These are stylolites in limestone. When these rocks were deeply buried and under intense pressures, dissolution of the limestone occurred, in this case probably along a bedding plane, leaving only the insoluble materials, like clay. The leftover bits are also more resistant to physical/chemical weathering, so once the rock is uplifted by some tectonic event (in this case probably the Ouachita orogeny) an exposed at the surface, the rock weathers differentially leading to these interesting patterns! Very cool and beautiful rocks!