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 (date unknown), only partially excavated, 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 to 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.