Wallen, R. N. (1992). Introduction to physical geography. WM C. Brown.
Lua i Stíl Chicago (17ú heag.)Wallen, Robert N. Introduction to Physical Geography. Boulevard: WM C. Brown, 1992.
Lua MLA (9ú heag.)Wallen, Robert N. Introduction to Physical Geography. WM C. Brown, 1992.
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