As dawn arrives on Dec. 21 at Cahokia Mounds, the sun will rise directly behind a pole hand-hewn from red cedar. Forty-eight such posts form a large circle, but only one of them will align with the sun. Winter will begin.
Four times a year, people gather at Woodhenge, a reconstruction of an ancient calendar somewhat like Stonehenge, its famous English cousin, to witness the change of seasons. The ritual dates more than 1,000 years to when as many as 20,000 so-called Mississippian people lived here in thatched huts nestled around more than 100 earthen mounds, some of them crowned with majestic temples.
Other, smaller communities took root nearby, where St. Louis and East St. Louis now sit. But the seat of power was in Cahokia, about 6 miles east of the Mississippi River. It was the largest prehistoric city north of Mexico.
Amateur archaeologists began digging for relics in the late 1800s. The first sophisticated excavations followed in the 1920s. But through much of the past century, many locals failed to appreciate the significance of the man-made mounds. As scientists winced, a subdivision of houses sprouted. So did a drive-in movie theater. The state built a campground on the site. Picnickers feasted and children frolicked where, a millennium earlier, men foraged for food using weapons of wood and stone.
By the 1960s, an interstate bisected the site. But as construction crews prepared for a second such highway, they made a discovery that would change public opinion: the rotted, red cedar remains of the original Woodhenge.
People began to take notice. Cahokia became one of just two UNESCO World Heritage Sites in mid-America. (The other is Kentucky's Mammoth Cave.) Cahokia Mounds State Historic Site was created. The houses and outdoor theater were removed. Interstate Highway 255 was relocated 3 miles to the south.
Thanks to preservation efforts, visitors can learn about the people who built this remarkable community. In the summer, they can even participate in archaeological digs.
"There was nothing else like it, before or after, in this country," said Bill Iseminger, an anthropologist and assistant site manager.
"It seemed to really start to flourish around A.D. 1050," he said. "All of a sudden, all these people were moving in. All these mounds were being built. Something happened. We don't know what exactly (but maybe) it was a charismatic leader of some kind."
Guests get their bearings at the impressive Interpretive Center. It's wise to begin by viewing the informative "Cahokia: City of the Sun," which shares the long-buried secrets of this ancient civilization.
When the movie ends, eyes pop as the screen seems to magically disappear, revealing a replica of what Cahokia probably looked like 1,000 years ago. Children romp between dwellings as men and women go about their labors. A vast mural depicts the amazing scale of the community. Actual implements — from clay pots to flint hoes and granite axes — highlight the ongoing archaeological work.
"As a result of the excavations, we're learning more about parts of Cahokia we didn't know about," Iseminger explained. "Our focus the last few years has been following what we call the stockade, or palisade, a defensive wall that went around the center part of the site."
Anyone 18 or older can volunteer to help with the digs during June and July.
"I grew up on a farm and like playing in the dirt," Rachel Kreher, of Smithton, Ill., said with a laugh. An anthropology major at Washington University in St. Louis, Kreher spent part of last summer sifting through Cahokia's soil for sometimes tiny pieces of the puzzle.
As she toiled in the rich, black earth, she admitted the work can be tedious.
"There was one unit I was working that I took down about 20 centimeters (8 inches) and found a couple of things about pea size," she said. "That was it."
"They can participate for a few days, a week, several weeks or up to the full six weeks," Iseminger said. "They get experience working on excavations, mapping, surveying, sifting and also working in the lab, washing, sorting and cataloging artifacts."
Even day-trippers can see the enormity of Cahokia as they wander through it. Many of the walkways, such as the one leading to the Grand Plaza, are paved.
One spot only hardy visitors can reach is the top of Monks Mound, which Iseminger described as "the largest prehistoric earthwork in the Americas." It rises 100 feet, and the flat terrace at the top is accessed by climbing 156 steps.
Flat-topped earthworks such as Monks were built to elevate important buildings, such as temples and the homes of leaders. Other, rounded mounds were burial sites. Some contain signs of human sacrifices.
Daybreak on the spring and autumnal equinoxes arrives with the sun rising directly over Monks Mound. Nearby, in a tradition begun in the 10th century, Woodhenge heralds the arrival of a new season.
For the winter solstice
Bill Iseminger will lecture on Woodhenge's role in Cahokia's culture on Dec. 22. His presentation will begin at 7 a.m., about 15 minutes before sunrise.
Admission to Cahokia Mounds State Historic Site (60 Ramey St., Collinsville; 618-346-5160; cahokiamounds.org) is free, but a donation of $7 for adults, $2 for children or $15 for families is suggested.
The grounds are open 8 a.m. to dusk daily. The hours and days for the Interpretive Center vary by season.