Nature’s medals

By Sarah Joomun, Documentation officer

In the 1820s a young geologist named Charles Lyell travelled around France studying the landscape and rock formations to try and work out the processes that created them.

In between these field-trips, he met the people who had been studying the geology of France and from these discussions and his observations he created The Principles of Geology, one of the first significant popular science books on the subject and a foundation for the methods of modern geology.

Lyell collected many samples from the rocks he studied, amassing thousands of fossils during his lifetime. The Museum has a collection of some 16,000 of them, around 90 per cent of which are shells, mostly gastropods (snails) and bivalves (clams), many collected during his travels in France.

The reason for this prodigious collection of fossil shells, or testacea as they were then known, was that Lyell believed them to be the most useful clue to understanding the Earth’s history.

The testacea are by far the most important of all classes of organic beings which have left their spoils in the subaqueous deposits : they are the medals which nature has chiefly selected to record the history of the former changes of the globe.

– Lyell’s Principles of Geology, Vol III, 1833.

Fossil shells can show how the animal that lived inside the shell behaved, and whether it lived on the land, in freshwater or in the sea. Species of shelled animals have a wide geographical range and individual species survive for a long time, so they can be compared across time and space.

This allowed Lyell and his colleagues to determine the relative ages of the rock layers that the fossil shells came from. He looked at the proportion of shells that belonged to living species and determined that the rock layers with the lowest proportion of living species were likely to be older than rocks with higher proportions of living species.

And so three main groups of rock layers were found: the Eocene, containing fewer than 4% living species; the Miocene, with fewer than 18% living species; and the Pliocene, with more than a third of living species.

Although what is now known as the Eocene (from 56 to 34 million years ago), Miocene (23 to 5.3 million years ago) and the Pliocene (from 5.3 to 2.6 million years ago) don’t denote exactly the same periods as Lyell described, we still use these terms for some of the youngest geological epochs today.