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Layers of sediment are deposited approximately horizontally under the action of gravity
The
principle of original horizontality
states that layers of
sediment
are originally deposited horizontally under the action of
gravity
.
[1]
It is a
relative dating
technique. The principle is important to the analysis of
folded
and tilted
strata
. It was first proposed by the Danish geological pioneer
Nicholas Steno
(1638?1686).
From these observations is derived the conclusion that the Earth has not been static and that great forces have been at work over long periods of time, further leading to the conclusions of the science of
plate tectonics
; that movement and
collisions
of large plates of the Earth's
crust
is the cause of
folded strata
.
[2]
As one of Steno's Laws, the principle of original horizontality served well in the nascent days of
geological science
.
[3]
However, it is now known that not all
sedimentary layers
are deposited purely horizontally. For instance, coarser grained sediments such as
sand
may be deposited at angles of up to 15 degrees, held up by the internal friction between grains which prevents them slumping to a lower angle without additional reworking or effort. This is known as the
angle of repose
, and a prime example is the surface of
sand dunes
.
Similarly, sediments may drape over a pre-existing inclined surface: these sediments are usually deposited conformably to the pre-existing surface. Also, sedimentary beds may pinch out along
strike
, implying that slight angles existed during their deposition. Thus the principle of original horizontality is widely, but not universally, applicable in the study of
sedimentology
,
stratigraphy
, and
structural geology
.
See also
[
edit
]
References
[
edit
]
- ^
Levin, H.L. (2009).
The Earth Through Time
(9 ed.). John Wiley and Sons. p. 15.
ISBN
978-0-470-38774-0
. Retrieved
28 November
2010
.
- ^
Michael E. Brookfield, Principles of Stratigraphy, Blackwell Publishing, 2004, p. 116, ISBN 140511164X
- ^
Kardel, Troels, ed. (2013). Nicolaus Steno: Biography and Original Papers of a 17th Century Scientist. Berlin: Springer. ISBN 978-3-642-25078-1.