Sedimentary rocks form through the compaction and cementation of sediments. There are three main types: clastic rocks form from fragmented sediments like sand and gravel; chemical rocks form when dissolved minerals precipitate out of solution, like limestone and rock salt; and organic rocks form from the remains of once-living organisms, such as coal, petrified wood, and fossil-bearing limestone.

1. SEDIMENTARY ROCKS

2. Sedimentary Rocks
 Earth’s crust is made mostly of Igneous Rocks.
 But, most rocks on Earth’s surface are sedimentary rocks (75%).
 All sedimentary rocks form in an aqueous solution (in water).
 Sedimentary is derived from the Latin sedimentum, which means
“settling”.

3. Sedimentary Rocks
 There are 3 main types of Sedimentary Rocks
1. Clastic
2. Chemical
3. Organic
Sedimentary rocks are rocks that are
made of broken-down materials from
other pre-existing rocks on Earth.
Clastic: Rocks that formed from sediment fragments of other rocks.
Chemical: Rocks formed when dissolved minerals drop out of solution.
Organic: Rocks that form from the remains of once-living organisms.

4. Clastic Sedimentary Rock Formation
1. Weathering
2. Erosion
3. Deposition
4. Compaction
5. Cementation

5. Clastic Sedimentary Rocks: Weathering
 Rocks on Earth slowly break down under forces of nature.
 Wind, water, and ice break rock apart.
 This is called weathering (Think of weathering like a “sand-
blaster”).
When rock weathers, it breaks into fragments or pieces.
These rock fragments are called sediment.
Sediment includes: gravel, pebbles, sand, slit, and clay.

6. Clastic Sedimentary Rocks: Sediment
Gravel
Sand From Biggest to Smallest
Silt Based on its size…
Clay
 Sediment is given a name based on it’s size.

7. Clastic Sedimentary Rocks: EROSION
Strong Currents:
• Carries clay,
silt, sand, and
gravel.
Medium Currents:
• Carries clay,
silt, and sand.
Gentle Currents:
• Carries clay
and silt.
Quiet Currents:
• Carries mostly
clays and muds.
 Erosional currents (moving wind and water) have energy to carry sediment.
 If the sediment is large, it will need a stronger erosional current to carry it
away.
 As the current slows , the largest sediments begin to drop out first.

8. Clastic Sedimentary Rocks: EROSION
Strong Currents:
• In the desert when the wind
picks up sand and
sandblasts rock.
Medium Currents:
• At the beach when the
waves come in and erode
the beach, you feel sand
suspended in the water.
Gentle Currents:
• On a hill where gullies form
from water running over the
land.
 These sediments are carried away by wind and water.
 This process is known as erosion.
 Large sediments – Needs strong current to move it.

9. Clastic Sedimentary Rocks: EROSION
LIKE SANDPAPER
• The movement of the sediment acts like sandpaper to
grind it down and smooth it out.
 As these sediments are carried away, they are broken down more.
 The sediments start out large and jagged.
 As wind and water bounces them around, they get smaller.
 They also become more rounded the further away they are carried.

10. Clastic Sedimentary Rocks: Deposition
Deposition occurs when erosional currents slow down!
If the current slows down, there is not enough energy to
continue to carry the sediments in the current.
As a result, the sediments fall out and are deposited.
 Eventually these sediments will be deposited.
 Deposition: when sediments settle out of moving
currents.
Ex. 1 • When a stream
enters a lake.
Ex. 2 • When a river enters
an ocean.

11. Clastic Sedimentary Rocks: Deposition
 Sediments are usually deposited in layers that build up.
 Just like when you make “deposits” in a bank.
 Your money “builds-up” over time.
 Or, when you deposit your books in your locker throughout
the day.

12. Clastic Sedimentary Rocks: Deposition
 After sediment is deposited, clastic sedimentary
rocks form by:
 Compaction
 Cementation

13. Clastic Sedimentary Rocks: Compaction
 As sediment builds up, it begins to compact.
 Pressure from the layers above push down on the lower
layers.
 Think of a trash compactor.
 Compacting snow to make a snowball.
 If sediments are really
small (Silt or Clay) they
will stick together
forming rock.
 This rock forming process
is known as Compaction.

14. Clastic Sedimentary Rocks: Compaction
 Clay sediments compact to make the sedimentary rock
Shale.
 Silt sediments compact to make the sedimentary rock
Siltstone.

