Athro, Limited Earth Science Rock Types 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.

Proximal (near source)		Breccia Large angular clasts Breccia at the base of the Potsdam Sandstone, Cambrian, New York, USA
		Conglomerate Large rounded clasts Conglomerate in aluvial fan facies, Catskill Beds, Devonian, New York, USA
		Coarse Sandstone Millimeter sized grains. Close up of the Hawkesbury sandstone, Sydney, Australia.
		Fine Sandstone Visible grains. Crossbeds in the Chenango Sandstone, Devonian, New York.
		Siltstone Grains too small to see, but feels gritty when chewed. Outcrop of siltstones and shales in the Moscow Fm, Devonian, New York.
		Shale Made of very small clay mineral particles. Fossil graptolites in a black shale. Provenance unknown.
Distal (distant from source)		Limestone 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.