Atop Merrimac Butte The white stone atop is known as Moab Tongue, a part of the Curtis formation. The hard white rock acts as a capstone for the softer Entrada sandstone below, protecting it from weathering effects. Once the capstone wears away, however, the sandstone below quickly crumbles, a process which has chipped away to form these dramatic buttes, and in time will erase them altogether.