The fundamental principle is called
the law of superposition...
The last half-billion years of the Geologic Time Scale is divided
into three Eras. The time relevant to placer mining is
the most recent Era - the time since the dinosaurs became
extinct about 66 million years ago.
This Era is divided on the basis of a large
ice age that began 2.6 million years ago.
It included multiple Glacial Periods
separated by warmer Interglacial Periods -
the current Postglacial Period is the latest.
For more information, see
Ice Age
on the
Glossary
page.
In the latest Era, the time before this ice age began
is called the Tertiary Period and the time since then,
up to the present day, is called the Quaternary Period.
The postglacial part of the Quaternary Period is called
the Holocene Epoch.
Almost all placer deposits in BC that are
less than a couple of million years old
were deposited during the postglacial period or
one of the interglacial periods.
During a glacial period, glaciers can rework existing placer deposits.
This rarely results in a minable deposits, but exceptions exist where
the original deposits were very rich. This apparently happened on
Cunningham Creek in the Cariboo.
In undisturbed layers of sediment,
a layer is younger what is under it.
The Time Scale, The Ice Age and Placer Deposits
You can skip directly to the
Geologic Time Scale.
The Geologic Time Scale
The age ranges are written backwards - more natural for reading the
top half of this list from the top down -
the part relevant to placer mining.
The time since last mass-extinction of species
(when the dinosaurs became extinct)
The time since the last retreat of glaciers.
("recent" or "postglacial" placer deposits)
Alternating glacial and interglacial periods.
(almost all placer deposits are "interglacial")
After dinosaurs, but before the recent ice age -
(oldest placer deposits in BC - Tertiary deposits)
Officially,
"Tertiary Period" is the old term for the time of the...
In geological reports, this name isn't used as often as
its two epochs...
Mammals become dominant (on land)
These divisions aren't relevant to placer mining, but interesting
anyway...
Dinosaurs dominant until end of the period,
when all non-avian dinosaurs became extinct
(yeah - non-avian -
birds are now considered to be a kind of dinosaur)
Dinosaurs become dominant; first birds appear
Dinosaurs and Mammals first appear
beginning with the Cambrian Period in which there was
a vast increase in kinds of life - the "Cambrian Explosion"
before there were (many) hard-shelled creatures