Horizons: BC, C/Bt, Ct, or C horizon with thick clay coatings
Saprolite (the light-colored area) is weathered bedrock which still retains the original lithic fabric and characteristics. The nature of the saprolite is influenced by the type of rock from which it develops, and it determines the chemical and physical properties of the associated soils. A common characteristic of the highly weathered finer textured granitic saprolite is a light, fluffy feel (low bulk density).
In soil science, the "C" horizon is the soil layer consisting of more or less weathered parent rock or deposited material that is little affected by pedogenesis (soil formation). However, if an overlying horizon contains a significant amount of clay, over time, the clay may be transported into and along vertical cracks or along channels within macropores creating thick clay coats or clay flows.
The question for this layer is the appropriate horizonation?
Clay films are a coating of oriented clay on the surface of sand grains (clay bridging), soil aggregates, or peds. Clay films also line pores or root channels. This form of orientated clay is considered a pedogenetic process resulting in diagnostic soil features and is most commonly associated with a structured "B" horizon.
The zones of clay accumulation (smooth brown area) in this substratum appears to be inflows at their thickest and weathered in situ where thinly layered--both areas absent of any ped formation or structure.
The "t" designation is most commonly associated with an argillic horizon. It indicates an accumulation of silicate clay that either has formed within a "horizon" and subsequently has been translocated within the horizon or that has been moved into the horizon by illuviation, or both. At least some part of the horizon shows evidence of clay accumulation, either as coatings on surfaces of peds or in pores, as lamellae, or as bridges between mineral grains.
However, is the "t" designation appropriate with any layer where clay coats (films) are present? It has been recognized with non-pedogenic materials such as paralithic materials where the faces of pararock fragments are coated with clayey material (Crt). Therefore, is a "Ct" designation appropriate where clay coats are present on plains of separation or vertical cracks. (See footnote--Keys to Soil Taxonomy, p. 340; "Indicates weathered bedrock or saprolite in which clay films are present.")
A C/B horizon has discrete, intermingled bodies of two horizons: C material dominates, with lesser but discrete bodies of B material; however, is this horizonation appropriate if the "B" part is entirely structureless translocated clay?
This condition leads to a possible separation of the historical pedogenic clay films from in-filling of clayey material, i.e., "clay flows".
For more information about describing and sampling soils, visit:
www.nrcs.usda.gov/resources/guides-and-instructions/field...
or Chapter 3 of the Soil Survey manual:
www.nrcs.usda.gov/sites/default/files/2022-09/The-Soil-Su...
For additional information on "How to Use the Field Book for Describing and Sampling Soils" (video reference), visit:
www.youtube.com/watch?v=e_hQaXV7MpM
For additional information about soil classification using USDA-NRCS Soil Taxonomy, visit:
www.nrcs.usda.gov/resources/guides-and-instructions/keys-...
or;
www.nrcs.usda.gov/resources/guides-and-instructions/soil-...
Horizons: BC, C/Bt, Ct, or C horizon with thick clay coatings
Saprolite (the light-colored area) is weathered bedrock which still retains the original lithic fabric and characteristics. The nature of the saprolite is influenced by the type of rock from which it develops, and it determines the chemical and physical properties of the associated soils. A common characteristic of the highly weathered finer textured granitic saprolite is a light, fluffy feel (low bulk density).
In soil science, the "C" horizon is the soil layer consisting of more or less weathered parent rock or deposited material that is little affected by pedogenesis (soil formation). However, if an overlying horizon contains a significant amount of clay, over time, the clay may be transported into and along vertical cracks or along channels within macropores creating thick clay coats or clay flows.
The question for this layer is the appropriate horizonation?
Clay films are a coating of oriented clay on the surface of sand grains (clay bridging), soil aggregates, or peds. Clay films also line pores or root channels. This form of orientated clay is considered a pedogenetic process resulting in diagnostic soil features and is most commonly associated with a structured "B" horizon.
The zones of clay accumulation (smooth brown area) in this substratum appears to be inflows at their thickest and weathered in situ where thinly layered--both areas absent of any ped formation or structure.
The "t" designation is most commonly associated with an argillic horizon. It indicates an accumulation of silicate clay that either has formed within a "horizon" and subsequently has been translocated within the horizon or that has been moved into the horizon by illuviation, or both. At least some part of the horizon shows evidence of clay accumulation, either as coatings on surfaces of peds or in pores, as lamellae, or as bridges between mineral grains.
However, is the "t" designation appropriate with any layer where clay coats (films) are present? It has been recognized with non-pedogenic materials such as paralithic materials where the faces of pararock fragments are coated with clayey material (Crt). Therefore, is a "Ct" designation appropriate where clay coats are present on plains of separation or vertical cracks. (See footnote--Keys to Soil Taxonomy, p. 340; "Indicates weathered bedrock or saprolite in which clay films are present.")
A C/B horizon has discrete, intermingled bodies of two horizons: C material dominates, with lesser but discrete bodies of B material; however, is this horizonation appropriate if the "B" part is entirely structureless translocated clay?
This condition leads to a possible separation of the historical pedogenic clay films from in-filling of clayey material, i.e., "clay flows".
For more information about describing and sampling soils, visit:
www.nrcs.usda.gov/resources/guides-and-instructions/field...
or Chapter 3 of the Soil Survey manual:
www.nrcs.usda.gov/sites/default/files/2022-09/The-Soil-Su...
For additional information on "How to Use the Field Book for Describing and Sampling Soils" (video reference), visit:
www.youtube.com/watch?v=e_hQaXV7MpM
For additional information about soil classification using USDA-NRCS Soil Taxonomy, visit:
www.nrcs.usda.gov/resources/guides-and-instructions/keys-...
or;
www.nrcs.usda.gov/resources/guides-and-instructions/soil-...