Sedimentary rocks are formed in places where there has
been water at one time. Dead animals, plants, and pieces
of rocks or minerals are carried to these places by wind, water,
ice, or even gravity. They are usually dropped off in
bodies of water where they sink to the bottom. Little by
little they make layers that build on top of each other.
This puts pressure on the lower layers. An example of this
would be tackling in a football game. The person with the
ball is tackled to the ground and player after player piles on
top of him. The person at the bottom of this pile of
people, will feel the weight of all of the people on top of him.
The person at the top will not feel that until someone else
piles on top of him. So, the weight of all of the layers
of
rocks,
minerals,
dead animal skeletons, and plants pushes down the lower layers
until they harden into rock. This takes millions of years to
happen. During that time, the ocean, lake, or other body of water
dries up. This makes the sedimentary layer, that used to be
under water, become a surface layer.
Each layer of the rock might be different if erosion and weathering drop different things to the bottom of the water for each layer. This is why you see streaks of different colors or textures in sedimentary rock.
Scientists can tell how old the rock is by fossils that were trapped there and by studying how the layers formed. This is how they figure out what it was like in the time of the dinosaurs. Since sedimentary rock is found where there used to be oceans and seas or other bodies of water, finding it in the desert tells the scientist that there was water there at one time. The Grand Canyon [on the right] is an example of this.
The picture on the left is of
the Sideling Hill Road Cut that is in Maryland. They call
it a road cut because the mountain was cut so that a road could
go through it. When they cut the rock, it let us see the
sedimentary layers below the surface.
The layers are of
conglomerate,
sandstone, siltstone,
shale, and
coal. It is
interesting to see that some of the layers have marine life in
them. When geologists studied the area, they found out
that the shorelines in Maryland had changed at one time and this
place used to be covered in sea water. It's kind of weird to
realize this when it's at the side of a road in the mountains
now!
Examples of Sedimentary Rocks:
Examples of minerals found in sedimentary rocks: