Sedimentary Rock Classification

How Do You tell One Sedimentary Rock from Another

Sedimentary rock is classified into two groups based on how they form. They are clastic and chemical.

Clastic sedimentary rock is formed as bits of weathered rock become cemented together. Because all kinds of rock are subject to weathering many different minerals can make up this group of rocks. Clays and quartz are the most common.

Classification of clastic sedimentary rocks is done according to the size of the sediments that make up the rock. The following table contains the major groups of clastic sedimentary rocks with their characteristics:

Clastic Sedimentary Rocks
Name of Rock	Sediment Type	Texture
Conglomerate	gravel - rounded fragments	course over 2 mm
Breccia	gravel - angular fragments	course over 2 mm
Sandstone	sand	medium 1/16 to 2mm
Siltstone	mud	fine 1/256 to 1/16 mm
Shale	mud	very fine less than 1/256 mm

Chemical sedimentary rocks form from dissolved minerals that are precipitated or separated from water. This happens most frequently when water evaporates leaving the minerals behind.

You can see this process taking place in your own home. The white deposits that form around the faucets in your bathroom or kitchen are from minerals left behind as water evaporates.

Below is a list of chemical sedimentary rocks with some of their characteristics:

Chemical Sedimentary Rocks
Name of Rock	Composition	Texture
Crystalline Limestone	Calcite - CaCo3	corse to fine Crystalline
Fossiliferous Limestone	Calcite - CaCo3	visible fragments of shells
Chalk	Calcite - CaCo3	microscopic shells and clay
Chert	Quartz - SiO2	very fine crystalline
Gypsum	Gypsum - CaSO4-2H2O	fine to course crystalline
Rock Salt	Halite - NaCl	fine to course crystalline
Bituinous Coal	Organic Matter	fine

Learn more about Sedimentary Rock

Examples of Sedimentary Rock

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