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Glossary <br />Many of the terms relating to landforms, geology, and geomorphology are defined in <br />more detail in the "National Soil 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 of <br />nearly static glacial ice, either contained within or accumulated on the surface of <br />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 aerated <br />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, such <br />as granules, blocks, or prisms, are called peds. Clods are aggregates produced <br />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, narrower, <br />and steeper than a fan and is composed of coarser and thicker layers of material <br />deposited by a combination of alluvial episodes and (to a much lesser degree) <br />landslides (debris flow). The coarsest materials tend to be concentrated at the <br />apex of the cone. <br />76 <br />