There are two general methods for determining the ages of geologic features and events: absolute and relative dating. The first method, called absolute dating, gives a specific age in years. Absolute dating of rocks and fossils is based on the principle that certain radioactive elements (isotopes) break down over the course of time. One technique widely used for obtaining ages of late-glacial and postglacial fossils is called radiocarbon dating. This method resulted from the discovery of radioactive carbon-14 (14C) in plants and animals. The amount of time that has passed since an organism died can be determined by measuring the amount of breakdown of the unstable carbon. This, in turn, helps determine the age of sediments in which the fossil was buried. Radiocarbon dating was pioneered by W.F. Libby in 1950. This method allows age determinations for organic remains back to about 40,000 years ago.

All of the absolute ages shown on the map are from fossil organisms, mostly mollusk shells, from the emerged glacial-marine deposits, or from terrestrial plant remains preserved in lake sediments and peat deposits. Their ages first were measured by radiocarbon dating, and then the ages were converted to actual (calendar) years.

The other method of determining age relationships is called relative dating, in which one event is determined to be younger or older than another. For example, in an undisturbed sequence of layered sediments, the oldest layer is at the bottom and the other layers are progressively younger toward the top. No specific age can be assigned by this method other than older or younger. The general relative ages of Ice Age sediments in Maine have been known for a long time, but much remains to be learned about the absolute ages of glacial events and how they relate to global climate history.

To read more about Maine in the Ice Age, check out these more indepth explanations: Eskers and Deltas, Marine Clay, Salt Marshes and Recent Sea Level Rise, Glacial Retreat, Moraines, Abrupt Climate Change