Fossil bivalves. Debris in a Miocene core sample from an exploratory oil well in Libya. The debris includes some fossilised shells (white) of bivalve molluscs. Fossils are impressions formed in rocks of objects buried in the original deposits from which the rocks formed. Under the right conditions, the details and anatomy of the objects, such as animals and plants, can be preserved. The Miocene epoch is the time period ranging from 23 million years ago, to just over 5 million years ago.