Tuesday, January 21, 2014

Geology Concepts - Superposition

Superposition is a concept in geology that states that the layers at the bottom of a rock column are older than the layers at the top of a rock column.  This is especially true for layers of sedimentary rock that remain in their original horizontal position.  These individual layers are known as beds.

Sedimentary rock formations at Pictured Rocks National Lakeshore - layers at the the top of the formation are older than layers at the bottom

Pictured Rocks National Lakeshore - Sandstone deposited in distinct layers with the oldest layers at the bottom of the formation.

If you think of a rock column as a layer cake this concept makes perfect sense.  When a baker makes a layer cake, they add each new layer to the top of the cake.  In nature, when sediment is deposited at the bottom of a lake or at the base of a hill new layers form over the top of older layers.

Bedding in a dune a Sleeping Bear Dunes National Lakeshore - the upper beds are younger than the lower beds.

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