Sedimentary Rock
Sedimentary rock forms when layers of sediment are compacted and cemented together, or when minerals precipitate from water.
Definition
Sedimentary rock forms when bits of sand, mud, or shells pile up in layers and get squeezed and glued together over a long time, and it is the kind of rock most likely to hold fossils. It forms at or near Earth's surface through deposition, compaction, and cementation of sediment (clastic rocks such as shale, sandstone, and conglomerate) or through chemical and biological precipitation (limestone, rock salt, coal), and it is layered in strata. Clastic rocks are classified by grain size (clay under $0.004\ \text{mm}$, silt, sand from $0.0625$ to $2\ \text{mm}$, gravel above), sorting, and rounding, all of which record transport energy and distance. Chemical and biochemical rocks precipitate from solution, with limestone ($CaCO_3$) and dolostone dominating. Lithification proceeds by compaction, which expels pore water, and by cementation with silica, calcite, or iron oxide precipitated from groundwater.
Example
Sandstone is made of sand grains cemented together, and limestone often forms from the shells of tiny sea creatures. The Coastal Plain of eastern North Carolina is built of sedimentary layers, the Castle Hayne limestone near Wilmington holds marine fossils, and the Grand Canyon's walls expose about $1{,}500$ meters of stacked sedimentary strata. Cross-bedded sandstone in the Navajo Formation of Utah records ancient dune migration, and the dip direction of the cross-beds reveals the paleowind. Evaporite sequences, gypsum and then halite, form as seawater loses more than $90\%$ of its volume to evaporation.
Key Insight
If you can see stripes or layers in a cliff, you are probably looking at sedimentary rock, and the bottom layer is the oldest. Sedimentary rocks cover about three quarters of the continents' surface but make up only a thin skin, a small fraction of the crust by volume. They are Earth's archive of surface conditions: climate, sea level, biology, and atmospheric oxygen are all read from them. Banded iron formations, for instance, record the first rise of free oxygen roughly $2.4$ billion years ago.