ABSTRACT
Sedimentary rocks were deposited in layers, the youngest being at the top. They contain the remains of organisms, that is, fossils, which represent the life of past geological times and permit the age of the sediment to be defined. Each layer of a sedimentary rock represents a particular event in geological time and the sequence of layers in a pile of sediments thus records a series of events in geological history. Various tests that can be applied include observations of internal structures formed during the deposition of the sediments. The sequence of rocks which has formed during geological time is represented by the stratigraphical column, which lists the rocks in their order of age: the oldest rocks are at the base of the column and the youngest at the top. Three geological periods, the Cambrian, Ordovician and Silurian make up the Older Palaeozoic rocks.
