Decomposition of granodiorite underlying basal Fountain (Pennsylvanian) formation on Flagstaff Mountain near Boulder, Colorado, was the result of pre-Fountain, probably post-Madison weathering. Chemical and mineralogical studies indicate that the weathering took place in a humid, warm or hot climate and that laterization was the dominant process.

The results of pre-Fountain and recent weathering are compared and contrasted.

The red color of the Fountain formation is regarded as inherited from a deeply weathered extensive red-stained pre-Fountain regolith.

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