rocks are formed from pebbles, sand, silt, or clay. They are
made from the end products of erosion,
which breaks down rock into tiny pieces.
Streams carry the tiny eroded fragments of stone to lakes and
oceans, or spread them out over the flood plains of of rivers.
Layers of these materials are built up over millions of years.
More and more sediment is added. Dissolved minerals in
the water are gradually precipitated, filling in the spaces between
the rock fragments. The layers of sediment turn into rock
materials are are also deposited by wind or glaciers.
strata of sedimentary rocks are often colored by minerals. A
reddish color, as in the picture above, is caused by iron oxide,
also called rust. Notice the more distant hills: some of their
strata are colored by other minerals, such as manganese.
of sedimentary rocks is a cool process compared with the formation
of igneous rocks. Igneous rocks begin
with molten material. Sometimes sedimentary rocks are heated
up when they are deep in the earth, and this can help to fuse
the particles of sediment together. Such rocks retain their layered
is how sedimentary strata are formed:
A stream is carrying silt and clay into a
large lake or sea.
Millions of years have gone by and there are
many layers of deposited sediment now.
Some of the sediment
granules have been cemented
together by minerals that
were dissolved in the
water. They have
turned into layers of rock.
This part of the earth
has been sinking, and the sea
has become much larger.
More sediment has been deposited. The layers
on the bottom have a lot of weight on them. They will heat up
from the pressure.
from underneath the sediments is causing them to bend. The rock
is so hot that it is somewhat plastic, and so will bend slowly
if enough force is acting on it.
A big crack has occurred in this huge block of sediment.
One part of the block slid downwards past the
This crack is called a fault.
The sliding produced an earthquake.
You can see the layers are no longer lined up.
earth movements stretched and bent the rock layers even
more. One end of the layers is above ground, and it shows
the stripes of the very
can see examples of bent and tilted layers in many places in
the world. Sometimes the strata run straight up and down. Scientists
learn about the age of the earth from studying these strata.
Fossils are found in
Scientists find skeletons,
leaves, footprints, and
shapes of animals and plants
that lived long ago.
Think about it!
Igneous rocks would burn up
these organic remains, but
they can be fossilized if
they are buried in soft mud.
Like pages in a book, the
stratified layers of rock
hold information about the
past of our planet and its
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1999, 2003. Elizabeth Anne
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