In western Illinois, many soil profiles developed into upland loess deposits (Peoria Silt) contain Archaic period artifacts greater than 3500 yr B.P. in stone zones below plow level. For artifacts in prairie and prairie-forest transition soils (not forest soils), depth distribution curves suggest they were buried in biomantles by small soil fauna. Artifacts of sizes archaeologists routinely collect generally move down while retaining fine-scale horizontal integrity. The process results in stratigraphic separation of Archaic and Woodland period components that otherwise would commingle at the surface. The characteristic distribution of known upland sites, narrowly rimming stream valley headwaters, reflects incomplete burial of materials on steeper forested slopes. On adjacent gently sloping upper shoulder segments of valleys where grassy cover more strongly influences soil development (and by inference on broad level uplands where true prairie soils occur), Archaic artifacts will be buried in the biomantle and go undetected by surface surveyors. © 2002 John Wiley & Sons, Inc.