Trail trees (or signal trees or thong trees) are trees "modified by the American Aboriginal peoples in order to signify trails, campsites, or special locations (water supply, food, safety, etc.). Often these were oak saplings that are given a unique bend, usually pointed in the direction of the point of interest." The scope and spread of these trees seems to be a recent discovery and a certain hesitance to affirm them as man-made abounds. There is a (or has been, the site is dated) fabulous project attempting to map the location of all known trees and the trails they collectively mark in order to confirm them as human artefacts rather than flukes of nature. Source.This reminded me of Levi-Strauss' book The Savage Mind.