Found this rock on Lake Michigan along the Frankfort shoreline. I thought it would be fun to experiment with some acid, so I put it in 10% or so bath for a day. I washed it off this morning and viola... little bird feet! Or, foot prints. Or maybe not. This area was a tropical, shallow sea for a long time, so maybe bird prints don't make sense. Also, other than the ID of this... I wonder how common this is?
Any help would be appreciated.