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cross bedding

Cross beds are sedimentary structures that form at the interface between loose sediment and a fluid, in general water (in a river, at a beach, in turbidity currents, etc.) but also air (for instance in sand dunes). In the case of this picture, given the presence of scattered coarse pebbles at the top of the ripple, along its slope, and at its foot, a river is the most likely culprit. Grains the size of gravel can not generally be picked up by the wind; if they are, then this sand would not be forming this particular structure.

 

The color of this sandstone, a dark, ocher-like yellow, indicates the presence of limonite, a hydrous iron oxide that also indicates a terrestrial environment. This oxidation is very likely primary because different cross beds are characterized by different colors. There tends to be a correlation, barely visible in this image, between the size of the grain and its oxidation state: the bigger the grain size, the darker the tonality of yellow, indicating abundance of limonite.

 

Fillmore, Ventura county, California

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Taken on April 14, 2012