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