Geo-pic of the week: Frankenstein scar on stylolite

Frankenstein boxwork on stylolitic surfaceBoris-Karloff-Frankenstein

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.

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