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Mapping of landtype associations (LTAs) in Region 1 was undertaken in order to provide continuous coverage of landform and geologic materials across all Forest Service lands in northern Idaho, Montana, North Dakota and the northwestern portion of South Dakota.
LTAs allow biophysical stratification at the landscape level for characterization of watershed properties important to management.
REFERENCE: Ford, G., C. Maynard, J. Nesser, D. Page-Dumroese. 1996. Landtype Associations of the Northern Region, A First Approximation. (PDF**)
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The use of landforms and geologic materials as differentia is based on the assumption that these are the two variables which can be mapped consistently at the landscape level that most closely predict significant changes in inherent watershed, stream and riparian properties.
The 10 major geologic categories, as mapped, are:
Geologic categories are subdivided according to landform and soil-forming process.