Effigy Tumuli is Michael Heizer's only 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: a catfish, a water strider (insect), a frog, a turtle, and a snake. These animals were chosen because they are indiginous to the region. The work, part of which can be seen in the photo to the right, is located in Buffalo Rock State Park, near Ottawa, Illinois (approximately 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 from head to tail (though 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.