Methods
Soil LSA classifications are made according to their OSD characteristics, a GIS review of their topographic position, comparison with related or adjacent soils, and available geomorphological studies. The 2024 NRCS soil geodatabase (USDA 2025b) was used to determine the landform and geographical position of the soils.
Soil Series LSA Classification Codes
Soils Formed in Holocene Alluvium, Primarily Found in River Valleys
- Camp Creek < 150 cm (e.g. Caneek)
- Camp Creek deposits of less than 1.5 m depth of recent alluvium with minimal soil development (e.g. C, AC horizons) on top of buried soils (e.g. Ab, Bb horizons).
- Found on modern floodplains and low terraces.
- Often historic in age but can bury older intact soils.
- Some Ab horizons will still consist of post settlement alluvium.
- Camp Creek > 150 cm (e.g. Nodaway)
- Camp Creek deposits of recent alluvium of more than 1.5 m depth with minimal soil development (e.g. C, AC horizons) throughout, but keep in mind that the NRCS description typically only goes to ca. 150–200 cm.
- Found on modern floodplains and low terraces.
- Often historic in age but can bury older intact soils.
- Roberts Creek (e.g. Colo)
- Well-expressed soils with dark (organic enriched), overthickened (cumulic) A horizons in the upper part of the unit.
- 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 (e.g. leaves, wood, stems, charcoal) can occur in the lower part.
- Commonly found on modern floodplains, generally in a narrow belt paralleling the modern channel.
- Usually Late Holocene in age.
- Roberts Creek/Gunder (e.g. Coland)
- This is an informally defined lithostratigraphic unit describing DeForest Formation sediments characterized by a conformable (gradational) contact between two Members.
- A dark (organic enriched), overthickened (cumulic) A horizon similar to Roberts Creek.
- Gray to olive brown Bg or Btg horizons similar to poorly drained facies of Gunder.
- In simplest terms, these soil series are interpreted as a Roberts Creek Member overlying a Gunder Member, but the contact between the two is gradational rather than abrupt.
- In pedogenic terms, the Roberts Creek Member is “welded” to the Gunder Member.
- Gunder (e.g. Festina)
- Oxidized (light colored) brown, yellowish brown, or grayish brown loamy sediments that grade to sand and gravel at depth.
- Usually early-middle Holocene in age. Early Gunder (ca. 10,500 to 4,500 BP) is strongly oxidized and Late Gunder (ca. 4,500 to 2,000 BP) is moderately oxidized.
- Poorly drained soils may exhibit reduced colors at depth.
- Unit lacks stratification except in the lower, coarser part.
- Well-developed soils formed into the upper part with strongly expressed subsoil (e.g. Bt and E) horizons.
- Plant material (e.g. leaves, wood, stems, charcoal) can be found in the lower part.
- Found on higher terrace positions and also beneath low terraces that are slightly elevated above modern floodplains.
- Gunder or Corrington (e.g. Napier)
- No soil series is directly classified as Corrington, but some soils formed in colluvium are also included under this classification.
- Corrington Member soils have at least one buried soil present, typically at the top of fining upward sequences.
- Soil series classified as Gunder or Corrington are commonly mapped on footslopes and alluvial fans where Corrington Member deposits are typically found.
- On these landforms, if a buried soil is present, the soils are Corrington, if absent the soils are Gunder.
Soils Formed in Other Materials
- Eolian sand (e.g. Sparta)
- Formed in Pleistocene or early Holocene eolian (wind-blown) sand facies of the Peoria Formation.
- Occur as dunes and sandsheets.
- Dunes in Iowa range from ca. 1 to 20 meters in height.
- Found in all landform regions but are most widespread on the Iowan Surface and high, sandy Pleistocene terraces in eastern and central Iowa.
- On Wisconsinan age terraces, the upper ca. 1 m of eolian sand deposits may be Holocene in age, reworked from the original Wisconsinan sands.
- Glacial depression (e.g. Canisteo)
- Formed on ground moraines and till plains in and near the Des Moines Lobe.
- Closed topography, often former wetlands with deep soil development.
- Glacial lake (e.g. Waldorf)
- Formed in silty lacustrine (lake deposited) sediments in and near the Des Moines Lobe.
- Sediment deposition may be continuous throughout the Holocene.
- Glacial till (e.g. Clarion)
- Formed in Pleistocene till diamictons. Unsorted or poorly sorted conglomerates of rock fragments that are generally varied in lithology and size.
- E.g. Dows and Sheldon Creek Formations.
- Found primarily on the Des Moines Lobe and northwestern Iowan Surface.
- Glaciofluvial (e.g. Estherville)
- Formed in Pleistocene outwash sediments.
- Sometimes occurs as supraglacial deposits on uplands and is referred to as the Morgan Member (e.g. Dows Formation, Morgan Member).
- Also underlies late Wisconsinan terraces (e.g. Henry Formation in the Mississippi Valley and Noah Creek Formation in other Iowa valleys).
