Geo-pic of the week: Frankenstein scar on stylolite

I was working near Lake Fort Smith State Park this last week when I came across a peculiar mineral deposit resembling Frankenstein’s Scars (Fig. 2).  It was just in time for Halloween!  The resemblance is uncanny.  Despite the horror, there is a lot of geology illustrated in this rock. 

The mineral that forms the “scars” seen in the photo is called limonite, and it was deposited within a cavity in a stylolite.  A stylolite is a surface, typically a bedding plane, that has recrystallized due to pressure from the weight of overlying rock material.  Stylolites can be recognized by their rough, jagged appearance (it’s difficult to see in this photo, but trust me – it’s there).  The limonite “scars” formed in a pattern called boxwork and, surrounding the boxwork, limonite is also present in botryoidal form: a crystal shape resembling small round globs (the orange goosebumps around the scars).

At this time, rocks are not thought to celebrate Halloween, although more work needs to be done to verify that.

Geo-pic of the week: Conostichus trace fossils

Above are two pictures of a trace fossil, Conostichus, from the Ozark Plateaus region of Arkansas.  Like other trace fossils, Conostichus are structures found in sedimentary rock that represent the spot where an animal lived, fed, or travelled.  Despite their abundance, especially in rocks of the Carboniferous period (299 to359 million years ago), it’s not certain what kind of animal made Conostichus, because the animal’s body wasn’t preserved. 

The upper picture is the top of the Conostichus and shows the hole through which the animal entered or exited the structure.  The lower picture is the same Conostichus with the top facing down.  As you can see, they taper and come to a rounded point at the base, vaguely resembling a badminton birdie.  

At present, the most widely accepted theory for their origin is that Conostichus are burrow traces left by Sea Anemone.