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Glossary <br /> Many of the terms relating to landforms, geology, and geomorphology are defined in <br /> more detail in the following National Soil Survey Handbook link: "National Soil <br /> Survey Handbook." <br /> ABC soil <br /> A soil having an A, a B, and a C horizon. <br /> Ablation till <br /> Loose, relatively permeable earthy material deposited during the downwasting <br /> of nearly static glacial ice, either contained within or accumulated on the surface <br /> of the glacier. <br /> AC soil <br /> A soil having only an A and a C horizon. Commonly, such soil formed in recent <br /> alluvium or on steep, rocky slopes. <br /> Aeration, soil <br /> The exchange of air in soil with air from the atmosphere. The air in a well <br /> aerated soil is similar to that in the atmosphere; the air in a poorly aerated soil is <br /> considerably higher in carbon dioxide and lower in oxygen. <br /> Aggregate, soil <br /> Many fine particles held in a single mass or cluster. Natural soil aggregates, <br /> such as granules, blocks, or prisms, are called peds. Clods are aggregates <br /> produced by tillage or logging. <br /> Alkali (sodic) soil <br /> A soil having so high a degree of alkalinity (pH 8.5 or higher) or so high a <br /> percentage of exchangeable sodium (15 percent or more of the total <br /> exchangeable bases), or both, that plant growth is restricted. <br /> Alluvial cone <br /> A semiconical type of alluvial fan having very steep slopes. It is higher, <br /> narrower, and steeper than a fan and is composed of coarser and thicker layers <br /> of material deposited by a combination of alluvial episodes and (to a much <br /> lesser degree) landslides (debris flow). The coarsest materials tend to be <br /> concentrated at the apex of the cone. <br /> 31 <br />