82 13.2 Folds

Folds are a type of ductile deformation. They form when rocks bend in response to stress. The sides of a fold are its limbs (Figure 13.10). The limbs meet in a region of curvature called the hinge zone. A fold’s axial surface is an imaginary surface that runs along the hinge zone and cuts the fold in half. The line that forms when the axial surface intersects another surface, such as the top of a bed, is called the axial trace. Axial traces are sometimes marked on geological maps to show the location of the fold’s hinge zone.

Figure 13.10 The parts of a fold. A fold consists of limbs that meet at the hinge zone. An axial surface bisects the fold along the hinge zone. The axial trace is where the axial surface intersects another surface, such as the top of a bed. Source: Karla Panchuk (2018) CC BY-NC-SA 4.0. Photo: Ron Schott (2009) CC BY-NC-SA 2.0 view source

 

Fold Classification

Synclines and Anticlines

Folds can be classified according to the whether the limbs slope toward or away from the hinge zone. If the limbs slope toward the hinge zone (i.e., the hinge zone points downward), as in the fold in the left of Figure 13.11, the fold is called a syncline. If the limbs slope away from the hinge zone  (i.e., the hinge zone points upward), the fold is called an anticline.  There is an anticline on the right side of Figure 13.11. The fold in Figure 13.10 is also an anticline. Sometimes an anticline or a syncline will occur by itself, but they can also occur in a series of alternating synclines and anticlines, similar to the way the anticline and syncline share a limb in Figure 13.11. A sequence of linked anticlines and synclines is called a fold train.

Figure 13.11 An asymmetrical syncline linked to an anticline on a beach in Cornwall, United Kingdom. The beds slope toward the hinge at different angles on either side of the axial surface. Source: Karla Panchuk (2018) CC BY-NC-SA 4.0. Photo: Harry Soar (2014) CC BY-NC-SA 2.0 view source

Symmetrical, Asymmetrical, Overturned, and Recumbent

In a symmetrical fold, the limbs slope at approximately the same angle on either side of the axial surface. The fold in Figure 13.10 is symmetrical. In an asymmetrical fold, the limbs slope at different angles on either side of the axial surface. The syncline in Figure 13.11 is asymmetrical. The limb on the left side of the syncline slopes toward the hinge at a steeper angle than the limb on the right.

If the fold is sufficiently tilted that the beds on one side have been tilted past vertical, and are sloping in the same direction, the fold is overturned (Figure 13.12).

Figure 13.12 Overturned folds in Andalusia in southern Spain. Some limbs have been overturned far enough to be sloping in the same direction on either side of the axial trace. Source: Karla Panchuk (2018) CC BY-NC-SA 2.0. Photo: Ignacio Benvenuty Cabral (2017) CC BY-NC-SA 4.0 view source

It is possible for rocks to be folded so tightly that the fold limbs are nearly parallel. Folds with parallel limbs are called isoclinal folds. A recumbent fold is an isoclinal fold that has been overturned to the extent that the limbs are horizontal (Figure 13.13).

Figure 13.13 A recumbent fold has limbs that are nearly parallel, and an axial trace that is nearly horizontal. Source: Karla Panchuk (2018) CC BY-NC-SA 4.0. Photo: Ignacio Benvenuty Cabral (2017) CC BY-NC-SA 4.0 view source

Folds in the Landscape

Folds can be of any size, and it’s very common to have smaller folds within larger folds (Figure 13.14).  Large folds can extend over 10s of kilometres, and very small ones might only be visible under a microscope.

Figure 13.14 Folded limestone (grey) and chert (rust-coloured) in rocks of the Triassic Quatsino Formation on Quadra Island, British Columbia.  The image is about 1 m across. Source: Steven Earle (2015) CC BY 4.0 view source

When folded rocks are weathered and eroded, they can alter the landscape by forming long ridges and valleys (Figure 13.15). Ridges and valleys curve into V-shapes if the hinge of the fold is not horizontal. A fold with a hinge that slopes downward is called a plunging fold (Figure 13.16).

Figure 13.15 Ridges and valleys in central Pennsylvania formed from weathered and eroded folds. The V-shapes indicate the folds are plunging. Source: NASA on the Commons (2001) Public Domain view source

 

Figure 13.16 Plunging folds have sloping hinges. Plunging folds are described in terms of the plunge angle, the angle the hinge makes with a horizontal line. Inset- When a plunging fold intersects a surface, the result is a V-shaped pattern. Source: Karla Panchuk (2018) CC BY-SA 4.0. Photo- Dieter Mueller (2004) CC BY-SA 3.0 view source

Folds can create landforms, but anticlines are not necessarily expressed as ridges in the terrain. Likewise, synclines do not necessarily appear as valleys. When folded rocks erode, the landform that results depends how resistant individual layers are to erosion. For example, if the rocks in the interior of an anticline are more resistant to weathering than the surrounding rocks, a ridge will result (e.g., the low hill represented by units 4 and 5 in Figure 13.17, top). On the other hand, if rocks in the interior of the anticline are weaker, a valley will result (Figure 13.17, bottom, units d1 and d2). Similarly, a syncline with stronger rocks in the interior will weather to form a ridge, and a syncline with weaker rocks in the interior will weather to form a valley.

Figure 13.17 Cross-sections of eroded folds expressed as hills and valleys, from an early study on the geology of Wales, Devon, and Cornwall. Top- An anticline in Shropshire, England. Beds in the interior of the anticline form a gentle hill. Bottom- An anticline in Herefordshire, England in which beds in the interior of an anticline weathered to form a valley. Source: Symonds (1872) Public Domain. View source: TopBottom

 

Exercise: Fold Types

What kind of folds are shown here? If you are finding it difficult to see the folds, follow the trace line formed by the white beds through the outcrop.

Figure 13.18 Folds in the Rocky Mountains near Golden, British Columbia. Source: Steven Earle (2015) CC BY 4.0 view source

References

Symonds, W. S. (1872). Records of the rocks; or, Notes on the geology, natural history, and antiquities of North & South Wales, Devon, & Cornwall. London: J. Murray  Read the book