Basic Principles of Stratigraphy
Ordering (Arranging) Geologic Events
Before any techniques for establishing numerical ages of rocks or geologic events (Absolute
Dating) were known, sometimes it is required to place of events in their proper order, this is
known as Relative Dating.
There are several principles or laws in the topic of stratigraphy that are fundamental to order
geologic events.
1- Law of Superposition (fig- 1)
In an undeformed sequence of sedimentary rocks, each bed is older than the one above it and is
younger than the one below it. These beds must be unaffected by later folding, faulting or other
physical disturbances.
2- Law of Original Horizontality (fig. 2- A and B)
Sediments commonly tend to accumulate or deposit in approximately horizontal (Planar) layers.
Even if the sediments were settled over irregular surfaces, they keep taking the horizontal form.
3- Law of lateral continuity
At the time of deposition, a water – deposited layer of sediments will be continues until it
reaches the edges of sedimentary basin (shoreline, stream, banks, barrier…etc.) or thins out
(wedges out) to appoint of non- deposition.
4- Law of Uniformitarianism
This law relies on statement given by James Hotton (The present is the Key to the Past). This
means that realizing ancient processes is possible through observing new ones.
5- Law of Cross- Cutting Relationships (fig. 3)
This law implies that the cross is younger than the cutting. For example, when an igneous
intrusion or fault cut through other rocks, they are assumed to be younger than the rocks they cut.
6- Law of Faunal Succession (fig- 4)
Fossils also play a role in determination of relative ages. The concept that fossils in a sequence of
strata where older beds contain types of primary (or primitive) fossils (old) and the younger beds
contain young (or more evolved) fossils.
The theoretical basis for this law was later presented by Charles Darwin’s Theory of Evolution
and Natural Selection.
7- Law of Inclusion (fig- 5)
This law states that inclusions are older than the host or surrounding rocks in which they are
found. For example (in fig- 5) the limestone layer (X) is younger than its sandstone inclusions
that belong to older layer (Y).