Objects are said to be in association with each other when they are found together in a context which suggests simultaneous deposition.
This principle presumes that the oldest layer of a stratigraphic sequence will be on the bottom and the most recent, or youngest, will be on the top.
The earliest-known hominids in East Africa are often found in very specific stratigraphic contexts that have implications for their relative dating.
The majority of chronometric dating methods are radiometric, which means they involve measuring the radioactive decay of a certain chemical isotope.
They are called chronometric because they allow one to make a very accurate scientific estimate of the date of an object as expressed in years.
Cultural seriations are based on typologies, in which artifacts that are numerous across a wide variety of sites and over time, like pottery or stone tools.
If archaeologists know how pottery styles, glazes, and techniques have changed over time they can date sites based on the ratio of different kinds of pottery.
The most commonly used chronometic method is radiocarbon analysis.
It measures the decay of radioactive carbon (14C) that has been absorbed from the atmosphere by a plant or animal prior to its death.
This number is usually written as a range, with plus or minus 40 years (1 standard deviation of error) and the theoretical absolute limit of this method is 80,000 years ago, although the practical limit is close to 50,000 years ago.
Because the pool of radioactive carbon in the atmosphere (a result of bombardment of nitrogen by neutrons from cosmic radiation) has not been constant through time, calibration curves based on dendrochronology (tree ring dating) and glacial ice cores, are now used to adjust radiocarbon years to calendrical years.
These strata are often most visible in canyons or gorges which are good sites to find and identify fossils.