|MadSci Network: Earth Sciences|
I believe the misnomer is the word "type". The US Dept of Agriculture is the premier authority on soil classification, specifically the branch known as the Soil Conservation Service (or Natural Resource Conservation Service).
There are thousands of specific soil "types" determined by many factors, one of which is texture analysis. USDA texture analysis is based on percent sand and clay, or particle sizes below 2.0 mm. General soil types are described as "sandy loam" or "silty clay" for example, while specific types, based on location, scientist or local dignitary name, and characteristics, have descriptions such as "Windthorst fine sandy loam" and "Houston Black Clay". Gravel, cobbles, rocks and boulders are not used in this scheme. These may be generally classified as "rocks". Soils are made of living and nonliving materials, including weathered rock (clay, minerals and sand), silt, humic substances, bacteria and water. So, to be short, gravel is not a "type" of soil, but a size of rock, which can be weathered (broken down) into soil if added to other components.
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