A tool characteristic of a particular archaeological era a dated concept borrowed from geology. A particular artifact form used to define a specific period or culture such as an Acheulian handax; a specific artifact which serves to represent the taxon of which it is a member. Such an artifact would have a wide distribution in space but a restricted one in time. Its value is for correlating cultural sequences over large areas as in cross-dating. In archaeology the time taken for a type to spread by diffusion must be allowed for and if possible calculated from outside evidence.
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