An artifact is any object that people made, used, or altered. In addition to manufactured items such as buttons, bottles, and keys, artifacts can be stones worn by grinding corn, or trees on which landowners carved their initials. Neither stones nor trees were manufactured, but they show evidence of human use.
Artifacts wind up in the ground in a variety of ways.
The most common is as discard. Regular garbage collection is a modern concept. Not very long ago, most people disposed of their trash in their own backyards. As people walked on this trash, they broke it down into smaller bits that they eventually ground into the dirt. Discarded materials tended to collect in low spots on the landscape.