The brownish red color is caused by oxidation of iron content in the rock. The lighter white color may be caused by bleaching of surface rock from the rain and sun. Correction thanks to David M Browne:"The lighter color rock is not the result of sun bleaching or water leaching. It is inherent in the composition of the rock when first lithified. What you are looking at is a transition from two types of sandstones created in different environments. A water environment [darker rock] transitioning to a shore environment [lighter rock]."
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