Whirlpool Formation
Monroe County, New York

This sandstone is built from quartz grains (yellow, white, gray) and calcareous grains (dark) held together by a calcite cement. The calcareous grains that contain a distinctive internal structure are ooids. The prominent ooid in this view has both radial and concentric structure. Ooids grow in wave agitated water. Rolling sand grains acquire concentric shells of carbonate (imagine a snowball forming), and develop a structure like an onion's. The radial spokes are a secondary feature caused by recrystallization of the concentric material. Ooids form as spheres, but some get broken or deformed when compacted in a sandstone. Can you find any mangled ooids? Crossed polarizers

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