40 The Geologic Time Scale

Elizabeth Johnson

Using a combination of relative age dating techniques and absolute age dating (https://en.wikipedia.org/wiki/Absolute_dating), geologists have constructed a geologic time scale.

We will use the geologic time scale in geologic maps.  Rock units on geologic maps are often referred to by their relative geologic age  – usually the geologic time period or era.  Note that the absolute ages are marked on the time scale in millions of years (1 Ma = mega annum, or 1,000,000 years).  1000 Ma = 1 Ga, or 1 billion years.

The Geological Society of America has an official geologic time scale which is continuously updated by scientists.  You can view this time scale at: https://www.geosociety.org/documents/gsa/timescale/timescl.pdf.  It is more detailed than the time scale above, and the absolute ages may be slightly different as the GSA time scale is continually updated based upon current knowledge.

Question 19. A Devonian shale is ________ than an Ordovician limestone.

  • Younger
  • Older


Question 20. A Precambrian gneiss is ________than a Mesozoic granite.

  • Younger
  • Older



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