This layer averages about 530 million years old and is composed primarily of mudstone shale. It is also interbedded with small sections of sandstone and sandy limestone. The retreat of the Canyon rim is attributed primarily to the erosion of this layer which forms the top of the Tonto Platform. The plateau is much wider in the eastern portions of the Canyon where the Bright Angel Shale contains less sand and is more easily eroded. The color of this layer varies with its compostion but it is mostly various shades of green with some grey, brown and tan thrown in here and there. Fossils to be found in this layer consist of marine animals such as trilobites and brachiopods.
 
Grand Canyon Rock Layers
 

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