Cahokia also contains five "woodhenges," circles of erect posts that served as celestial calendars, marking the seasonal solstices and equinoxes.

Cahokia is exceptional for its size and complex city structure, but it is not unique. Seventeen centuries ago, the Midwest was covered with hundreds of such precisely aligned astronomical markers and mounds.




These structures survived for close to two millennia before most were plowed over in the 19th century, paved over in the 20th century or destroyed by archaeologists digging to recover artifacts such as pipes, pottery and other religious relics.

A team from the University of Cincinnati's Center for the Electronic Reconstruction of Historical and Archaeological Sites, has been virtually piecing together the fragments of the immense existing earthworks built by three other prehistoric Native American cultures -- the Adena, Hopewell and Fort Ancient peoples -- in the area that now comprises Ohio, Kentucky, Indiana and Illinois. The people who built Cahokia were of the Mississippian culture.

Using archaeological data gleaned from remote-sensing devices that can detect remains below the ground, and infrared aerial photographs and satellite images to figure out where the earthworks had been located and what they looked like, the University of Cincinnati team is virtually rebuilding the mounds, using standard architectural rendering software. The result will be interactive programs that show how the river valleys of the Midwest would have looked when the mounds were new.

At Cahokia, most of the mounds still exist, though some were destroyed before the site was protected. Two mounds that provided a clear view of a drive-in movie theater's screen several miles away were removed in the 1960s to stop people from watching films for free.

Anthropologists said it's critical to preserve the mounds, which contain many clues about Cahokian culture. While no longer in danger of being leveled for commercial purposes, the mounds are fragile and subject to environmental degradation. State budget cuts have made it difficult to ensure that rain doesn't wash away the remnants of what is the only known prehistoric Indian city north of Mexico.

A recent excavation of a small ridge-top mound -- Mound 72 -- exposed the bodies of nearly 300 people, mostly young women believed to be sacrificial victims, who'd been buried in mass graves. Nearby is the burial site of a man believed to have been a ruler, about 45 years of age, whose body lies on a blanket of more than 20,000 shell beads, surrounded by piles of arrow tips from tribes that inhabited the present-day states of Arkansas, Oklahoma, Tennessee and Wisconsin. They were presumably given as a tribute to the deceased.

Archeologists believe other bodies buried near the ruler are the remains of those who were sacrificed to serve him in the next life. But the skeletons of four men with their heads and hands missing were also found near the largest sacrificial pit, and no one is quite sure why these bodies were mutilated before being buried.

Certainly, a headless, handless body wouldn't make for a good servant.

Every new discovery here raises more questions than it answers about Cahokia, said Bill Iseminger, assistant site manager at Cahokia Mounds.

"I believe that new archeological technology will absolutely allow us to solve many of the mysteries of Cahokia," Iseminger said. "But right now, what with the budget cuts, we're focused mostly on keeping the site intact, just trying to survive so that we can make more people aware of the complexity and brilliance of Native American culture." source

Interesting information - don't know much about it though.

I have been to the Cahokia Mounds - extremely interesting. I was there just 1 day following the autumnal equinox, there was only one woodhenge when I visited which was right at 2 years ago. The museum is incredible, very lifelike dioramas depicting the daily life of the tribe. It is reported to be a vey mystical site.
Link to official site
http://medicine.wustl.edu/~mckinney/cahokia/cahokia.html

Fascinating information about the people who once built the great prehistoric city of Cahokia was revealed accidentally during excavations in the early 1960s. Professional archaeologists were trying desperately to save archaeological information which was to be destroyed by the construction of an interstate highway, which was later rerouted. After a summer of intense excavation, Dr. Warren Wittry was studying excavation maps when he observed that numerous large oval-shaped pits seemed to be arranged in arcs of circles. He theorized that posts set in these pits lined up with the rising sun at certain times of the year, serving as a calendar, which he called WOODHENGE. After further excavations by Wittry and other archaeologists, more post pits were found where predicted, and evidence that there were as many as five Woodhenges at this location. These calendars had been built over a period of 200 years (A.D. 900-1100). Fragments of wood remaining in some of the post pits revealed red cedar had been used for the posts, a sacred wood.

The first circle, only partially excavated, (date unknown) would have consisted of 24 posts: the second circle had 36 posts; the third circle (A.D. 1000), The most completely excavated, had 48 posts; the fourth, partially excavated, would have had 60 posts. The last Woodhenge was only 12, or possible 13 posts, along the eastern sunrise arc(if it had been a complete circle, it would have had 72 posts). Building only the sunrise arc might indicate that red cedar trees had become scarce.

It is not known why the size and location of the circles, and the number of posts was constantly changed --perhaps to include more festival dates or to improve and increase alignments.

Only three posts are crucial as seasonal markers -- those marking the first days of winter and summer (the solstices), and the one halfway between marking the first days of spring and fall (the equinoxes). Viewing was from the center of the circle, and several circles had large "observation posts" at that location, where it is likely the sunpriest stood on a raised platform. Other posts between the solstice posts probably marked special festival dates related to the agricultural cycle. The remaining posts around the circle have no known function, other than symbolically forming a circle and forming an enclosure to hold the sacred Woodhenge ceremonies. There have been suggestions some posts had alignments with certain bright stars or the moon, or were used in predicting eclipses, and others have suggested Woodhenge was used as an engineering "aligner" to determine mound placements, but none of this has been proven convincingly.

The most spectacular sunrise occurs at the equinoxes, when the sun rises due east. The post marking these sunrises aligns with the front of Monks Mound, where the leader resided, and it looks as though Monks Mound gives birth the sun. A possible offertory pit near the winter solstice post suggests a fire was burned to warm the sun and encourage it to return northward for another annual cycle and rebirth of the earth. This probably marked the start of the new year.

The third circle (A.D. 1000) was reconstructed in 1985 at the original location. The circle is 410 feet in diameter, had 48 posts spaced 26.8 feet apart (9 are missing on the west side, removed by a highway borrow pit). The posts were 15-20 inches in diameter and stood about 20 feet high. Red ocher pigment found in some of the post pits suggests the posts may have been painted. The post pits averaged 7 feet long and just over two feet wide, sloping from the surface at one end to a depth of four feet at the other, forming a ramp to slide the posts down to facilitate their raising.