Photos of Pacific Coast, Cascades, Columbia Plateau
Geology of the Pacific Northwest

Sedimentary Rock Classification Table
Clastic Sedimentary Rocks
Grain Size Original Sediment Grain Roundness Rock Name
coarse (> 2 mm) gravel angular breccia
coarse (> 2 mm) gravel rounded conglomerate
medium (0.06 - 2 mm) sand variable sandstone*
fine (0.004 - 0.06 mm) silt not visible siltstone
extra fine (< 0.004 mm) clay not visible shale
Chemical Sedimentary Rocks
Composition Common Depositional Environment Features Rock Name
calcite warm shallow ocean gray, softer than glass limestone
calcite warm shallow ocean white, powdery chalk
microquartz deep ocean floor conchoidal fracture chert
microquartz lakes in volcanic environments white, powdery diatomite
carbon heavily vegetated swamp black, soft coal
halite evaporating water body tastes like salt rock salt
gypsum evaporating water body softer than fingernail gypsum rock

As rocks undergo weathering and erosion on the surface of the earth, they turn into grains of sediment. Clastic sediment is the solid bits and pieces of rocks and minerals that have broken down from the source rock by a combination of physical and chemical weathering. Clastic sediment is sometimes called detrital sediment. Grains of clastic sediment range in size from microscopic (clay particles) to house size (large boulders), with sand and silt being two of the common sizes of clastic sediment.

As rocks on the surface of the earth undergo chemical and physical weathering, some of their chemical elements or ions become dissolved in water. Subsequently, the dissolved atoms or ions come back out of solution in water and crystallize into solid minerals, a process known as precipitation. Minerals that precipitate from solution comprise the chemical sediments. Common chemical sediments include the precipitated minerals halite (rock salt), gypsum, calcite, and various forms of ultra-microscopic quartz grains (cryptocrystalline quartz, listed in the table above as "microquartz").

Sediment turns into sedimentary rock by some combination of burial beneath more sediment, compaction into a smaller volume (due to being compressed by the weight of more sediment piled on top), and cementation by new mineral growth precipitating between the sediment grains. This is commonly summarized as "burial, compaction, and cementation."

*Three common types of sandstone are arenite, arkose, and graywacke.


Geology of the Pacific Northwest
Sedimentary Rock Classification Table
© 2001 Ralph L. Dawes, Ph.D. and Cheryl D. Dawes
updated: 7/16/13