The term allochthon is derived from the Greek allos 'other' and chthonous 'of the earth.' Allochthonous objects have been displaced from their original site of origin, in contrast to autochthonous (auto,' self') objects that remain indigenous or in situ. In geology, the term allochthonous has been used to characterize plant material transported from site of growth to site of deposition into coal seams (allochthony), for masses of redeposited sediments from distant sources, for constituents of derived magmas (cf. allogenic components), for transported and redeposited fossils (cf. remanié faunas), for impact crater ejecta, and for subterranean streams in karst regions.
In structural geology, an allochthon is a large allochthonous body of mappable, coherent rock such as the Taconic Allochthon of eastern New York (see Zen, 1967; Bird and Dewey, 1970) or smaller masses of chaotic, slumped rock and sediment, called olistostromes , such as those of the central and northern Apennines