Geologists interpret Earth history through the rocks they study. This task can be quite difficult given the number of geologic events that can occur to an area over the vastness of geologic time. These events include both dramatic and slow events. The dramatic events include volcanic eruptions, earthquakes, floods and landslides. The slow events include plate tectonic activity, which builds tremendous mountain chains, the steady erosional processes that reduce these mountains into sediments and the transportation of these sediments into depositional basins where they are subsequently lithified into rock again.

Of particular importance in the understanding of Earth history is the identification of the correct sequence of these geologic events or the relative time of each event.

Fortunately, early geologists recognized certain features within thick sequences of rocks that have come to be known as the fundamental laws of geology.

1. Superposition
2. Lateral Continuity
3. Original Horizontality
4. Inclusions (also known as Components)
5. Crosscutting Relationships
6. Uniformitarianism
7. Fossil Succession

Exercise 1

Exercise 2

This material was developed by Donald E. Clark of the Dept. of Engineering Science and Physics, CSI