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| Geological photographs from around the world | ||||||||
The Principle of Original Horizontality suggests that layers of sediments laid down in water are deposited as horizontal layers, parallel with the surface of the water and at right angles to the direction of gravity. (As usual in Geology, there are some exceptions to the rule!)
Sedimentary rocks - hardened, or lithified, sediments - which outcrop at the surface of the Earth's surface are often not horizontal and do show evidence of deformation.
If stress is applied steadily, the rocks will bend or fold.
When the stress is applied suddenly,
or if the rocks are relatively brittle, they will fracture. If there is enough stress for the rocks to be
displaced as well as fracture then the line of displacement is known as a fault. (With this sudden movement,
an earthquake is triggered.)
If the stress is used up in the process of fracturing so
that there is no displacement, the resulting pattern of cracks is called jointing.
As a general guide, in an exposure of strata such as a road cutting or a quarry:
- the beds, or strata, often slope (or dip) in one direction, suggesting that they have been tilted; in fact, the sloping beds will be just one part (or limb) of a larger fold, caused by compression;
- complete folds may be seen if the cutting is big enough;
- a fault will occur by itself, or sometimes with one or two related faults;
- joints occur as a regular pattern, with the fractures spaced about 1 - 2 metres apart.
Folding
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Faulting
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Jointing
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| It is perhaps surprising - with so much stress acting in the crust - that other rocks have remained undeformed for hundreds of millions of years. Although the rocks of the Grand Canyon remain horizontal, there is evidence in the photograph of movement of some sort. |
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The Study Topics
- Folding
- Faulting
- Jointing and Cleavage
- Unconformities
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