Process

Exploring Rocks: Sedimentation

Wind and rain, the agents of erosion, scour Earth's surface, breaking rocks into sand and mud. Winds, streams and rivers pick up the eroded debris and carry it to sediment sinks. As piles of debris accumulate and thicken, the buried layers become cemented into sedimentary rock.


 

The size and shape of sand grains give clues to the rock's story. Grains of two sizes make up this sandstone. The big grains were dropped by a strong pulse of current, while the little ones filled gaps as the current waned. The rounded quartz clasts tell of a long journey - a bumpy ride that knocked off their corners in the strong currents that brought them home.

 


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Sandstone, Potsdam Formation, St. Lawrence County, New York
Crossed polarizers with gypsum plate
 

The accumulated grains must be glued together to make a sedimentary rock. Here new quartz grew over the rounded quartz grains. The quartz in the overgrowths extended the crystalline structure of each grain until all the grains were cemented together. A film of dust on the grain surfaces preserves their original rounded outline.

 


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Sandstone, Day Point Formation, Grand Isle County, Vermont
Crossed polarizers with gypsum plate


To view other sedimentary rocks, view Recycled Sandstone, Fossils in Mud and Oolitic Sandstone.


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