Referring to something not native to a particular habitat: Allochthonous rocks or deposits have emigrated or been imported from another environment.

Allochthonous features of a landscape or elements of its geologic structure are those that have been moved to their current position by tectonic forces.

In geology, allochthonous substances in a present site are away from their place of origin.

A type of coal formed from accumulated plant material that was transported from its original place of growth and deposited elsewhere: Judy's teacher told his class about allochthonous coal that had been formed from plant substances that accrued from a place far from where the coal was formed.

A stream that flows in a channel it did not itself form: Mr. Smart showed his class a photo of a river, or allochthonous stream, rushing along in a man-made or artificial duct.