On the western side of Lesvos there are petrified tree trunks strewn over an area measuring 150,000 stremmata (37,000 acres) which comprise the Lesvos Petrified Forest. This is one of the most beautiful and rarest geological heritage monuments in the world, since it is a complete ecosystem that was petrified on account of the intense volcanic activity in the north Aegean region approximately 20 million years ago and has been preserved in an excellent state at present. The density of the petrified trunks in an upright position and their fully developed root systems demonstrate that these trees were prettified in their original position and were not carried away. This is an indigenous petrified forest which provides evidence not only for the volcanic rocks but also on the geological development of this particular area.