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 Sedimentary Rocks 

In general, sedimentary rocks form a gradient from near the source (mountains, where erosion is occurring) to far from the source. Very near the source, angular blocks of source material are eroded. These are quickly rounded by movement in water. Sediments deposited very near to the source area can form a breccia, composed of large angular clasts. Sediments deposited a little further from the source can form a conglomerate, composed of large rounded clasts.

As sediment is transported from its source, it is physically broken down by impacts with other particles in transport, and it is chemically broken down. Chemical weathering tends to turn mineral grains, especially feldspars into clays. Quartz is durable and resistant to both chemical and physical weathering. Thus in weathering, granites (composed mostly of feldspars and quartz) tend to break down into small grains of quartz and tiny clay mineral grains.

(near source)
decimeter sized angular clasts in breccia at base of the potsdam sandstone, cambrian, new york


Large angular clasts

Breccia at the base of the Potsdam Sandstone, Cambrian, New York, USA

centimeter sized rounded clasts in the catskill beds, devonian, new york


Large rounded clasts

Conglomerate in aluvial fan facies, Catskill Beds, Devonian, New York, USA

sand grains in hawkesbury sandstone, sydney, australia

Coarse Sandstone

Millimeter sized grains.

Close up of the Hawkesbury sandstone, Sydney, Australia.

crossbeds in fine grained chenango sandstone, devonian, new york

Fine Sandstone

Visible grains.

Crossbeds in the Chenango Sandstone, Devonian, New York.

shale and siltstone outcrop, devonian of new york


Grains too small to see, but feels gritty when chewed.

Outcrop of siltstones and shales in the Moscow Fm, Devonian, New York.

shale and siltstone outcrop, devonian of new york


Made of very small clay mineral particles.

Fossil graptolites in a black shale. Provenance unknown.

(distant from source)
massive blue grey lowville limestone, ordovician, new york.


Composed primaraly of calcium carbonate, derived from organisms.

Lowville Limestone, Ordovician, New York, USA

This is but an overview. There is lots more complexity to sedimentary rocks. There are, for example, other sorts of sedimentary rocks, such as coals and salts. Limestones have a wide range of textures, from those formed from carbonate muds, to those made up entierly of large fragments of the shells of organisms.

Sources: Press and Siever, 1978
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Copyright © 2000 Athro, Limited. All Rights Reserved.
Written by Paul J. Morris
Maintained by Athro Limited
Date Created: 2000 Dec 30
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