Answer: "With each hour-and-a-half orbit of Earth, Glenn watched the sun rise and set. "Five hours and 81,000 miles after he had taken off, John Glenn splashed down to a welcome..."
It is produced by the processes of living organisms which draw in calcite to construct their shells and structures. The shells and structures that remain(the clasts in this case) after the organisms have died become cemented together into the rock conquina.
