How can we even begin to understand what was happening on our planet millions of years ago? Our main source of information is rock formations that developed during those distant eras. By studying them, geologists can try to reconstruct the conditions that existed at the time of their formation. For example, if a layer of solidified lava or volcanic ash is found, it is evident that a volcano was erupting nearby. A layer of clay suggests that there was a sea, and quite a deep one. In addition, fossilized remains of ancient animals and plants are often preserved in sedimentary rocks, which can be used to restore their appearance.