Beaumaris Monocline
Melbourne is situated at the north end of the Port Phillip Sunkland, which is bounded on the west by the Rowsley Fault, running from north of Bacchus Marsh to near Geelong, and on the east by Selwyn Fault, running from east of Dandenong to near Cape Paterson. These faults were active from Late Tertiary into Quaternary time and there is still some slight movement on both producing earthquakes and tremors. Beaumaris Bay is situated on the downthrown side of the Beaumaris Monocline that runs north-east from Beaumaris to Clayton. The cliffs at Beaumaris are parallel with the turnover on the monocline.
Monocline
Monoclines are folds consisting of two horizontal (or nearly so) limbs connected by a shorter inclined limb. They can be compared to anticlines, which consist of two inclined limbs dipping away from each other, and synclines, which consist of two inclined limbs dipping towards each other.
Folds such as monoclines, anticlines, and synclines are defined solely on the basis of their geometry, and the names therefore have no genetic connotations. Monoclines are, however, characteristic of regions in which sedimentary rocks have been deformed by dip slip movement along vertical or steeply dipping faults in older and deeper rocks, such as the Colorado Plateau of the southwestern United States. An excellent example of a Colorado Plateau monocline is the Waterpocket fold in Capitol Reef National Park. Most monoclines are classified as drape folds or forced folds because the sedimentary rocks are draped or forced as a result of movement along the underlying faults. Drape folds and forced folds are not necessarily monoclinal, though, so care must be taken to distinguish between geometric and genetic names.
The shape of a monoclinal drape fold is controlled in part by the nature of the underlying fault (normal, reverse, or vertical) and in part by the mechanical nature of the strata being folded. A sequence of relatively thin strata, for example, is less resistant to folding than a single rock unit of the same aggregate thickness. Thick rock units such as massive sandstones may fracture during monoclinal folding because they are too stiff to be bent without breaking, whereas rock units such as shales may be easily folded because they consist of innumerable and thin laminae that are weakly bonded to each other. Likewise, the rock type of the lowermost layer being folded has an influence on the form of the fold. A weak rock such as shale, salt, or gypsum can attenuate much of the movement along the underlying fault and reduce the amplitude of the resulting fold.
Formation of Monocline
Monoclines may be formed in several different ways (see diagram)
- By differential compaction over an underlying structure, particularly a large fault at the edge of a basin due to the greater compactibility of the basin fill, the amplitude of the fold will die out gradually upwards.
- By mild reactivation of an earlier extensional fault during a phase of inversion causing folding in the overlying sequence.
- As a form of fault propagation fold during upward propagation of an extensional fault in basement into an overlying cover sequence.
- As a form of fault propagation fold during upward propagation of a reverse fault in basement into an overlying cover sequence.
What Are Folds?
We often think of rock as hard, brittle material. Throw a rock hard enough on the ground and it will likely break into pieces. But under the right conditions, rock can actually 'flow' in a way that keeps it from breaking. When rock deforms in such a way that it bends instead of breaking, we call this a fold. Folds come from pressure on rocks that occur over very long periods of time (think millions of years), so instead of a quick action like throwing the rock on the ground, it's more like standing on it for millions of years until the pressure is finally enough that it changes shape.
Types of Folds
Folds are created in rock when they experience compressional stress. This is when the rock is being pushed inward from both sides. This is like if you put a spring between your hands and push them together. As you push, you're compressing the spring, and rock can be compressed in the same way over long periods of time. There are different types of folds created by compressional stress depending on which way the rock bends.
An anticline is a fold that arches up as both sides of the rock are pushed inward. You can remember that the anticline creates this type of fold because the arch looks like an 'A' (for anticline).
A syncline is a fold that sinks down as both sides of the rock are pushed inward. You can remember that a syncline creates this type of fold because the fold 'sinks' downward, which sounds like 'syncline.'
A monocline is a fold where the rock layers form an S-shape as the sides of the rock are compressed. You can remember this type of fold because all the layers of rock are still horizontal, going in one direction instead of bending vertically upward or downward like anticlines and synclines. And since 'mono' means 'one,' monoclines are layers in only 'one direction.'
We also have domes, which are like anticlines but instead of an arch, the fold is in a dome shape, like an inverted bowl. Similarly, there are also basins, which are like synclines but again, instead of a sinking arch, the fold is in a shape of a bowl sinking down into the ground.
Lapstone Monocline
Another example of a Monocline is the Lapstone Monocline in the Blue Mountains in New South Wales.
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Sources:-
http://study.com/academy/lesson/geological-folds-definition-causes-types.html
http://www.encyclopedia.com/topic/Monocline.aspx
http://geology.wikia.com/wiki/Monocline