The Geoarchaeology of Iowa's Stream Valleys

Stream valley floors are underlain by deposits of alluvium, defined as sediment (mud, sand, and gravel) laid down by flowing water. The largest particles (sand and gravel) tend to accumulate within the channel itself. Particles of clay, silt, and fine sand are small enough to be suspended in flowing water. When the stream overflows its banks, these particles can be distributed across the valley floor. These overbank deposits are the most common contexts in which buried archaeological sites are found in the stream valleys of Iowa. 

During the last 10,000 years (a period of time known as the Holocene), streams have deposited considerable thicknesses of alluvium in Iowa’s valleys. The state’s Geological Survey Bureau has classified this alluvium into four lithostratigraphic units, referred to as the Camp Creek, Roberts Creek, Gunder, and Corrington Members of the DeForest Formation. The DeForest Formation provides archaeologists with a powerful model for finding, dating, and studying archaeological sites buried beneath valley floors in Iowa. Before beginning a survey project in a valley setting, the model can be use to predict the archaeological potential of the study area to yield sites of particular cultural periods. The model can also be used to predict anticipated depths of burial, and can be used to select geoarchaeological field methods for finding and studying buried archaeological sites.

General descriptions of the four members 

Camp Creek Member

Stratified Sediments
(Usually Historic in Age)

The sediments are layered in horizontal beds that vary in thickness.
  • In the upper 25 cm of the unit, stratification may be disrupted by plant rooting and animal burrowing.
  • Often contains artifacts dating to the historic period (tin cans, fence wire, glass, crockery.
  • Modification of the sediments by soil formation is minimal
  • Is thickest in areas adjacent to the modern channel.
  • Usually buries the presettlement soil.
  • Dates from 400 years before present to modern times.

Roberts Creek Member

Dark Colored (Organic-Enriched) Sediments
(Usually Late Holocene in Age)

Very dark gray to grayish brown loamy sediments grading to sand and gravel at depth
  • Unit lacks stratification except in the lower, coarser part.
  • Plant material (leaves, wood, stems, charcoal) is common in lower part
  • Well-expressed soils with overthickened A horizons (topsoil layers) are developed into the upper part of the unit.
  • Unit is found on modern floodplains, generally in a narrow belt paralleling the modern channel.
  • Sediments were deposited between 4000 and 500 years before present.

Gunder Member

Oxidized (Light-Colored) Sediments
(Usually Early-Middle Holocene in Age)

Brown, yellowish brown, or grayish brown loamy sediments that grade to sand and gravel at depth
  • Unit lacks stratification except in the lower, coarser part
  • Plant material (leaves, wood, stems, charcoal) is common in lower part
  • Moderately well-developed soils, formed into upper part, have well-expressed B and E (subsoil) horizons.
  • Unit is usually found beneath low terraces slightly elevated above modern floodplains. The surface expression of the terrace is often subdued because of an overlying mantle of Camp Creek Member sediments.
  • Radiocarbon ages from the unit range from about 10,500 to about 3,000 years before present. 

Corrington Member

Sediments with Buried Soils
(Usually Early-Middle Holocene in Age)

The unit consists of vertically stacked sediment packages, with a buried paleosol developed in the upper part of each package.
  • Each package is a fining upward sequence, with the coarsest textures (commonly sandy loam or sand) at the base, and finer textures (silt loam, loam, silty clay loam) at the top.
  • Unit is found beneath alluvial fans and colluvial slopes along the margins of large- and moderate-sized valleys.
  • Deposited between about 9000 and 2500 years before present.

Technical Descriptions

General Characteristics of DeForest Formation Members in Eastern Iowa
MemberLithologyLandscape PositionAge
Camp Creek
  • very dark grayish brown to yellowish brown (10YR 3/2-5/4)
  • silt loam to loam (sandy loam if sandy source materials are common) grading to sand and gravel in the channel belt;
  • horizontally stratified where greater than 0.25 m in thickness;
  • surface soils are Entisols (A-C profiles)

unit often buries pre-settlement surface soil;

thickest in and adjacent to modern channel belt and at the base of steep slopes.

400 years before present to modern
Roberts Creek
  • very dark gray to dark grayish brown (2.5Y3/0 to 10YR 3/1-3/2)
  • silt loam; silty clay loam and loam grading downward to sand and gravel
  • thick sections are stratified at depth
  • detrital organic matter in lower part
  • relatively thick Mollisol (A-C or A-Bw-C) profile developed in upper part
  • strong brown and yellowish red mottles may occur throughout unit

found within modern floodplain

usually parallels modern channel

4000 to 500 years before present
  • brown to yellowish brown to grayish brown (10YR 4/3-5/4 to 2.5Y5/2)
  • silt loam; clay loam; or loam grading to sand and gravel at depth
  • lower part may be stratified
  • detrital organic matter ofter present in lower, stratified, coarse part of unit
  • moderately well to somewhat poorly drained Mollisols and Alfisols (A-Bw-C, A-Bt-C, or A-E-Bt-C profiles) developed in upper part
  • C horizons usually contain strong brown, yellowish red or dark brown mottles

usually comprises low terrace that merges with valley wall sideslope in smooth concave upward profile

10,500 to about 3,000 years before present
  • Dark grayish brown to yellowish brown to olive brown (10YR 4/2-5/4 to 2.5Y4/2-4/4) loam and silty clay loam with sandy loam; pebbly sandy loam and gravelly interbeds
  • upper part of unit has thick Mollisol or Alfisol (A-Bw-C, A-Bt-C, or A-E-Bt-C profiles)
  • at least one and often several buried paleosols within unit
  • units consists of several fining-upward sequences, most having paleosols developed in their upper part
  • brown mottles common

found in alluvial fans and colluvial slopes along the margins of large to moderate-sized valleys

about 9,000 to 2,500 years before present

Source: Bettis, E. Arthur III, et al. (1991), Guidebook Series 12, Iowa Department of Natural Resources, Geological Survey Bureau, Iowa City.

Archaeological Potential

To determine where buried sites of a given cultural period will be found in a valley, it is first necessary to know where sediments of that age are preserved. As shown in the following table, buried sites of the PaleoIndian, Early Archaic, and Middle Archaic periods will be found only in alluvium of Early and Middle Holocene age (i.e., Corrington and Gunder Members). Buried sites dating from the Late Archaic through Late Prehistoric will be found primarily in the Late Holocene Roberts Creek Member. Historic sites may occur in the Camp Creek Member and in the upper part of the Roberts Creek Member.

  DeForest Formation Member
Cultural PeriodYears B.P.CorringtonGunderRoberts CreekCamp Creek
Paleoindian ++++--
Early to Middle Archaic ++++--
Lake Archaic +++++-
Woodland +--++-
Late Prehistoric +--+++-
Historic --+-++

- not possible; +- low potential; + moderate potential; ++ high potential

Source: Bettis, E. A., III, and D. W. Benn (1984) Plains Anthropologist 29:211-228.