In 1967 Smithson began exploring industrial areas around New Jersey and was fascinated by the sight of dump trucks excavating tons of earth and rock that he described in an essay as the equivalents of the monuments of antiquity. This resulted in the series of 'non-sites' in which earth and rocks collected from a specific area are installed in the gallery as sculptures, often combined with mirrors or glass. Works from this period include "Eight-Part Piece (Cayuga Salt Mine Project)" (1969) and "Map of Broken Clear Glass (Atlantis)" (1969). In September 1968, Smithson published the essay "A Sedimentation of the Mind: Earth Projects" in "Artforum" that promoted the work of the first wave of land art artists, and in 1969 he began producing land art pieces to further explore concepts gained from his readings of William S. Burroughs, J.G. Ballard, and George Kubler. The journeys he undertook were central to his practice as an artist, and his non-site sculptures often included maps and aerial photos of a particular location, as well as the geological artifacts displaced from those sites. Of these travels, several on-site works were produced including Mirror Displacements a series of photographs that illustrated his essay "Incidents of Mirror Travels in the Yucatan" (1969).