My Country Tis of Thee - Effigy Mounds National Monument

Effigy Mounds National Monument sits in the Northeast Corner of Iowa and is the largest collection of prehistoric mounds in the United States.

From Wikipedia:

Prehistoric mounds are common from the plains of the Midwest to the Atlantic seaboard, but only in this general area was there a culture that regularly constructed mounds in the shape of mammals, birds, or reptiles. The monument contains 2,526 acres (10 km²) with 206 mounds of which 31 are effigies. The others are conical, linear and compound. Woodland period Indians built mounds from about 500 BC until the early European contact period. When the American prairies were plowed under by European settlers for agriculture, many mound sites were lost. Effigy Mounds National Monument is the largest known collection of mounds in the United States.