The Mystery of ‘Woodhenge’ and Its Connection to the Equinox

Nearly 1,000 years ago, immense circles of soaring wooden poles towered over the fertile Mississippi River floodplains, several miles east of present-day St. Louis. Known today as Woodhenge, the timber monuments erected by inhabitants of the ancient city of Cahokia appear to have marked the seasons and tracked movements of the sun, moon and stars.

Archaeologists have unearthed traces of at least five vast wooden rings, numbering between 24 and 72 red cedar posts, built during the height of Cahokia, one of North America’s largest pre-Columbian settlements. While their exact purpose remains unknown, the Woodhenge circles are believed to have been a focal point for religious ceremonies that drew pilgrims from hundreds of miles away during significant celestial events such as the autumnal equinox.

Centuries before the European settlement of North America, a great metropolis flourished in the continent’s heartland. Home to some 20,000 people at its peak—a population rivaling those of medieval London and Paris—Cahokia sprawled over 6 square miles that included residential neighborhoods, farms, pyramids, expansive public plazas and more than 100 massive terraced earthen mounds.

The largest pre-Columbian city north of Mexico, Cahokia was built by people of the Mississippian cultural tradition, which emerged across the American Midwest and Southeast before A.D. 1000. Mississippian societies developed complex social structures, a sophisticated agricultural system and intricately planned cities aligned with the heavens.

By orienting homes and temples to the position of cosmic bodies, Cahokia’s planners integrated their worship of the sky into the city’s physical layout, explains Timothy Pauketat, a professor emeritus of anthropology at the University of Illinois and author of Cahokia: Ancient America’s Great City on the Mississippi.

“The stars, moon and sun were viewed as supernatural entities," says Pauketat. "People gathered in places to commune with gods and heavenly bodies moving around the sky."

After decades of decline, Cahokia was mysteriously abandoned by 1350. Prairie grass eventually carpeted its ruins, which lay largely undisturbed for centuries.

Although the first detailed account of Cahokia was written in 1814 by amateur historian Henry M. Brackenridge, the abandoned city attracted little investigation. That changed in the early 1960s when plans for a pair of interstate highways slicing through Cahokia prompted extensive archaeological excavations.

In addition to unearthing evidence of Cahokia’s extensive footprint, crews led by archaeologist Warren Wittry discovered a curious collection of oval-shaped pits. Further investigation found the placement of those pits was anything but random. The holes formed at least five perfect circles, four of them overlapping, with diameters ranging from 240 to nearly 480 feet.

Additional digs led by archaeologist Robert Hall and the discovery of a huge log embedded in one of the pits helped Wittry recognize the holes as foundations for upright marker posts. The postholes—averaging 7 feet long and 2 feet wide—included sloped ramps that allowed Cahokia’s builders to maneuver 20-foot-tall red cedar poles into place.

The circles appear to have been rebuilt on an increasingly larger scale. The earliest circle contained 24 posts and later versions added posts in increments of 12. Wittry dubbed the timber rings “the American Woodhenge” because of their similarities to Stonehenge in England.

Traces of red ochre pigment suggest the posts, made from red cedar trees native to the area, were likely painted. “The trees probably came from the bluffs next to Cahokia, which is an important area,” says Pauketat. “It's a place where you see many burials, so the cedars from those bluffs would have been sacred.”

With no written records left by its builders, however, archaeologists are left to surmise the purpose of these massive circles.

Wittry surmised that Woodhenge functioned as a solar calendar, tracking the sun’s movement and marking sunrise on key occasions such as equinoxes and solstices. Those alignments might have helped Cahokia’s inhabitants to time agricultural cycles and mark dates for sacred ceremonies, communal feasts and sporting events that drew travelers from far beyond the city.

From the vantage point of the single post that stood at Woodhenge’s center, the winter solstice sun rose over a perimeter post aligned with Fox Mound, one of the many earthen mounds built at Cahokia. The equinox sun rose due east above a pole midway between the solstice posts. That sightline likely aligned with the 100-foot-tall Monks Mound. Archaeologists theorize that as Monks Mound grew taller and altered views of the rising sun, Cahokians might have built progressively larger Woodhenge circles to preserve those celestial sightlines.

Since only three posts served as seasonal markers and the posts were arranged in multiples of 12, archaeologists such as Hall speculated that Woodhenge might have tracked more than the sun. Some have proposed alignments with the moon, the Milky Way and other celestial bodies.

“Many of the houses, especially temples, of Cahokia are very rigidly aligned but very seldom to the sun. They're aligned mostly to a position of the moon or to the Milky Way,” says Pauketat, who notes that many questions about Woodhenge’s purpose remain unanswered. “Part of the reason is that we don't have a good chronology of the big mounds sitting to the east of it, which matters when interpreting observations of the rising sun or moon.”

Centuries after its construction, Woodhenge still draws visitors to Cahokia. In 1985, the Cahokia Mounds World Heritage and State Historic Site erected a reconstruction of the third timber circle, placing its 48 posts in their original locations. Today, the site hosts sunrise observations on each equinox and solstice.