Rate of Accumulation is a relative dating technique which uses stratigraphy to determine the relative ages of artifacts. The basic principle of Rate of Accumulation is based on the geologic theory that the rock layers in the earth accumulated over time and that the deeper the layer, the older. In archeology, the artifacts are the basis on which the rate of accumulation is determined over time. So it would follow that the deeper the artifact, the older it is.

This is extremely useful for a site that has been occupied a number of times throughout history. For example, a hearth is found in a layer of earth and chalk 1.5 meters deep. Then, a second hearth is found at 2.5 meters deep in a layer of ash. It follows then that the second hearth is older than the first, assuming of course that there has been no external disturbance.

This method can also help to determine the relative age of artifacts from two different sites. However, those sites must be located close enough to each other that they share the same geographical stratigraphy. For instance, at one site a pot is found in a clay layer 1 meter deep. At a nearby site, a few beads are found in the same clay layer. One could then logically assume that the pot and the beads are from about the same time period.

There are two problems with dating through this method. First, it is only relative. Unless there is a dated object (such as a coin) within a layer, a second technique must be used to determine an approximate chronological date. The second problem arises when the layers are disturbed, such as by a post hole, storage cache or animal. This disturbance allows artifacts of the same age to be found in different layers, which could lead one to wrongly assume an older date for the deeper object.

Source:

Harris, Edward C. Principles of Archaeological Stratigraphy. Academic Press. London, 1979.