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The Geologic Time Scale for Placer Miners

The Geologic Time Scale divides up the Earth's past on the basis of the age of rocks - particularly sedimentary rocks - sediment deposited in water.

The fundamental principle is called the law of superposition...
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 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.

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.

• Cenozoic Era   (sen-uh-zoh-ik)   Present to 66 mya (million years ago)
  The time since last mass-extinction of species
  (when the dinosaurs became extinct)
   
  • Quaternary Period   (kwuh-tur-nuh-ree)   Present to 2.6 mya
     
    • Holocene Epoch   (hol-uh-seen  ee-pok)   Present to 11,000 years ago
      The time since the last retreat of glaciers.
      ("recent" or "postglacial" placer deposits)
    • Pleistocene Epoch   (ply-stuh-seen)   11,000 to 2.6 mya
      Alternating glacial and interglacial periods.
      (almost all placer deposits are "interglacial")
  • Tertiary Period   (tur-shuh-ree)   2.6 mya to 66 mya
    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...
     
    
    • Neogene Period   (nee-uh-jeen)   2.6 mya to 23 mya
      In geological reports, this name isn't used as often as its two epochs...
      • Pliocene Epoch   (ply-uh-seen)   2.6 mya to 5.3 mya
      • Miocene Epoch   (my-uh-seen)   5.3 mya to 23 mya
    • Paleogene Period   (pay-lee-uh-jeen)   23 mya to 66 mya
      Mammals become dominant (on land)
• Mesozoic Era   (mez-uh-zoh-ik)   66 mya to 252 mya
  These divisions aren't relevant to placer mining, but interesting anyway...
   
  • Cretaceous Period   (kri-tay-shus)   66 mya to 145 mya
    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)
  • Jurassic Period   (juh-rass-ik)   145 mya to 201 mya
    Dinosaurs become dominant; first birds appear
  • Triassic Period     (try-ass-ik)   201 mya to 252
    Dinosaurs and Mammals first appear
• Paleozoic Era   (pay-lee-uh-zoh-ik)   252 to 541 mya
  beginning with the Cambrian Period in which there was
  a vast increase in kinds of life - the "Cambrian Explosion"
• The Precambrian   (pre-kame-bree-uhn)   Before 541 mya
  before there were (many) hard-shelled creatures