Crete Senesi are literally the "clays of Siena": the distinctive grey colouration of the soil gives the landscape an appearance often described as lunar. This characteristic clay, known as mattaione, represents the sediments of the Pliocene sea which covered the area between 2.5 and 4.5 million years ago. The landscape is characterized by barren and gently undulating hills, solitary oaks and cypresses, isolated farms at the top of the heights, stretches of wood and ponds of rainwater (commonly referred as fontoni, literally "big springs") in the valleys.