Rock layers are formed through sedimentary processes over long periods of time. Stratified rocks are formed from sediments laid down in layers, and their formation depends on stratigraphy and stratification. There are several laws of stratigraphy that help geologists determine the relative ages of rock layers based on principles like original horizontality, superposition, cross-cutting relationships, inclusions and unconformities. Geologists use physical features of the rock layers as well as index fossils to correlate and match rock layers across different regions in order to reconstruct Earth's history.

1. FORMATION OF ROCK LAYERS
Prepared by: Engr. Ariel S. Motas
Engr. Andreana Amor M. Gulay

2. LESSON 1: ROCK LAYERS
- How are rock layers formed?
Stratified Rocks
• Also known as derivatives rock, maybe fragmental or crystalline.
• These rocks are product of SEDIMENTARY PROCESSES.
• These are made of visible layers of sediments.
• Formation on rock layers depend on its stratigraphy and stratification
Stratigraphy
• It is the branch of geology that deals with
the description, correlation, and
interpretation of stratified rocks on and
within the Earth.
Stratification
• It is also known as bedding, which is the
layering that happens in sedimentary and
igneous rocks formed at the surface of the
Earth that comes from lava flows or other
volcanic activity.
• It is expressed by rock layers (units) of a
general tabular or lenticular form that differ
in rock type

3. Law of
Unconformomities
Law of Cross Cutting
Relationship
Law of Inclusions
Law of Original
Horizontality
Law of Faunal
Succession
Law of Superposition
LAWS OF
STRATIGRAPHY

4. Law of Superposition
• The largest and heaviest rock layer that settled first at the bottom is the oldest
rock layer
• The lightest and smallest that settled last is the youngest rock layer.
Sandstone & Shale
Shale & Siltstone
Sandstone
Limestone & Sandstone
Limestone

5. Principle of Original Horizontality
• sediments are deposited in flat layers, if the rock maintains in
horizontal layers, it means it is not yet disturbed and still has its
original horizontality
1
2

6. Law of Inclusions
• A rock mass that contains pieces of rocks called
inclusions are younger than the other rock
masses.
• Stated that if a rock body (Rock B) contained
fragments of another rock body (Rock A), it
must be younger than the fragments of rock it
contained. The intruding rock (Rock A) must
have been there first to provide the fragments.

7. Law of Cross Cutting Relationship
• Stated that if a fault or other body of rock cuts through another body
of rock then it must be younger in age than the rock through which it
cuts and displaces.
• When magma intrudes to the rock, that fault or magma is younger
than the rock
dike or dyke, in
geological usage,
is a sheet of rock
that is formed in a
fracture of a pre-
existing rock body

8. Law of Faunal Succession
• Different strata contain particular assemblage of fossils by which rocks
may be identified and correlated over long distances

9. Law of Unconformities
• Rock layers that are formed without interruptions are conformable.
• Describes a layer of rock that have been deformed or eroded before
another layer is deposited, resulting in rock layer mismatching
• An unconformity represents a long period during which deposition
stopped, erosion removed previously formed rocks, and then deposition
resumed
• Three types of unconformities: Disconformity, Angular Uncorformity and
Non-conformity

10. Three Types of Unconformities
Disconformity
Originates from horizontal
sedimentary rock layers that is
lifted and the top layer eroded.
New sediments are deposited
when they are submerged
beneath a freshwater or
saltwater
Angular Unconformity
It originates from horizontally
parallel strata of sedimentary
rock that are deposited on
tilted and eroded layers.
It produced an angular
discordance with underlying
horizontal layers.
Non-conformity
It originates between
sedimentary rocks,
metamorphic or igneous rock
when sedimentary rock lies
above and was deposited on the
pre-existing and eroded
metamorphic or igneous rock.

11. EXAMPLES:
• Arrange the rock layers from OLDEST to YOUNGEST

12. EXAMPLES:
• Arrange the rock layers from OLDEST to YOUNGEST
C, E, D, A B

13. EXAMPLES:
• Arrange the rock layers from OLDEST to YOUNGEST
E, B, C, D, A

14. EXAMPLES:
• Arrange the rock layers from OLDEST to YOUNGEST
A,C, B

15. EXAMPLES:
• Arrange the rock layers from OLDEST to YOUNGEST
G, C, A, F, B, H,
D, I, E

16. LESSON 2: Correlation of Rock Layers
- The process of showing that rocks or geologic events occurring at different locations are of the same age
Two types of correlation:
1. Physical
2. Fossil
- Geologists need to correlate rocks from one place to another to get more complete record of Earth’s history over time.
- They try to determine the relative age of widely separated strata or rock layers.
- They used correlation trying to fit together sedimentary strata in different places just like a cut out puzzle.

17. 1. Physical Correlation
• Is accomplished by using number of criteria such as color, texture, and types of
minerals contained within a stratum which make it possible for geologists to classify a
particular stratum specifically
2. Fossil Correlation
• Is a principle that geologists use
to determine the age of rock. It
uses fossil with unique
characteristics, such as
geologically short lifespan and
easily identifiable features and
use this information to estimate
the age of rock layer in other
areas that contain the same type
of fossil or group of fossils

18. Criteria to be considered in identifying index fossils includes:
• Easily recognizable and it must be easy to identify because of its
uniqueness.
• Must be geographically widespread
• Must lived for only a short time
There are fossils which are used to date the layers of rock that they are found in. Fossils
that can be used in this way are called index fossils, and rock layers with the same
index fossils in them can be correlated.

19. Match correlated rock layers

20. Answer:

21. Match correlated rock layers
There are four columns from neighboring regions. The numbers represent different
fossils found in the rock layers. Unconformities (buried erosional surfaces) indicate that
material is missing. Try to find a fossil number that is common to all four columns and
use it to line them up for correlation in the previous example.

22. Answer: