Answer:
Sedimentary rocks are formed from pre-existing rocks
or pieces of once-living organisms. They form from
deposits that accumulate on the Earth's surface.
Sedimentary rocks often have distinctive layering or
bedding. Many of the picturesque views of the desert
southwest show mesas and arches made of layered
sedimentary rock.
Common Sedimentary
Rocks:
Common sedimentary rocks include sandstone, limestone,
and shale. These rocks often start as sediments
carried in rivers and deposited in lakes and oceans.
When buried, the sediments lose water and become
cemented to form
rockClastic Sedimentary Rocks:
Clastic sedimentary rocks are the group of rocks most
people think of when they think of sedimentary rocks.
Clastic sedimentary rocks are made up of pieces (clasts)
of pre-existing rocks. Pieces of rock are loosened by
weathering, then transported to some basin or
depression where sediment is trapped. If the sediment
is buried deeply, it becomes compacted and cemented,
forming sedimentary rock. Clastic sedimentary rocks
may have particles ranging in size from microscopic
clay to huge boulders. Their names are based on their
clast or grain size. The smallest grains are called
clay, then silt, then sand. Grains larger that 2
millimeters are called pebbles. Shale is a rock made
mostly of clay, siltstone is made up of silt-sized
grains, sandstone is made of sand-sized clasts, and
conglomerate is made of pebbles surrounded by a matrix
of sand or mud.
Biologic Sedimentary
Rocks:
Biologic sedimentary rocks form when large numbers of
living things die, pile up, and are compressed and
cemented to form rock. Accumulated carbon-rich plant
material may form coal. Deposits made mostly of animal
shells may form limestone, coquina, or chert.
Chemical Sedimentary
Rocks:
Chemical sedimentary rocks are formed by chemical
precipitation. The stalactites and stalagmites you see
in caves form this way, so does the rock salt that
table salt comes from. This process begins when water
traveling through rock dissolves some of the minerals,
carrying them away from their source. Eventually these
minerals can be redeposited, or precipitated, when the
water evaporates away or when the water becomes
over-saturated with minerals.
