The Geoarchaeology of Iowa's Uplands

Stream valley floors are an obvious place to look for buried archaeological sites. Floods, by periodically spreading fresh layers of mud and sand across the valley floor, provide an excellent opportunity for archaeological remains to become buried. 

The hills and ridges of the uplands, however, are usually several meters higher in elevation than the valley floors, well out of the reach of flooding on the streams. Most of these landforms in Iowa are underlain by thick deposits of silty, windblown sediment known as loess, most of which was deposited prior to about 14,000 years ago, during the last Ice Age. 

In the 14,000 years since the loess stopped falling, Iowa's upland landscapes have been greatly affected by the actions of wind, water, plants, and animals. Wind and water have eroded, transported, and redeposited loess materials, wearing down hilltops, cutting back hillsides, and carving and filling gullies. All the while, the upper part of the soil has been churned by burrowing organisms and by the growth and decay of plant roots.

In the past 20 years, geoarcheological research in Iowa and neighboring states has demonstrated that many archeological sites are shallowly buried in uplands. The processes by which this burial occurs are not clear. Some think that geological processes have been most important, while others believe that the activities of organisms have played a dominant role. 

Some interesting facts about sites buried in uplands have emerged. As first noted in 1986 by Larry Abbott and Joe Tiffany (then both of the Office of the State Archaeologist), the depth at which sites are buried in uplands is dependent on age. 

Sites younger than about 1000 years 

Cultural Periods: Late Woodland, Late Prehistoric sites, historic 
Maximum Depth of Burial: 20-30 cm 
Soil Horizons Found In: A, E horizons 

Sites older than 1,000 years 

Cultural Periods: Middle, Early Woodland, Archaic 
Maximum Depth of Burial: 50-60 cm 
Soil Horizons Found In: AB, BA, BE, EB horizons 

Sites older than about 9,000 years 

Cultural Periods: PaleoIndian 
Maximum Depth of Burial: greater than or equal to 50 cm 
Soil Horizons Found In: Bt horizon