1. Billings, W. D. 1979. High mountain ecosystem: evolution, structure, operation and maintenance. In High altitude geoecology, ed. P. J. Webber, 97-125. Boulder: Westview Press.
2. Cole, D. N. 1983. Assessing and monitoring back-country trail conditions. USDA Forest Service Research Paper INT-303: 1-10.
3. Cole, D. N. 1991. Changes on trails in the Selway-Bitterroot Wilderness, Montana, 1978-1989. USDA Forest Service Research Paper INT-450: 1-5.
4. Gerrard, A. J. 1990. Mountain environments. Lon-don: Belhaven Press.
5. Hammitt, W. E., and Cole, D. N. 1987. Wildland recreation. New York: Wiley Interscience.