Sedimentary Rocks

Sedimentary rocks are formed by the physical or chemical weathering and erosion of older rocks or, sometimes animal or plant matter.. 

The particles can be bound together by a type of cement (for example, water circulating through sand can deposit lime,  silica and iron oxide, which cements the grains of sand into sandstone) or they can be formed by compression (for example, when mud is compressed by the weight of sediment above, water is squeezed out leaving shale).

 


Detrital Rocks

Rocks formed by the physical weathering of older rocks are called "detrital". They are classified according to the size of the particles of the predominant mineral.

If these particles are pebble-sized, the rocks are called conglomerates. If they are the size of grains of sand, the rocks rare called sandstone. Rocks with the finest particles are called shale.


Main Types of Detrital Sedimentary Rocks

PARTICLE SIZE  
Coarse Conglomerate rounded pebbles
Breccia angular pebbles
Medium Sand-
stone
Quartz sandstone quartz sand grains
Arkose quartz and feldspar sand grains in clay
Graywacke various sand grains in clay
Fine Mud-
stone
Siltsone gritty, silt-sized particles
Very Fine Claystone smooth, clay-sized particles
Shale clay-sized particles in sheets

 

Conglomerate Arkose Mudstone

 

Chemical Rocks

Rain water percolating through soil and rock can dissolve certain chemicals which collect as a sediment and re-crystallize into new rocks. Chemical sedimentary rocks are classified according to their main chemical.

Main Types of Chemical Sedimentary Rocks

COMPOSITION  
Halite Rock Salt
Gypsum Rock Gypsum
Calcite Limestone
Dolomite Dolostone
Quartz Chert

 

Dolomite Rock Gypsum

 

Organic Rocks

Organic sedimentary rocks are formed when plant or animal remains are compressed or cemented together. Thus coal is formed from plant remains and limestone from fossil shells and corals.

Main Types of Organic Sedimentary Rocks

COMPOSITION    
Fossil
Fragments
(Calcite)
Limestone Fossiilferous Limestone well-cemented
Coquina loosely cemented
Chalk powdery
Plant Matter Coal  

 

Limestone Coal

 

Sedimentary rock specimens now available (at eBay):

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