- On terraces, a surface mantle of eolian (wind-blown) sediments or fine grained DeForest Formation sediments may overlie the coarse sediments.
- Glaciofluvial/lacustrine (e.g. Medary)
- Formed in Pleistocene alluvium that includes lacustrine (lake deposited) or slack water sediments from Lake Superior sourced meltwater floods.
- Found primarily in the Mississippi River Alluvial Plain.
- Loamy sediments (e.g. Churchtown)
- Formed in eolian (wind-blown) or hillslope deposits.
- Surface texture ranges from silt loam to sandy loam.
- Can be of any age.
- Loess-mantled terrace (e.g. Monona)
- Formed in loess in the south half of the Southern Iowa Drift Plain. Wind-blown fine silty deposits.
- Occurs on terraces and benches that have a surface mantle of late Wisconsinan loess.
- These map units can be underlain by early to middle Holocene Gunder Member alluvium instead of loess beneath low terraces.
- Can be of any age.
- Loess-mantled terrace, thick loess (e.g. Fayette, Clinton, Tama)
- A combined LSA, widespread in most of Iowa, except in the Des Moines Lobe and Iowan Surface.
- Soil series express as a loess-mantled terrace in river valleys and thick loess in uplands.
- Mucks/peats (e.g. Blue Earth)
- Formed in organic sediments (peat and muck), primarily in the Des Moines Lobe and Iowan Surface (e.g. Woden Member).
- Late Wisconsinan through Holocene age sediments filling semi-closed and closed depressions.
- Alternating zones of fine-grained colluvium, peat, and muck.
- Can be buried by Camp Creek deposits.
- Mineral sediment is usually a reduced light gray color at base, grading upward to black, organic-coated sediment.
- Shallow to bedrock (e.g. Nordness)
- Shallow soils where bedrock occurs within 150 cm of the surface.
- A surface mantle of loamy hillslope or erosional surface sediments may be present.
- Pre-Wisconsinan paleosols may be present.
- The bedrock may be overlain by residuum from the in-situ weathering of bedrock.
- Shallow to pre-Wisconsinan paleosol (e.g. Keswick)
- Formed in pre-Wisconsinan hillslope and alluvial sediments in the Southern Iowa Drift Plain.
- Parent material is a pre-Wisconsinan paleosol. An ancient, weathered, buried former stable land surface.
- Shallow to till (e.g. Lindley)
- Shallow soils where pre-Illinoian till occurs within 150 cm of the surface.
- Very common throughout most of Iowa, but less in the Des Moines Lobe and Paleozoic Plateau.
- A surface mantle of hillslope or erosional surface sediments.
- Thick loess (e.g. Ida)
- Formed in loess deposits that are more than 150 cm thick. Wind-blown fine silty deposits.
- Parent material is Peoria Formation silt facies.
- A very common upland LSA in the Southern Iowa Drift Plain and Loess Hills.
Classifying Soil Names to Geologic Materials
- Data Source: The NRCS Soil Survey Geographic Database (SSURGO). The first edition of Ackmore to Zwingle used its predecessor, the Iowa Soil Properties and Interpretation Database (ISPAID).
- SSURGO identifies the parent material and landscape position of soils for individual soil map units (SMU). Values for these two variables are usually sufficient to classify SMUs to a geologic landform/sediment assemblage (LSA), or minimally to identify the geologic material in which the soil formed.
- LSA classifications were aided by previous classifications of soil data compiled for parts of southeastern Iowa and the Des Moines Lobe by Deb Quade and others (Iowa Geological Survey), and for parts of western Iowa by Joe Artz.
- The remaining SMUs, located primarily in northeast and northwest Iowa, were classified using the database landscape/parent material variables and OSD information.
- The soil series formed in Holocene alluvium were classified to DeForest Formation units using the above sources, landscape position/parent material values, and taxonomic classification and horizonation data from OSDs. Michael Perry (Office of the State Archaeologist) assisted in the classification.
This is a generalized account of the methods used. Many of the procedures entailed intermediate steps of data cleaning and normalization.
Version Log
- v 1.0 (08/01/05). HTML version with hyperlinks to OSDs; not released to public.
- v 2.1 (09/12/05). ASP version, linked to database, with classifications by parent materials. Alphabetic menu and soil parent material menus display series subsets. Purpose, Appropriate Use, and Methods sections added.
- v 2.2 (11/23/05). Substantial reclassification of parent materials, particularly for upland series. Addition of checkboxes to display MLRA, Parent Materials, and Landscape Positions from ISPAID.
- v 3.0 (08/03/17). Redesign of website, including major changes to styles and layout. Content and linkage to OSD remains the same.
- v 4.0 (9/25/25). In addition to their LSA classification, each soil series now has a summary of their Natural Resources Conservation Service (NRCS) official series description (OSD) (USDA 2025a) and horizonation in the table.
62 new soil series have been added to the site.