Ankarana’s limestone massif formed on the ocean bed during the age of the dinosaurs, rose, then cracked, resulting in the steep scarp on the west and the gentler slopes to the east. This tectonic movement created and then sank a huge network of caves. Over the millennia, the range cracked further, from east to west, breaking open the cave systems and creating channels for several rivers. Mildly acidic rainwater dissolved into the softer strata of limestone from above, and carved it out from underneath, eroding the solid rock both vertically and horizontally, and wearing it away on every surface to a sponge-like, porous honeycomb of limestone cells. It’s also believed that the snails whose shells you see everywhere may have contributed over the aeons to the process of erosion, by rasping the limestone surface for algae growing in rain puddles in the rock.