Whenever I'm at Mannum in South Australia, I'm always fascinated at the thick layer of fossils mixed in the rocks in the roadside cuttings. A little searching on the internet tells me that it is "a fossil-rich tertiary sandy limestone of some 15 to 20 million years of age. This limestone is part of what is known as the Mannum Formation: coral engendered limestone, crowded with the remains of urchins, shells and other marine fossils." That's a pretty good description.