Building up a Time-Scale

THIS ability to re-clothe the past at successive intervals of time has enabled the geologist to extend the realm of history for hundreds of millions of years and to picture, still incompletely and with many a tantalising gap, the story of life as it developed on land and in the sea.

As he studies the succession of fossils, he sees the grand outlines of organic evolution emerge. And combining this with what he learns from the study of rocks in their relative order, he builds a time-scale such as the one on the opposite page.

The time-scale can be followed easily if it is remembered that the earliest intervals of time are at the bottom, exactly as the oldest layers of rock are at the base of a cliff. Working up gradually we come to recent times, which correspond to the sprinkling of earth at the very top of the cliff. The geological terms used in the scale are long and slightly alarming, but when explained they make geology as simple as history.

To begin with the geologist divides the earth’s story into great intervals of time called eras. There is first of all an immensely long era, not repre­sented by rocks in New Zealand, which can be called Pre-Cambrian because it comes in the time-scale before the Cambrian Period. Then follow two eras named from a Greek word zo-on, mean­ing animal — the Paleozoic, or Era of Ancient Life, and the Mesozoic that is Era of Middle or Intermediate Life. Next come the third and fourth eras since Pre-Cambrian times—the Tertiary and Quaternary Eras.

But these vast eras, extending as they may for hundreds of millions of years, have to be split up into smaller intervals, or periods, which are usually named after the places where the rocks were first examined. Hence we find such terms as Devonian, after the County of Devonshire in England, and Jurassic, after the Jura Mountains of Europe. Other terms are descriptive, like Carboniferous, used to describe the age when the greatest beds 5f coal were formed.

The periods which concern us in this story

Cambrian - Named after the Roman word for Wales.

Ordovician - Named after a part of Wales whose people were known to the Romans as Ordovices.

Silurian - Named from another part of Wales where the people were called Silures by the Romans

Devonian - Named after the County of Devonshire in England.

Carboniferous - The period when the forests flourished this formed the earth’s coal beds.

Permian - Named after the Perm district in Russia.

Triassic Jurassic - The ‘ period with three divisions. Named after the Jura Mountains in Europe.

Cretaceous – The chalk period

Pleistocene - The Great Ice Age

Recent -. The period in which we now live

Equipped with this chart, we can now start a journey of exploration, covering millions years, through the course of New Zealand’s ancient history.