SEDIMENTARY ROCK
Sedimentary rocks are classified based on their origin or how they form. There are three classifications of
sedimentary rocks: Clastic, Chemical and Organic.
CLASTIC
Clastic sedimentary rocks are the type of rocks most people associate with sedimentary rocks. These rocks form
from pieces or sediments from other rocks. These sediments or rock fragments occur as a result of weathering and
erosion. Weathering is the breaking down of rock into smaller pieces by wind, water, ice and gravity. The
transporting or moving of these smaller pieces by wind, water, ice and gravity is called erosion. Clastic rocks are
differentiated based on the size of the sediments.
These sediments are either compressed together or cemented together by minerals. The size of the sediments will
determine the type of rock that is created. The smallest type of sediment is clay. When clay sized particles are
compressed together, shale is formed. The next size sediment is called sand. When sand sized particles are
cemented or compressed together, a rock called sandstone is formed. When the largest sediments (which are pebble
size or larger) are cemented together, they will form either conglomerate or breccia rock. The difference between
conglomerate and breccia is not the size of the rock fragments, but rather their shape. Conglomerate rocks are made
II fragments that are smooth and round, similar to M&M candy. Breccia rocks are comprised of rock
up Part
of rock
fragments that are sharp and angular, similar to shards of broken glass.
CHEMICAL
Chemical sedimentary rocks are rocks that formed due to evaporation. When water evaporates, the minerals that
were dissolved in the water remain behind and solidify into rock. Halite and gypsum are the two most common
ORGANIC
Some sedimentary rocks are formed from material that was once alive. Coal is a type of sedimentary rock that
formed from ancient plant material. Limestone is another example of a rock that formed from something that was
once alive. Limestone is made from the shell of organisms that lived in the seas millions of years ago. Chalk is
another example of an organic sedimentary rock. Chalk is a special type of limestone that also came from sea
shells.