Clastic sedimentary rocks are
formed from other, eroded rock material.
The rock material that
is eroded from existing rock is transported by
water and wind before it is deposited in the bottom of water bodies, in
deserts or on beaches where it will finally turn into sedimentary rock
under the pressure of more and more layers.
During the transport
grain sizes are sorted.
sedimentary rocks can be classified
according to clast size, but also composition, cement character, and
how well sorted the clast sizes are. Below are the rocks of this
from largest to finest clast size.
Conglomerate by Spencer Garness via Flickr.com
is a rock easy to recognise because of the large clasts
which are round - they have been pebbles. The clasts are mostly quartz,
but can be a variety of different minerals or rock such as limestone
and others in polygenetic conglomerate. The matrix is usually
sandstone. Conglomerate is often found in association with
sandstone. There are no fossils.
also coarse grained
but the difference is that breccia's
large fragments are not round but anguar. This shows that they
have not been transported very far from their source. The large
fragments can be of any
but they are made of limestone in
Matrix is usually made of sandstone.
Fossils are uncommon in breccia,
but more common in limestone breccia.
has a medium grain size
and is one of the most common sedimentary rocks. Like sand, it contains
a lot of quartz - a hard mineral which requires that sand is cemented
together to turn into sandstone. Other minerals may be feldspars
grains are cemented by
calcite, silica, or iron oxides.
Bedding layers are often visible. In micaceous sandstone, large mica
minerals are present. It can vary in colour between red, brown, white
Siltstone. By James St. John via Flickr.com
appearance to mudstone but it contains more quartz. Its grain size is
fine, between the sizes of sandstone and mudstone. Like sandstone, it
can have different colours and textures. It can also form in different
environments. It is most often grey or reddish in colour. It is
laminated, and it often contains carbonates
and sometimes fossils.
Shale rock by Tom via Flickr.com
is an interesting rock that is known for its fantastic layers and the
ability to break along them. It has consequently been widely used when
building roofs, making bricks, and tiles. Without its
fissility it would resemble mudstone when it comes to colour and grain
size. It can contain fossils,
and its main minerals are quartz, clays
and mica. Shale is the most common of clastic sedimentary rocks.
a black or grey rock with a very
grain size. Although the grains cannot be seen with bare eyes, it
contains quartz, mica, feldspars, different clay minerals and sometimes
carbonates, iron oxides and/or fossils. It can be similar to shale,
except that it is not nearly as well laminated as shale is. It forms
from mud sediments in the bottom of waterbodies.