stromatolite

Mound produced in shallow water by mats of algae that trap mud particles. Another mat grows on the trapped mud layer and this traps another layer of mud and so on. The stromatolite grows to heights of a metre or so. They are uncommon today but their fossils are among the earliest evidence for living things - over 2,000 million years old.

The oldest stromatolites have been dated at 3.46 billion years old. They were discovered in 1999 in Western Australia, near the town of Marble Bar (1,200 km/745 mi north of Perth).