In June 1976, the Native Americans’ former winter camp (known to archaeologists as the Rainbow Site) was discovered while planning for the Held Creek Watershed, an erosion and gully-control project in southwestern Plymouth County managed and funded by the U.S. Department of Agriculture, Natural Resources Conservation Service (NRCS). Since passage of the National Historic Preservation Act of 1966 and the National Environmental Policy Act of 1969, environmental impact statements (EIS) are required for federally funded projects. The purpose of an EIS is to ensure that the environment is not being adversely impacted by the project, or if it is, to mitigate the impact. Part of the environmental assessment involves inventory and evaluation of the historic and prehistoric cultural resources within a project area.

Archaeologists usually walk over plowed fields and dig shallow test pits to search for artifacts and other evidence of prehistoric human activity. These techniques work well in upland locations and other portions of the landscape where prehistoric sites are not deeply buried. In valleys, however, deep burial is common, and the difficulty of the archaeologists’ job is compounded.

Since 1976, geologists working with archaeologists in Iowa have begun to unravel the sequence of geologic deposits in which the archaeological record is preserved. This work has brought to light little-known aspects of culture history and has raised questions about the distribution and abundance of archaeological sites.

It is important to realize that the archaeological record is a product of both cultural and geologic factors. Where and when people engage in activities and leave behind artifacts is a cultural phenomenon. Once a site is abandoned, however, whether or not it is preserved and becomes part of the archaeological record is a geologic phenomenon. This aspect of preservation is especially important in valleys, where stream erosion regularly removes older deposits. Equally important in assessing the archaeological record is the potential for younger deposits to bury sites and prevent their detection. These two geologic factors, erosion (destruction) and burial, profoundly shape the archaeological record as well as our perceptions of that record.

Charcoal, ash, and fired earth are seen in the cross-section of the shelter’s hearth.

In western Iowa, the inventory of known archaeological sites is dominated by those less than 2,000 years old. Scattered evidence, however, indicates that the region was occupied at least 8,000 years ago. Following discovery of the Rainbow Site, the NRCS initiated a study aimed at dating episodes of gully growth and filling during the last 10,500 years and tracing distinct gully fills throughout the region. Six distinct fills were present in the area. Each of these accumulated during a specific interval of time and therefore has specific archaeological associations.

Mapping the distribution of these deposits permits assessment of the geologic potential for a valley to contain archaeological remains from the various culture periods defined by archaeologists. This assessment enables archaeologists to determine which methods are needed to locate cultural resources in an area, and also helps planners avoid impacting high-potential areas, thereby decreasing the need for costly mitigations.

The western Iowa studies demonstrated that abundant remains of pre-2,000-year-old occupations are deeply buried in valleys and alluvial fans. The systematic locating of these sites and our subsequent increase in knowledge of these early inhabitants represent a frontier in Iowa archaeology.

Changing styles in decorative patterns of prehistoric pottery can date both cultural sites and geologic deposits enclosing them.

Another example from this rapidly expanding field of archaeological geology is the combined archaeological and geological investigations of the central Des Moines River valley, undertaken to provide the U.S. Army Corps of Engineers with cultural resource information needed for planning recreational development and interpretive programs in the Saylorville Lake area. Since the 1960s, many prehistoric sites have been recorded in this area, but most date from the last 2,000 years. Few deeply buried and stratified sites were recorded prior to the 1984 geologic studies. Stratified sites are especially important to archaeologists because they can show successive changes in diagnostic artifacts that can be used to date sites that are not stratified. In addition, bone, ceramics, and earthen features such as storage pits are better preserved in buried sites.

Five valley-landform areas were recognized in the Saylorville Lake area; each of these contained deposits that accumulated during a specific portion of the last 11,500 years. Just as in the western Iowa gully fills, archaeological associations and the geologic potential for buried sites from individual culture periods varied in each landform area. Combining geological mapping with the archaeological study revealed that sites older than 2,000 years are not rare, but are rarely evident at the present land surface. Now archaeologists know where in the valley these sites are likely to be preserved and that subsurface methods are needed to find them. It was also discovered that even the youngest sites in the valley can be buried and thus “invisible” from the surface using traditional site-locating techniques. Geologic investigations revealed that extensive deposits of historic floodplain alluvium covered previously undiscovered sites of the Oneota Culture, the most recent prehistoric occupants of the valley above Des Moines. These studies have improved our understanding of the culture history of the Saylorville Lake area and have pointed toward productive avenues of future research.

Archaeological geology continues to grow in its applications and scope in Iowa and elsewhere. It holds promise for unraveling some enigmas of archaeological site distribution and culture history. The results of archaeological-geology studies benefit archaeologists, planners, conservationists, and, through more effective use of federal funds, all taxpayers.