GeoPic of the Week: Skeletal Limestone — Happy Halloween!

Skeletal limestone – Happy Halloween!

Limestone varies in color from shades of gray to white and red.  This limestone gets its red color from iron oxide in the rock and contrasts nicely with the white crinoids.  Crinoids are a group of marine invertebrates sometimes referred to as “sea lilies”.  The animal was attached to the ocean floor by a main stem or stalk with long arms flowing outward from a central head on top of the stem.  These animals were abundant in the Mississippian Period in Arkansas.  It is unusual to find a complete crinoid fossil; however, pieces of the stalk (commonly called stem or column) are abundantly preserved in the rocks in northern Arkansas.  This limestone is fossiliferous or since it contains the skeletons of animals can be referred to as a skeletal limestone.  This adjective is especially fitting for the rock considering how it has weathered to resemble a skeleton face and the fact that it is Halloween week.  Happy Halloween!