They are works of art that can be considered works of art but don't have to be in the collection of the Metropolitan Museum.
Effigy Tumuli is Michael Heizer's only known representational work. Designed in the tradition of Native American mound building, Effigy Tumuli consists of five different mounds of earth that resemble geometrically abstracted animals, each chosen because they are indigenous to the region: a catfish, a water strider (insect), a frog, a turtle, and a snake. The work is located in Buffalo Rock State Park, near Ottawa, Illinois (approx. 85 miles southwest of Chicago), on Buffalo Rock, a bluff overlooking the Illinois River.
Buffalo Rock was once the site of a mine, and as part of its conversion into a state park, the Ottowa Silica Company Foundation comissioned Heizer in 1983 to complete the large-scale tumuli at Buffalo Rock. The work was completed in 1985.
The tumuli, like most of Heizer's earthworks, are massive in scale. Water strider is 685 feet long, frog is 340 feet long, turtle is 650 feet long, catfish is 770 feet long, and snake is 2,070 feet long measured from head to tail (it curves). Whereas frog, turtle, and catfish are essentially mounds built upon the flat bluff of Buffalo Rock, turtle and snake utilize the natural geography to their advantage, with the turtle's shell being formed by a mound as the rock dips to the river level, and the snake curving around and also dipping down 90 feet to the river level.
Given the nature of the work and the vegetation that has occupied the site over time (itself a function of the site's intended reclamatory purpose), it can be difficult today to get a sense of the works. Water strider and catfish, however, are visible from both the U.S.G.S. satellite imagery below (taken in the late 90s or early 2000s) and in current views from Google Maps.
Buffalo Rock State Park, where Effigy Tumuli is located, is open to the public. The park is a popular spot for recreation and, among other facilities, houses two live bison. It is operated by the Illinois Department of Natural Resources, which maintains a website containing information about the park. Information about the park is also available through the park's interpretive office, at (815) 433-2224.