Mudstone vs. Sandstone

The most important geological job was to start at the bottom of the rock layers and measure how thick each layer of sandstone and mudstone was. As we took the measurements, we drew in our field notebook a picture of the different rock layers. This picture is called a stratigraphic section. This drawing is important for telling time because layers of rock are laid down one on top of the other. The lowest layer is the oldest and the highest layer is the most recent.

Fossils from lower layers lived earlier than fossils from higher layers.