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Geoarchaeology in Iowa
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.
Links to General Descriptions and Photographs of the
four members
Camp Creek
Member:
Stratified Sediments
(Usually Historic 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 DESCRIPTION
General Characteristics of DeForest Formation Members
in Eastern Iowa
|
Member
|
Lithology
|
Landscape Position
|
Age
|
| 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 |
| Gunder |
- 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 |
| Corrington |
- 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 9000 to about 2500 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 Period |
Years Before Present |
Corrington
|
Gunder
|
Roberts Creek
|
Camp Creek
|
| PaleoIndian |
|
+ +
|
+ +
|
-
|
-
|
| Early to Middle Archaic |
|
+ +
|
+ +
|
-
|
-
|
| Late 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.
Goearchaeology in Iowa | Stream Valleys | Geoarchaeology of the Uplands in Iowa
| Geoarchaeological Methods in Iowa
Updated by Tricia R. Bender, Web Designer
Mary De La Garza, Web Master
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