English: Map with facsimile of horse dispersal from a map at: Haines, Francis. "The Northward Spread of Horses Among the Plains Indians." American Anthropologist New Series, Vol. 40, No. 3 (Jul. - Sep., 1938), pp. 429-437 at p. 430
online version. Depictions of the Great Basin and Continental Divide from USGS hydrographic .shp files. Depiction of the Great Plains from
this map
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Used EPA and WWF GIS ecoregion files to make desert boundaries rather than Great Basin hydrographic boundariy. Also used ecoregion boundarie to define the Great Plains