Although there are no gigantic earthworks or monuments, the Koster site is one the richest sites in information about several groups and their dependence on the environment. Koster gives us a complex record of the technologies and food sources for a wide range of inhabitants through time.
The Koster site is located in Section 21, T 9 N, R 13 W Greene County in the lower Illinois Valley. The Site was test excavated in the summer of 1969, under the direction of Stuart Struever of Northwestern University.
Originally it was defined on the basis of debris from a Late Woodland occupation on a ridge top. The 1969 tests at Koster also identified possible older occupations beneath the Late Woodland occupation. Little did they know at the time, that they would find several occupations dating back to the Early Archaic Period.
Dates of the Koster Horizons (Struever 56).
| Horizon | Date of Occupations | Period |
| 1 | A.D. 1000-1200 | Mississippian period |
| 1 | A.D. 400-1000 | Late Woodland period |
| 2 | 200-100 B.C. | Early Woodland period |
| 3 | 1500-1200 B.C. | Late Archaic period |
| 4 | 2000 B.C. | Late Archaic period |
| 5 | ||
| 6 | 3900-2800 B.C. | Middle Archaic period |
| 7 | ||
| 8 | 5000 B.C. | Middle Archaic period |
| 9 | ||
| 10 | 6000-5800 B.C. | Early Archaic period |
| 11 | 6400 B.C. | Early Archaic period |
| 12 | ||
| 13 | 7500-6700 B.C. | Early Archaic period |
There were actually 26 occupations at Koster. Only 13 were originally recognized.
Early Occupation (6400 - 3900 BC)
By 6400 BC, Early Archaic people had established a semi permanent village, probably used on an annual cycle. Horizon 11 was occupied for a duration, with over thirty fire hearths discovered and several metates found. This is the earliest evidence for grinding technology in North America. Stone adzes were also found in this level, preceding the 5000 BC dates at other sites. It is speculated that these adzes could have been used to make simple canoes. Bone awls and needles also suggest that they may have engaged in Horizon 10 shows a different type of site at Koster - stone tool manufacturing. Koster was used almost exclusively as a stone tool site from 6000-5800 BC. Koster is located near two major sources of chert - an essential material for stone tools.
Horizon 8 contains the earliest permanent houses in North America. Posts for these houses are larger than 10 inches in diameter. More advanced stone tools appeared in this site, many of which were used for woodworking. Plant and animal remains also suggest a possible year-round habitation.
Middle Occupation (3900 - 100 BC)
Around 3900 BC, a long-term village was again established at Koster. During a period of about 1000 years, villages flourished in this area, gathering and harvesting the annual resources of ducks, geese, fish, nuts and cereal like seeds. Communities grew to about 100-150 people at this time. Skeletal remains in horizon 6 indicate that some members of the community lived to be 60-70 years. Copper and other trade goods were found in abundance, along with technological improvements in metates. Although clay hardened fire pits were discovered in this level, pottery wasn't developed in this area until almost 3000 years later.
After a period of little inhabitation, Koster became a deer butchering base around 2000 BC. The climate was becoming cooler and wetter around this time. This meant richer varieties of game in the area. At horizon 3, Late Archaic people left little debris. Horizon 2 yields the first true pottery of the area.
Recent Occupation (100 BC - AD 1400)
During the Middle Woodland period (circa 100 BC - 450 AD), there was a large increase in population at Koster. This can be attributed to agriculture. Although the inhabitants at Koster did not depend primarily on agriculture until around 900 AD, it provided a more stable food source. The number of settlements increased, and the village covered a much larger area than in previous periods. Over 1000 inhabitants lived at the site (horizon 1) around AD 800. Around AD 1400 however, there was a substantial decrease in population.
Struever and Holton, Koster: Americans in Search of Their Prehistoric Past.
Edwin R. Hajic, Koster Site Archeology I: Stratigraphy and Landscape
Houart, Koster: A Stratified Archaic Site in the Lower Illinois Valley. Illinois Valley Archeological Program, 1971.