Owl Creek Mounds were built between 800 and 900 years ago and were used for only about 100 years. Their age was determined by dating charcoal samples collected at the site and by studying artifacts and building remains found in the mounds. Artifacts help establish a site's time period, and pottery works especially well; people of the Mississippian culture made pots with crushed mussel shell added to the clay as temper, and pottery tempered with crushed mussel shell was found in all five mounds. Building evidence also points to the Mississippian period, because wall trenches typical of Mississippian cultures were found in Mounds I, II, and IV. In this method, builders dug narrow trenches for each wall, set posts in them, sometimes in postholes at the bottoms of the trenches, and filled the trenches with dirt to hold the posts in place. Archaeologists also used radiocarbon dating of charcoal, which measures how much carbon-14 remains after living organisms such as trees or corn plants die and stop absorbing it from the atmosphere. Small charcoal samples collected from a wall trench and postholes in Mounds I, II, and V produced five radiocarbon dates spanning from A.D. 1133 to A.D. 1219.
