Reflecting on Dream Lake Telling roof from floor is a bit tricky when the spring water in a part of Luray Cavern (in Virginia) is so still that it takes on mirror like qualities, so the old question of which are stalactites (holding on tight to the ceiling) or mites is open to debate. The rocks here are Ordovician magnesium rich limestones called dolomites deposited in a long gone ocean some 480 million years ago. They were then folded into a mountain range (whose American remnants are known as the Appalachians) when the Iapetus Ocean closed up during the assembly of the supercontinent Pangaea. Later acidic waters hollowed out networks of caves in a set of processes called karst erosion, whereby the limestone is dissolved and carried away to be reprecipitated. Loz Image credit: David Jones