Sedimentary Rocks

Sedimentary Rocks

Sedimentary rocks are formed from layers of deposits laid down in

lakes or seas. Over millions of years the layers of sediment build up

resulting in the weight pressing downwards, creating high pressure,

which squeezes the water out. As the water is squeezed out, salts form

between the particles of sediment due to crystallization and this

cements the particles together. Sedimentary rocks can either rise to

the surface again to be discovered, or they can descend into the heat

and pressure below.

Sandstone is a type of sedimentary rock and is formed from sand.

Sandstone is fine particles of sand cemented firmly together due to

heat and pressure, which is why it looks very much like sand. However,

there is red sandstone as well as yellow. Sandstone is commonly used

for buildings.

Limestone, which can also be called chalk, is formed from seashells.

It mostly contains calcium carbonate as seashells are made from

calcium carbonate. It is usually white or grey in colour. The original

shells are mainly crushed down how some fossilised shells can still be

found in limestone.

Mudstone, which is also known as shale, is formed from mud. Mud is

basically finer particles of sand cemented together due to heat and

pressure. Mudstone is often grey in colour and unlike other rocks; it

tends to split onto its original layers very easily.

Coal is a fossil fuel formed from the remains of plants and

vegetation. Most of our coal was formed about 300 million years ago

when the earth was covered by steamy swamps. As plants and trees died,

their remains sank to the bottom of the swampy areas building up in

layers. Due to ...

... middle of paper ...

... the rocks over the years and the

origin of the rocks. Such features include:

* The order of the layers, as the deeper the layer is, the older the

rock is.

* Discontinuous deposition, where layers of different rocks are laid

down at different times.

* A younger layer of rock could cut across an older one.

* Ripple marks in the rock can show that it has been formed from the

seabed due to waves or currents.

* When rock formations are tilted, it causes large-scale movements

and the angles can be followed over distances to show the

relationship between distant rock formations.

* The folding of rocks shows the compression of layers due to plate

movement.

* When fractures or fault signs are present in rocks, it shows

evidence of earthquakes in the past.