Radioactive dating geologic time scale
This is the same principle used to determine relative age in the trash pits discussed previously.
In fact, sedimentary rocks are, in a sense, trash from the Earth's surface deposited in basins.
Radiometric Dating Radiometric dating relies on the fact that there are different types of isotopes.
Calculations gave estimate of 100 million years for the age of the Earth.
In 1896 radioactivity was discovered, and it was soon learned that radioactive decay occurs at a constant rate throughout time.
The nature of an unconformity can change with distance.
Notice how if we are only examining a small area in the figure above, we would determine a different type of unconformity at each location, yet the unconformity itself was caused by the same erosional event.