned it dark, and convoluted sediment (not flat lines)is caused by pressure and upheaval.

[carlowe.livejournal.com]

I've seen stuff in the Malmsbury area that looks similar, and would support that hypothesis. I can send a copy to my sister who is a geo, and ask?

[reynardo.livejournal.com]

Please do!