Basics of Environmental Science

74 / Basics of Environmental Science

fossil assemblages can be used to identify particular strata wherever those strata are found. In other

words, animal species have come into existence, lived for a while, and then have disappeared and

their places have been taken by others, newly arrived: we would say evolved. From this it is possible

to construct a ‘stratigraphic column’, a vertical section through sedimentary rocks in which each

stratum is shown in chronological order. The Cuvier and Brogniart study inspired geologists all over

Europe to apply the method to their own localities and eventually to divide geologic time into distinct

episodes on the basis of the animals associated with them. Though much amended, the geologic

time-scale used today is derived from this work, as are most of the names.

Geologic time is divided into eons, eras, suberas, periods, and epochs. Table 2.4 shows the present

arrangement, starting with the oldest, although some of the dates are revised from time to time (Ma

means millions of years ago). Priscoan, Archaean, and Proterozoic together comprise the eon formerly

known as the Precambrian. The term ‘Precambrian’ is still widely used, but not in a formal sense (all

it means, after all, is ‘before the Cambrian’).

The order in which historical episodes should be arranged having been established, the next

step is to allot dates to them. The thickness of strata is no help with this. Sedimentation is an

Table 2.4 Geologic time-scale