Zone within the pedo-stratigraphic column that represents a change in lithology or sediment type. It may also mark a zone of nondeposition or erosion. Soil horizons below a lithologic discontinuity are designated as such by adding an "2" in front of the horizon designation (for the second parent material), e.g., 2Bt. If two lithologic discontinuities are present, a "3" is used for the third parent material, counting from the surface downward.