This large mound is a trace fossil called a stromatolite. It formed when mats of bacteria repeatedly grew up through layers of sediment. It was found in Lester Park just outside of Saratoga Springs. There, 490 million years ago, these mounds grew upward in shallow water and provided a safe haven for snails and trilobites that lived among them. At 3.5 billion years old, stromatolites from Australia are the oldest undisputed fossils known on Earth.