Competitive Exams: Identification of Sedimentary Rocks

Hardness

Grain Size

Composition

Other

Rock Type

hard

coarse

clean quartz

white to brown

Sandstone

hard

coarse

quartz and feldspar

usually very coarse

Arkose

hard or soft

mixed

mixed sediment with rock grains and clay

gray or dark and “dirty”

Wacke/Graywacke

hard or soft

mixed

mixed rocks and sediment

round rocks in finer sediment matrix

Conglomerate

hard or soft

mixed

mixed rocks and sediment

sharp pieces in finer sediment matrix

Breccia

hard

fine

very fine sand; no clay

feels gritty on teeth

Siltstone

hard

fine

chalcedony

no fizzing with acid

Chert

soft

fine

clay minerals

foliated

Shale

soft

fine

carbon

black; burns with tarry smoke

Coal

soft

fine

calcite

fizzes with acid

Limestone

soft

coarse or fine

dolomite

fizzing with acid unless powdered

Dolomite rock

soft

coarse

fossil shells

mostly pieces

Coquina

very soft

coarse

halite

salt taste

Rock Salt

very soft

coarse

gypsum

white or pink

Gypsum

Sedimentary rocks are the second great rock class. Whereas igneous rocks are born hot, sedimentary rocks are born cool at the Earth's surface, mostly under water. They usually consist of layers or strata, hence they are also called stratified rocks. Depending on what they're made of, sedimentary rocks fall into one of three types.