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.
Nonradioactive absolute chronometers may conveniently be classified in terms of the broad areas in which changes occur—namely, geologic and biological processes, which will be treated here.
During the first third of the 20th century, several presently obsolete weathering chronometers were explored.
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.