STRATIGRAPHIC LAWS

Stratigraphic Laws are basic principles that all geologists use in deciphering the spatial and temporal relationships of rock layers. These laws were developed in the 17th to 19th centuries based upon the work of Niels Steno, James Hutton and William Smith, among others. Stratigraphic laws include the following:

1. Original Horizontality- all sedimentary rocks are originally deposited horizontally. Sedimentary rocks that are no longer horizontal have been tilted from their original position.

"Strata either perpendicular to the horizon or inclined to the horizon were at one time parallel to the horizon." Steno, 1669           

For more information refer to Original Horizontality

2. Lateral Continuity- sedimentary rocks are laterally continuous over large areas. A useful way for Wisconsinites to consider this law is to think of snowfalls. As snow falls, it is not limited to the intersection of Main and Division streets, nor UWSP campus, but falls over a larger area such as Central Wisconsin. Sediments also "rain" down in a simialr fashion such that sedimentary layers are laterally continuous over an area similar to, or greater than, Central Wisconsin. 

"Material forming any stratum were continuous over the surface of the Earth unless some other solid bodies stood in the way." Steno, 1669

For more information  refer to Lateral Continuity 

3. Superposition  

"...at the time when any given stratum was being formed, all the matter resting upon it was fluid, and, therefore, at the time when the lower stratum was being formed, none of the upper strata existed." Steno, 1669.

Check out the following webpage:   Superposition

4. Cross-Cutting Relations

"If a body or discontinuity cuts across a stratum, it must have formed after that stratum." Steno, 1669. 

For more information refer to Cross-cutting relations

5. Law of Inclusions- this law states that rock fragments (in another rock) must be older than the rock containing the fragments. Refer to Inclusions. 

6.Law of Faunal Succession- This law was developed by William "Strata" Smith who recognized that fossil groups were succeeded by other fossil groups through time. This allowed geologists to develop a fossil stratigraphy and provided a means to correlate rocks throughout the world.

 

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