The color of Recent fluvial sediments in tropical Mexico supports the hypothesis that the hematite pigment in ancient red beds forms in situ after deposition and is not derived from erosion of red tropical (lateritic) soils, as is commonly claimed. Although red soils do occur in the region's hot humid source areas, the Recent alluvium that is transported and deposited by rivers is grayish brown because the red detritus is masked by more abundant nonred material. Red coloration occurs only in soils resulting from prolonged weathering on well-drained upland surfaces of Pleistocene age or older.

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