The largest known protozoans in terms of volume are the extinct calcareous foraminifera (Foraminiferida) of the genus Nummulites. Individuals measuring up to 150 mm (6 in) wide were found in the Middle Eocene rocks of Turkey. The calcareous foraminifera existed in the Eocene geological period, which lasted from about 55 to 38 million years ago. During this time the Earth's climate as a whole was warmer and wetter than today. Areas as far north as Greenland, ice-locked today, were dominated by trees which nowadays exist much further south. Vast quantities of dead foraminifera make up the chalk white cliffs of Dover, in England;and, elsewhere in the world, deposits made up from these tiny creatures provide clues to the locations of oil and to the nature of the Earth's climate in ages past.