Photo of the Ordovician–Silurian boundary. Due to the folding of the Caledonian mountain range, the layers have been inverted (turned right over). This left the earlier Ordovician limestone sitting on top of the later Silurian brownish mudstone.

Geology is the study of the nonliving things that the Earth is made of. Geology is the study of rocks in the Earth's crust. People who study geology are called geologists. Some geologists study minerals (mineralogist) and the useful substances the rocks contain such as ores and fossil fuels. Geologists also study the history of the Earth.

Some of the important events in the Earth's history are floods, volcanic eruptions, earthquakes, orogeny (mountain building), and plate tectonics (movement of continents).