Coral and absolute dating
When certain evidence suggested 25,000 years to be an appropriate figure, factors became years—namely, 75,000, 150,000, and 200,000 years.
And, if glacial time and nonglacial time are assumed approximately equal, the Pleistocene Epoch lasted about 1,000,000 the time since the surface examined was freshly exposed.
Furthermore, artifacts reused repeatedly do not give ages corresponding to the culture layer in which they were found but instead to an earlier time, when they were fashioned.
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In spite of this tremendous spread, most geologists felt that time in the hundreds of millions of years was necessary to explain the sedimentary record.
Castile Formation, for example, consists of alternating layers of gypsum and calcite that may reflect an annual temperature cycle in the hypersaline water from which the minerals precipitated.
Sometimes human observation can be maintained long enough to measure present rates of change, but it is not at all certain on a priori grounds whether such rates are representative of the past.
This is where radioactive methods frequently supply information that may serve to calibrate nonradioactive processes so that they become useful chronometers.