Water and wind not only weather the rocks and soils but also move the
weathered materials (the sediments) and deposit them in other places.
Whenever you see a dust or sand storm, or a muddy creek or river, you
are observing the movement of sediments by wind and water to other land
areas or to the sea. The combination of weathering and movement is
called _erosion_.
[Illustration: Conglomerate from Webb County, Texas, is composed of
rounded gravel that has been cemented together.]
Some of the rock fragments carried by water are still fairly large when
they reach their destinations. On the basis of size, they are called
_boulders_, _cobbles_, _pebbles_, and _granules_. Loose deposits of
these larger-size sediments make up what is known as _gravel_. Nature
cements gravels together to form rocks such as _conglomerates_ (made up
of rounded gravel) and _breccias_ (made up of sharp-cornered gravel).
The finer sediments are called _sand_, _silt_, _mud_, and _clay_. When
cemented, the sand grains become _sandstones_, the silt particles become
_siltstones_, and the mud and clay particles become _shale_. The
sedimentary rocks that are made up of these rock fragments are called
_clastic_ or _fragmental_ rocks.