Laingsburg folds are a drag EarthCache
Laingsburg folds are a drag
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Folding is a geological phenomenon that occurs in layered rock types whereby the layers are contorted or bent due to some sort of permanent deformation (Wikipedia, 2016). This Earthcache aims to tell you a little more about folding, the interpretation of which is important to geologists wishing to decipher the past history of the rocks. At the listed co-ordinates you will need to safely pull over onto the shoulder of the N1 to view the folding exposed by the road cutting here.
Folding occurs on all scales, in all rock types and at all levels in the crust; all arising from a variety of causes (Wikipedia, 2016). Folds can occur in soft sediments before they are lithified or they can occur in solid rock. The forces required to fold rock are massive, and are generally found only where tectonic plates collide, large igneous chambers intrude or where strata are deeply buried.
Simply folded strata are shaped into synclines (troughs) and anticlines (domes). In nature, folds very seldom occur individually. Folds we see in road cuttings or exposures are almost without exception parts of bigger structures that are kilometres or tens of kilometres across, called synclinoria and anticlinoria (Norman & Whitfield, 2006).
The axial plane of a fold is an imaginary plane cutting through the middle of the fold, separating the opposing limbs while the axis is the line that the axial plane forms where it intersects any bed (Norman & Whitfield, 2006). If the axis of a fold is not horizontal, the fold is said to plunge (see below).
If the axial plane is vertical and bisects the fold, it is said to be symmetrical or upright. Whereas if the axial plane is inclined, the fold is described as asymmetrical or inclined (Whitten & Brooks, 1972). Furthermore, if the axial plane is curved, the fold is said to be an overfold, with a recumbent fold being where the axial plane becomes horizontal (see below).
Folds are also classified according to the relative angle of the opposing limbs. Where the opposing limbs have a shallow dip less than 45o, the fold is termed open while if the limbs have a steeper dip greater than 45o, the fold is considered to be tight. Isoclinal folds are a special type of tight fold where opposing limbs are so tight that they are almost parallel (Norman & Whitfield, 2006) (see below).
Often larger folds have smaller subsidiary folds within the larger fold structure. These are called drag folds. At the listed co-ordinates for this Earthcache you will see a fine example of a drag fold. Looking at the road cutting on the northeastern side of the N1 you will see towards your left hand side an open, symmetrical synclinal fold. On the right hand side of this fold is a subsidiary tight, asymmetrical anticlinal fold. Further to the right of this drag fold, things get rather complicated, with faulting and other deformations having disturbed the strata quite a lot. So we will not attempt to explain what is there, but you are welcome to inspect this further and see if you can develop your own conclusions.
In order to qualify to log this Earthcache, please send your answers to the following questions to the CO by email or using the message board within a reasonable time frame after logging your find.
1. In what rock types can folding occur?
2. The rock in the cutting at GZ is layered, with each of the layers (strata) being of different thickness. What are the minimum and maximum thickness of the layers that you can see in the cutting?
3. Using your own words, explain what the plunge of a fold is?
4. Look closely at the drag fold at GZ. Estimate the plunge of this fold?
5. Although optional, it would be great if you could post a picture of yourself at GZ with the road cutting in the background.
REFERENCES
Norman & Whitfield (2006) Geological Journeys
Whitten & Brooks (1972) The Penguin Dictionary of Geology
Wikipedia (2016) https://en.wikipedia.org/wiki/Fold_(geology)
Additional Hints
(Decrypt)
Erzrzore gb fraq lbhe nafjref!
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