Adena Complex ceremonial site in the Ohio Valley, the focus of which is a gigantic earthen mound in the form of a serpent with an egg or frog in its jaw and a tightly coiled tail. Overall, the serpent is 382 m long from the tip of the upper jaw to the end of the tail, 6 m wide, and up to 1.5 m high. The serpent itself does not appear to have been a funerary monument, although a burial mound stands nearby.
R. Fletcher et al., 1996, Serpent mound: a fort or ancient icon? Midcontinental Journal of Archaeology, 21.1, 105–43
http://ohsweb.ohiohistory.org/places/sw16/index.shtml Complete guide to the most famous of all effigy mounds, in Ohio, USA.