Tag Archives: oolitic limestone

Geopic of the week: Oolitic Limestone

oolitic pitkin

This is a close-up picture of a hand-sized specimen of oolitic limestone.  It’s called “oolitic” because it’s composed chiefly of ooliths which are the round, sand-sized grains that make up the majority of the rock.  An Oolith is a grain of marine sediment formed by repeated precipitation of minerals from sea water around a nucleus; the nucleus is typically a tiny fossil fragment or speck of sand.  They form in very shallow marine shoals where waves are agitating the grains on the sea floor causing them to tumble around.  As they tumble they accrete concentric mineral layers (usually calcium carbonate but sometimes other minerals) around them and grow larger.  Once formed, ooliths can be transported by currents in the same way as sand grains, accumulate in various marine environments and form rocks.