Robert G. Bailey's ecoregions were first mapped in 1976. This map was important for the first Forest Service roadless area analyses, RARE I and RARE II. In 1993, the ecoregions were adopted for use in ecosytem management planning as part of the National Hierarchical Framework of Ecological Units. Bailey's ecoregions should be regarded as very generalized, as they were drawn at a scale of 1:7,500,000.
Reference: Bailey, Robert G. 1995. Description of the ecoregions of the United States. Miscellaneous Publication No. 1391. Second edition, revised. Washington, DC: USDA Forest Service.