At the time Australia was settled by Europeans the age of the Earth was agreed, at least in principle, and based on biblical genealogy reaching back to Adam and Eve. However, the fact that fossils reflect past life disturbingly different from that of the present became increasingly apparent during the 1800s. Darwin's On the origin of species, of 1859, combined with geological concepts already developed in Europe challenged the biblically based view. Geological mapping, particularly in England, established sediment thicknesses of many kilometres in what we now recognise as accumulations of sedimentary basin configuration. The time needed for such accumulations of this scale, based on observed sedimentation rates, changed our perspective of geological time. From multiplying sediment thicknesses established from mapping sedimentary rock layers by observed sedimentation rates, it became clear that biblical estimates of the age of the Earth departed from reality by several orders of magnitude. The sedimentary rock layers shown by the Grand Canyon, in the USA, well over 1 km thick, did not form in a mere 4000 years or so.

DEVELOPING A TIMESCALE

Individual sedimentary layers accumulate at the surface of the Earth. The accumulation of multiple layers, the younger on top of the older, registers the passage of time like the sequential pages of a novel as they are read. This simple fact is an important principle of geology, referred to as superposition: stacked sedimentary layers record the passage of time. Where the stacks are kilometres in thickness the time registered has been of very long duration (see Figure 3.1).

The contents of the very many sedimentary basins in Australia, Europe and elsewhere likewise represent very long spans of time.