There's a great variety of free essays by and about Robert Smithson on his website. Below is Smithson's brief thoughts on non-sites. Poke around more on Smithson's site here.
By drawing a diagram, a ground plan of a house, a street plan to the location of a site, or a topographic map, one draws a "logical two dimensional picture." A "logical picture" differs from a natural or realistic picture in that it rarely looks like the thing it stands for. It is a two dimensional analogy or metaphor - A is Z.
The Non-Site (an indoor earthwork) is a three dimensional logical picture that is abstract, yet it represents an actual site in N.J. (The Pine Barrens Plains). It is by this dimensional metaphor that one site can represent another site which does not resemble it - this The Non-Site. To understand this language of sites is to appreciate the metaphor between the syntactical construct and the complex of ideas, letting the former function as a three dimensional picture which doesn't look like a picture. "Expressive art" avoids the problem of logic; therefore it is not truly abstract. A logical intuition can develop in an entirely "new sense of metaphor" free of natural of realistic expressive content. Between the actual site in the Pine Barrens and The Non-Site itself exists a space of metaphoric significance. It could be that "travel" in this space is a vast metaphor. Everything between the two sites could become physical metaphorical material devoid of natural meanings and realistic assumptions. Let us say that one goes on a fictitious trip if one decides to go to the site of the Non-Site. The "trip" becomes invented, devised, artificial; therefore, one might call it a non-trip to a site from a Non-site. Once one arrives at the "airfield", one discovers that it is man-made in the shape of a hexagon, and that I mapped this site in terms of esthetic boundaries rather than political or economic boundaries (31 sub-division-see map).
This little theory is tentative and could be abandoned at any time. Theories like things are also abandoned. That theories are eternal is doubtful. Vanished theories compose the strata of many forgotten books.
*Non-Site #1. Smithson changed the title for this text which was initially "Some Notes on Non-Sites." It has been partially excerpted by Lawrence Alloway in "Introductions 1: Options, Milwaukee Art Center, 1979, p. 6
from Unpublished Writings in Robert Smithson: The Collected Writings, edited by Jack Flam, published University of California Press, Berkeley, California, 2nd Edition 1996