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.
Have you ever noticed a rock that had numerous concentric layers like the sandstone pictured to the left? If so then you’ve seen liesegang bands Strange as they may look these bands occur naturally within many sedimentary rocks and less commonly within igneous and metamorphic rocks. At some point after the rock formed it was buried and saturated with iron-rich ground water and the bands, which are typically an iron-rich mineral such as limonite, precipitated within the rock in repeating rings. Later, it was exposed at the surface and the softer sandstone eroded away leaving the harder rings standing in relief on the rock.
Liesegang bands can form an array of visually stunning patterns. Sometimes they form near-perfect cylinders that resemble iron pipes.