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?