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Modal slope is defined as the most common class of slope occurring in a landform pattern.
Where slope classes have been obtained by systematic sampling, define the classes
using equal increments on a scale of the logarithm of the slope tangent, a procedure
intended to normalise frequency distributions of observed slope (Speight 1971). Where
the most common slope class is estimated by direct observation, it is thought that
the estimate will compare with that calculated using the log-normal model.
Modal slope class determines the use of certain adjectives applied to landform patterns
that are characterised by alternating crests and depressions. These are: rolling for
moderate modal slopes (10–32%); undulating for gentle slopes (3–10%); and gently undulating
for very gentle slopes (1–3%) (compare with Soil Survey Staff 1951, pages 161–165).
The other slope classes, precipitous, very steep, steep and level, are to be applied
as they stand. The terminology for simple erosional landform patterns based on relief
and modal slope is given in Table 5.
Table 5 defines the category badlands by various combinations of high slope values
and low relief values. These combinations imply extremely close spacing of streams
or valleys. Specifically, if one assumes a sawtooth terrain profile, the valley spacing
implied is less than 100 m in areas with 50 m relief and less than 30 m in areas with
5 m relief; these values appear to accord with usage.
Table 7 lists types of landform pattern in order of their typical class of modal slope.
This table should not be regarded as definitive, for slope within each type of landform
pattern may vary widely.
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