erratic

1. (geology) A rock moved from one location to another, usually by a glacier.Examples: "The term for a displaced boulder is an erratic, but in the nineteenth century the expression seemed to apply more often to the theories than to the rocks."; "During the last ice-age, massive stones were carried for miles by the scouring glaciers, only to be left, like passengers at the end of the line, when the glaciers retreated. Stranded in their new surroundings with rocks with which they share no common geology, their out-of-place-ness is evoked by their name: “erratics”."Synonyms: dropstone