Michael V said:

ruby said:

I have rock questions MV.
One of my nursery buddies took pictures at Bouddi National Park of a couple of interesting rocks and was pondering on how unusual they are. I said ‘I know just the person to ask about them!’. He sent me the pictures, and now I can ask you for all the interesting facts :)))

Both photos show ironstone concretions in sandstone.

Iron travels through rock in low temperature water solutions in the Fe2+ (soluble, reduced) state. If the fluid encounters a change in oxidation in the rock (many reasons this can occur), iron can be oxidised to the Fe3+ state, which is insoluble. The iron drops out of solution as iron oxides and iron oxy-hydroxides (effectively rust) deposits.

Oh, lovely detail. The fellow is a retired chemical nerd so he’ll like this.

What made the curious looking indentations in the first picture? Everyone that saw the pics of course said ‘aliens’. Excluding the possibilty of little green Martians seeing a tempting piece of damp iron deposits and writing the Martian equivalent of Foo was here, how would this form?

The words ironstone concretions….I take it the iron has some of the surrounding rock mixed in with it?