William Henry Jackson's "Phantom Curve" (late 1800's) and Robert Adams' "Clear Creek Canyon, near Idaho Springs" (ca. 1970). If you click on the Jackson photograph to enlarge it you will be able to see the man standing at the base of (what's left of) the rock formation to the right of the tracks.
Robert Adams is the most literate of photographers, and his photographs of Colorado and the West remain the most important model of landscape photography for me. This image called "Burning Oil Sludge, North of Denver, Colorado" (1973) is part of a show held at the J. Paul Getty Museum in Los Angeles in 2006 called "Robert Adams: Landscapes of Harmony and Dissonance."
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