Parent MaterialsSection: Introduction  -  Next

Definition

Parent material is the geologic material from which soil horizons form. There are seven variations of parent material: Weathered bedrock, Till, Outwash and lacustrine deposits, Eolian Sand, Loess, Alluvium, and Local overwash. Here are the rules for distinguishing which one to pick on the scorecard.

Types of Parent Material

Weathered Bedrock (including bedrock):
  1. More than 80% of the material in the diagnostic zone is so hard that roots will not grow into it (qualifies as bedrock limiting layer), OR
  2. Less material is that hard, AND:
    1. Texture varies greatly depending on kind of rock, AND
    2. Sedimentary rock fragments, if present, are mainly angular or flat or both.
(Click for larger)
Till:
  1. Calcareous in C horizons.
  2. Pebbles, if present, are mainly igneous and mainly rounded.
  3. Not stratified (layered).
  4. Usually has platy depositional structure in C horizons.
  5. Common texture and color combinations in C horizons:
    1. Medium texture (loam) and brownish color (10YR 5/4, 5/6, 5/3), OR
    2. Moderately clayey texture (clay loam, silty clay loam) and near the brownish/gray boundary (10YR 5/3, 5/4, 5/2).
(Click for larger)
Outwash and lacustrine deposits:
  1. Pebbles, if present, are mainly rounded.
  2. Material is either:
    1. In the sandy texture group and the sands are mainly greater than 0.5 mm in diameter (qualifies as a coarse sand and gravel limiting layer if it is thick enough), OR
    2. In any texture group and is stratified (consists of layers that are clearly visible and/or are in different texture groups).
  3. Subsoil has strong soil development (to distinguish it from alluvium and local overwash).
Note: If official judges determine that the parent material is outwash or lacustrine material, but the material does not show clear stratification or the material is not coarse sand and gravel, they will give the parent material on the site card.
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Eolian sand:
  1. Sandy texture with no gravel or pebbles.
  2. Sand grains are mainly 0.1 to 0.5 mm in diameter (between 150-grit and 40-grit sandpaper).
  3. Not stratified, but may have lamellae.
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Loess:
  1. May or may not be calcareous.
  2. No or very few pebbles.
  3. Silt loam or silt texture in lower part of profile (BC or C horizon).
  4. Not stratified (in contrast to water-deposited material).
  5. Not dense (in contrast to most till).
(Click for larger)
Alluvium:
  1. Two or more distinct layers that differ in texture and/or organic matter content (color).
  2. Soil horizons in and above the parent material zone have very weak development.
(Click for larger)
Local overwash:
  1. More than 20 inches thick (but the parent material zone may not include all 20 inches).
  2. Buries a dark horizon.
  3. Soil horizons in and above the parent material zone have very weak development.
 Image N/A. See alluvium image for reference.
Note: If the material in the parent material zone does not fit any of the guidelines above, official judges will list the name of the parent material on the site card.

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