4 Geological time

Reading the age of rocks

• Layers of sedimentary rocks form one over another, so in an undisturbed sequence the lower layers are older and the upper layers are younger.
• Fossils (studied by paleontology) show the geological age and the environment in which a layer formed.
• In folds, the order of the layers shows the age: the oldest layers are in the centre of an anticline and the youngest are in the centre of a syncline.
• An unconformity is a surface that records a gap in deposition or erosion of tens of millions of years, so it marks missing geological time.
• A sudden change in the fossil sequence between two layers also points to a time gap.

The scale of geological time

• Earth history is divided into two eons: the Precambrian (the oldest and longest part) and the Phanerozoic (the time of obvious life).
• The Phanerozoic is divided into three eras, from oldest to youngest: Paleozoic, Mesozoic and Cenozoic.