Methods And Principles Of Geology
                     METHODS AND 
PRINCIPLES OF GEOLOGY
Principles of Geology
• There are several basic principles that geologists use to figure out the history of a rock:
– Uniformitarianism
– Original horizontality
– Superposition
– Cross-cutting relationships
– Walther’s Law
Source: www.e-education.psu.edu
Uniformitarianism
• The principle of uniformitarianism states that processes that alter the earth’s crust are the same 
processes that occurred millions of years ago. Furthermore, the results of processes today are 
the same as the results of the same processes millions of years ago. 
Source: www.e-education.psu.edu
Original Horizontality
• The principle of original horizontality states that sediment is deposited horizontally. This is 
sometimes easier to envision with liquids: imagine pouring water into a cup. The surface of the 
water is perfectly flat - horizontal. 
Source: www.e-education.psu.edu
Superposition
• This principle states that a sequence of rocks in their original orientation will have the oldest rock 
on the bottom and the youngest rock on the top. 
• A simple way to think about this is that for something to be on top of something else, for example 
in order to put a book on top of a table, the table has to be there. 
Source: www.e-education.psu.edu
Walther's Law
• Walther’s law is a little different from the previously discussed geologic principles, but it is just as 
important. Instead of dealing only with relative time, Walther’s law deals with relative space 
through time. 
• Walther’s law states that depositional environments that are laterally adjacent on the surface of 
the earth will also appear in succession in a stratigraphic sequence. 
Source: www.e-education.psu.edu
Lateral Continuity 
• Lateral Continuity suggests that all rock layers are laterally continuous and may be broken up or 
displaced by later events. 
• This can happen when a river or stream erodes a portion of the rock layers. This can also happen 
when faulting occurs. Faulting causes displacement in rock units. 
Source: timescavengers.blog
Cross-cutting relationships
• Cross-cutting relationships also helps us to understand the timing of events. Younger features 
cut across older features. Going back to the fault on this image, we know that these rock layers 
were involved in the fault movement because they are all offset. 
Source: timescavengers.blog
Faunal Succession
• The Principle of Faunal Succession states that a species appears, exists for a time, and then 
goes extinct. Time periods are often recognized by the type of fossils you see in them. Each 
fossil has a ‘first appearance datum’ and a ‘last appearance datum’.
Source: timescavengers.blog
Angular unconformities
• Angular unconformities are represented by an older group of rock layers has been tilted, eroded, 
and another younger set of rock layers were deposited on top of this erosional surface. 
• The tilting process is commonly by a mountain building event, it doesn’t necessarily have to be in 
the mountains but the effects of mountain building processes are long reaching.
Source: timescavengers.blog
Disconformities
• Disconformities are an erosional surface between two sets of rock layers. Unlike with angular 
unconformities, there is no tilting of the older rock layers. This makes disconformities difficult to 
recognize because the erosional surface is often very difficult to find.
Source: timescavengers.blog