land art or earthworks, art form developed in the late 1960s and early 70s by Robert Smithson, Robert Morris, Michael Heizer, and others, in which the artist employs the elements of nature in situ or rearranges the landscape with earthmoving equipment. The resulting work, often vast in scale, is subject to all natural changes, such as temperature variations, light and darkness, wind, and erosion. The technique was in part an attempt to counter the perception of art as an acquirable commodity, although as the movement developed such items as site photographs, cartographic studies, and artists' notebooks were made available to collectors. Smithson's Spiral Jetty (1970), a huge spiral of rock and salt crystal in the Great Salt Lake, Utah, is a characteristic example of the land art form. Because of the fluctuating water level of the lake, Spiral Jetty is not always visible. Heizer's vast City (1971–) in the Nevada desert is probably the largest land art project yet attempted. Another monumental work is James Turrell's Roden Crater, an extinct volcano near Flagstaff, Ariz., the interior of which he has transformed since the 1970s into an enormous work of art with rooms, tunnels, and openings to the sky. Robert Morris constructed outdoor earthworks in the 1960s and 70s, but land art is just one of the movements with which he has been associated. Among other artists who have worked in the land art genre are Dennis Oppenheim, Alice Aycock, Nancy Holt, Richard Long, Walter De Maria, Newton and Helen Harrison, and Andy Goldsworthy.