5
$\begingroup$

enter image description here

I found this rock by the shore of Lake Ontario. It's light weight and it was grouped with some other rocks with the same patterns.

$\endgroup$
  • $\begingroup$ Not OT IMHO - as edited, it's not asking what the rock is, it's asking what the circular features are. $\endgroup$ – Semidiurnal Simon Aug 27 '19 at 20:46
8
$\begingroup$

Your rock contains crinoid stem fossils.

Wikipedia has a good summary of crinoids

$\endgroup$

Not the answer you're looking for? Browse other questions tagged or ask your own question.