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Folds in rocks are bends caused by stress.

They develop gradually over time and their folds can be categorized based on the shape, symmetry, and orientation.

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There are three types of folds based on the shape, and they are:

Anticline

Syncline

Monocline

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An anticline is an upward-aching fold in lock layers, meaning that it resembles an "A" shape.

It forms due to compressional forces that push rock layers together, causing them to bend others.

Key Characteristics:

The oldest rock layers are found in the middle/center of the fold, and can be symmetrical or asymmetrical, depending on the intensity of deformation.

They can appear as large-scale structures that stretch for kilometers, and is often associated with oil and gas traps, as hydrocarbons can accumulate in the crest of the fold.

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A syncline is a downward-curved fold where rock layers dip toward the center, which causes the formation of a "U" shape.

Synclines form due to compressional forces that push the layers together, causing them to bend into a U-shaped fold.

They are commonly found in mountain belts and deformed sedimentary basins.

Some key characteristics of this type of folding is the fact that the limbs dip toward the center of the fold (this can be seen in the diagram).

Additionally, the youngest rock layers are in the core, and they are the opposite of an anticline in terms of rock layer orientation.

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A monocline is a type of fold characterized by a steep, stairstep-like bend in otherwise horizontal or gently dipping rock layers.

Some characteristics that differentiate monocline from the other folds are that the fold consists of one steep limb, unlike anticlines and synclines that have two limbs.

The steepness can range from a gentle flexure to having nearly vertical bends.

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Anticlines, or rocks that have an upward-facing bend, can range to a few meters across in rock formations (small-scale) to hundreds of kilometers in mountain ranges (large-scale).

The Appalachian Mountains have massive anticlines formed during continental collisions.

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A syncline, or rocks that have a downward-facing bend, also vary in size.

The size of synclines can range from minor bends in rock layers to broad, basin-like depressions spanning vast areas.

In mountain belts, many valleys result from the erosion of synclines, where the softer rock layers in the fold core are worn away over time.

The Illinois Basin is one example of a syncline that stretches for hundreds of kilometers and affects sediment accumulation and regional drainage.

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Monoclines are step-like folds where rock layers bend steeply in one direction before leveling out again.

Even though monoclines are often smaller than synclines and anticlines, they can sometimes reach tens of kilometers, particularly in fault-influenced areas.

Unlike symmetrical anticlines and synclines, monoclines often form due to vertical displacement along deep-seated faults, creating unique landscape features.

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As previously mentioned, there are three types of folds that occur when a rock experiences stress, and they are: anticline, syncline, and monocline.

Anticline:

A fold that arches upward, with the oldest rock layers at the core and younger layers on the flanks.

Anticlines are often associated with mountain ranges and can serve as traps for oil and natural gas.

Syncline:

A fold that curves downward, with the youngest rock layers at the center and older layers on the outer edges.

Many valleys in folded mountain regions are the result of eroded synclines.

Monocline:

A step-like fold in which rock layers bend steeply in one direction before leveling out.

These are typically caused by faults deep underground.

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Each fold has distinct structural components: crests, troughs, limbs, hinges, and axial planes.

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Crest (for anticlines) and Trough (for synclines): The highest and lowest points of a fold.

Limbs: The sloping sides of the fold extending from the crest or trough.

Hinge: The point of maximum curvature where the fold bends the most.

Axial Plane: An imaginary surface that divides the fold into two symmetrical or asymmetrical halves.