15. Clastic Sedimentary Rocks: Cementation
 Larger sediments cannot just be squeezed together to make
rock.
 There needs to be something else that holds the rock together.
 Water, carrying dissolved minerals, runs through the pore spaces between sediment.
 Water drains or evaporates, leaving minerals behind.
 Minerals in the water act like glue (quartz and calcite).
 The minerals harden and cement the larger sediment together as rock.
 If sediments are large (Sand and Gravel) they will
not stick together through compaction.

16. Clastic Sedimentary Rocks: Cementation
This rock forming process is known as cementation.

17. Clastic Sedimentary Rocks: Cementation
 Sand makes Sandstone
 Sand and Gravel makes Conglomerate
 Large and sharp rock fragments make Breccia

18. Clastic Sedimentary Rocks: Cementation
 Think of cementation like fruit cake.
Fruit cake has sediments
• Nuts
• Cherries
• Raisins
• Pineapple
• Cloves
 But those sediments do not stay together on their own.
 The sediments would be loose if they were not cemented
together with – Batter.

19. Clastic Sedimentary Rocks: Classification
 Sedimentary rocks are classified by sediment size:
 Clay, Silt, Sand, and Gravel are the major types of sediment.
 They are compacted and cemented to form sedimentary rocks.
 Each sediment forms a different type of rock.
Sediment Clay Silt Sand Gravel
Example Shale Siltstone Sandstone Conglomerate
or Breccia
Size Range < 0.004
mm
0.004 –
0.063 mm
0.063 –
2.0mm >2.0 mm

20. Clastic Sedimentary Rocks
 Summary of Clastic Sedimentary Rocks:
 Sedimentary rocks form from pre-existing rocks.
 Rocks on Earth weather and break into sediments.
 The sediment is eroded and carried away.
 The carried away sediment is later deposited.
 As sediment is deposited, it builds up layer upon layer.
 The layers compact over time.
 Minerals dissolved in water start to glue sediment
together.
 Finally, a clastic sedimentary rock is formed.

21. Chemical Sedimentary Rocks
 Chemical sedimentary rocks form from dissolved
minerals.
 Water contains dissolved minerals
 When water evaporates, those minerals precipitate.
 Precipitate means to come out of solution.
 The mineral crystals grow together to make the rock.
 Types of Chemical Sedimentary Rocks:
1. Limestone (most common)
2. Rock Salt

22. Chemical Sedimentary Rocks: Limestone
 Travertine Limestone:
Limestone often found in
caves.
 Limestone
 Forms when Calcium Carbonate precipitates from
solutions.
 The calcium Carbonate is essentially the mineral
Calcite.
 How can we tell if a rock is Limestone?

24. Chemical Sedimentary Rocks: Rock Salt
 Rock Salt
 Water rich in salt evaporates to leave salt crystals
behind.
 Those crystals are especially the mineral Halite.
 Halite forms the rock, Rock Salt.

25. Gypsum Crystals

26. Organic Sedimentary Rock
 Rocks that come from the remains of organic matter.
 Types:
 Petrified Wood
 Coal
 Fossiliferous Limestones

27. Petrified Wood
 Petrified wood is a fossil.
 It forms when plant material is
buried by sediment and protected
from decay by oxygen and
organisms.
 Then, groundwater rich in dissolved
solids flows through the sediment
replacing the original plant material
with silica, calcite, pyrite or another
inorganic material such as opal.

28. COAL
 Coal is formed from vegetation that previously existed in
swampy and marshy soils which prevented their full decay
after their death.
 As their remains piled up and were covered by more and
more deposits, they gradually underwent compaction and
cementation.

29. Chalk and Fossiliferous Limestones
 Chalk and Fossiliferous Limestones - formed from the
skeletons of marine organisms
 Chalk is a soft, white, porous sedimentary carbonate rock, a
form of limestone composed of the mineral calcite. Calcite is
calcium carbonate or CaCO3

30. Chalk and Fossiliferous Limestones
 Fossiliferous limestone is any type of limestone, made mostly
of calcium carbonate (CaCO3) in the form of the minerals
calcite or aragonite, that contains an abundance of fossils or
fossil traces.

31. SUMMARY
 Clastic
 Solidifies through the lithification process.
 Chemical
 Solidifies through solution evaporation.
 Organic
 Solidifies through the re-mineralization of organic
material.