www.letstonesspeak.wikispaces.com
Sedimentary rocks
Sediments carried by water or wind may build up in deep layers, either on land or more
comonly on the sea bed. As more material is added, its weight compresses the lower layers.
Over milions of years minerals dissolved in seawater of groundwater cement the
compressed grains of sediment together to form sedimentary rocks. Most of these are made
of rocks debris, but typical limestones consist of the skeletons and shell of marine organisms,
while coal is made of the remains of long-dead plants.
Clay
Clay is the softest of rocks, created from tiny flakes of minerals released b the weathering of
hard rocks such as granite. The flakes combine with at least 17 per cent water. In this
condition clay feels dry and brittle, but if it absorbs more water is becomes sticky. Heating
clay in a kiln drives of the water and turns it into a stony form.
Shale
Clay particles that settle in deep, still water form layers of mud. Over time the weight of
more sediment drives out the water, turning the mud to shale. This is a relatively soft rocks
that splits easily along the boundaries between the original layers. Scientist can expose
fossils in shale by prising the layers apart with a chisel.
www.letstonesspeak.wikispaces.com
Conglomerate
Most sedimentary rocks are made of small rock fragments or mineral grain, but
conglomerate consist of big pebbles that have been cemented together. The rounded
pebbles were once transported by water, and they are often the remains of an ancient
beach. A similar rocks, breccias, is made of sharp-edged stones once carried by ice.
Evaporites
These rocks are formed by the evaporation of water that contains a lot of dissolved minerals.
As the water vapour is driven off, the minerals stay behind. Evaporating seawater, for
example, can leave thick salt deposits, which may then be buried and compressed into rock
salt.
.
Limestones
Marine organisms absorb dissolved chalky minerals from seawater and use them to build
their skeletons and shells. When they die the chalky, calcareous materials survives decay and
www.letstonesspeak.wikispaces.com
build up in layers on the sea bed, Over time, the layers may be compressed into chalk or
limestone, which often contains visible shell fragments.
Flint
Chalk is a soft white limestone made of the calcareous skeletons of countless microscopic
marine organisms that live roughly 100 milion years ago in a shallow tropical sea. In often
contains nodules of hard black flint, which probably formed from the glassy skeletons of
other organisms such as sponges. Flint is very like glass, fracturing to produce razor-sharp
edges, and was widely used by early humans to make stone tools
.
Coal
If plant remains accumulate in waterlogged, airless conditions, they turn into peat. If the
peat is buried deep beneath more sediments, it can be compacted and heated to form coal-a
black, shiny rock then can be burnt as fuel. The oldest, hardest coal, formed from plants that
lived some 300 milion years ago, is almost pure carbon.
www.letstonesspeak.wikispaces.com
.
Answer the questions:
1. Name some of the most known sedimentary rocks.
_________________________________________
2. How are sedimentary rocks formed?
_________________________________________
3. Which rock consist of big pebbles ?
_________________________________________
4. What is chalk?
_________________________________________
5. Which of sedimentary rocks can we burn?
_________________________________________