"The mind seemed to grow giddy by looking so far into the abyss of time". John Playfair, Professor of Mathematics and Natural Philosophy at the University of Edinburgh in 1788 on the enormity of geological time.

The term "deep space" conjures up the vast distances between stars and galaxies measurable in millions of light years, but way beyond comprehension by the human mind. In his book Basin and Range the American writer John McPhee coined an appropriate parallel term, "deep time", to capture the full dimension of geologic time, sweeping back beyond the paltry crust of recent history that is calibrated by mere human records.

Deep time extends deeper in Canada than anywhere else. The oldest rocks on Earth have been identified in the Slave Province north of Great Slave Lake in the Northwest Territories -- fully four thousand million years old (4 billion or 4 Ga). The rest of the granitic, volcanic, sedimentary and metamorphic rocks exposed across the rest of the Canadian Shield provide a patchy history of geologic events for the next three billion years. Crude chronologies of these rocks of the Precambrian Eon are provided by different radiometric dating techniques. In younger rocks assigned to the Phanerozoic Eon, the succession of multicellular animals and plants becomes the basis for much more precise relative age determinations.

The geologic time scale of eons, eras, systems and series calibrated in millions of years

The Phanerozoic Eon is divided into three eras which, in turn, consists of systems -- Paleozoic Era (Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian systems), Mesozoic Era (Triassic, Jurassic and Cretaceous systems) and Cenozoic Era (Paleogene, Neogene and Quaternary systems). The Ediacaran or Vendian has variously been considered the terminal system of the Precambrian or the first of the Phanerozoic. The names of these time units are part of the basic working vocabulary of paleontologists and stratigraphic geologists for referring to the age of rocks across Canada and elsewhere.

Based on fossil content, each of the twelve or thirteen Phanerozoic systems has, in turn, been divided into series, stages, and zones. Zones provide the most accurate measurement of geologic time; in most cases approaching a precision of a million years, and sometimes as fine as a quarter of a million years.

Further reading:

Albritton, Claude C. Jr., 1986. The Abyss of Time: Unraveling the Mystery of the Earth's Age. Jeremy P. Tarcher, Inc., 251 p.