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Irregular tonguing horizons and those that occur sporadically in soil sections are not only difficult to describe consistently but also pose problems in correlating and classifying soil series. Average measurements of the frequency of occurrence and of the maximum and minimum depths of horizon variations can be used to describe and compare these variations in soils. Theoretical values of these parameters are combined and grouped into five variability classes that serve as guidelines with which actual field data can be compared.

A method for establishing a modal profile and separating inclusions in mapping units is presented. The statistical concepts and implications are discussed. Horizon thicknesses of 100 soil profiles in a 10-by-10 grid on a stable, remnant, upland terrace were used as the basic data to demonstrate the method. All horizon thicknesses did not follow normal frequency distributions, the Ae and AB were skewed and the depth to carbonates exhibited a tendency to a bi-modal distribution, whereas the Ah and B horizons approached normal distributions.