Grand Canyon was formed by exploring four possible hypotheses. Initially the superimposition mechanism intuitively seemed to be the more plausible explanation for Grand Canyon formation, but the weight of the evidence from the field trip lies with the overflow mechanism. Though it still leaves some questions unanswered, and some formations and processes unaccounted for, overall the overflow mechanism offers the best explanation.

To begin with, the first requirement for being able to explain Grand Canyon formation was determining whether the Colorado River was older or younger than the bedrock high of the Kaibab Plateau. Based on information provided at the various stops, one can conclude that the Colorado River is younger than the Kaibab Plateau.

At stop 2e there is evidence showing that the topography of the bedrock high controls the location of the transverse drainage incision into low elevation saddles. The topographical map shows how Marble Canyon is controlled...
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