9.2 Folding

When a body of rock, especially sedimentary rock, is squeezed from the sides by tectonic forces, it is likely to fracture and/or become faulted if it is cold and brittle, or become folded if it is warm enough to behave in a ductile manner.

The nomenclature and geometry of folds are summarized in Figure 9.2.1.  An upward fold is called an anticline (or, more accurately, an antiform if we don’t know if the beds have been overturned or not), while a downward fold is called a syncline, (or a synform if we don’t know if the beds have been overturned).  In many areas it’s common to find a series of antiforms and synforms (as in Figure 9.2.1), although some sequences of rocks are folded into a single antiform or synform. A plane drawn through the crest of a fold in a series of beds is called the axial plane of the fold. The sloping beds on either side of an axial plane are called the limbs of the fold. An antiform or synform is described as symmetrical if the angles between each of limb and the axial plane are generally similar, and asymmetrical if they are not. If the axial plane is sufficiently tilted that the beds on one side have been tilted past vertical, the fold is known as an overturned antiform or synform.

If the limbs dip away from one another, they form an antiform. If the limbs dip toward one another, they form a synform.

Figure 10.2.1: Examples of different types of folds and fold nomenclature. Axial planes are only shown for the anticlines, but synclines also have axial planes.
Figure 9.2.1: Examples of different types of folds and fold nomenclature. Axial planes are only shown for the anticlines, but synclines also have axial planes.
Figure 9.2.2: An isoclinal recumbent fold.

A very tight fold, in which the limbs are parallel or nearly parallel to one another is called an isoclinal fold (Figure 9.2.2). Isoclinal folds that have been overturned to the extent that their limbs are nearly horizontal are called recumbent folds.

Folds can be of any size, and it’s very common to have smaller folds within larger folds (Figure 9.2.3).  Large folds can have wavelengths of tens of kilometres, and very small ones might be visible only under a microscope.

Figure 10.2.3: Folded limestone (grey) and chert (rust-coloured) in Triassic Quatsino Formation rocks on Quadra Island, B.C.  The image is about 1 metre across.
Figure 9.2.3: Folded limestone (grey) and chert (rust-coloured) in Triassic Quatsino Formation rocks on Quadra Island, B.C.  The image is about 1 metre across.

Antiforms are not necessarily, or even typically, expressed as ridges in the terrain, nor synforms as valleys. Folded rocks get eroded just like all other rocks and the topography that results is typically controlled mostly by the resistance of different layers to erosion (Figure 9.2.4).

Figure 10.2.4: Example of the topography in an area of folded rocks that has been eroded. In this case the blue and green rocks are most resistant to erosion, and are represented by hills. The pale cream-coloured rocks are the least resistant to erosion, and are represented by valleys.
Figure 9.2.4: Example of the topography in an area of folded rocks that has been eroded. In this case the blue and green rocks are most resistant to erosion, and are represented by hills. The pale cream-coloured rocks are the least resistant to erosion, and are represented by valleys.

As folded rocks are eroded away, anticlines and synclines can be recognized not only by the dip directions of their limbs, but also by examining their map patterns in plan view (Figure 9.2.5). Eroded anticlines expose older rocks near the surface trace of the axial plane, and the rocks get progressively younger as you move away from the axial plane in either direction. Eroded synclines have the youngest rocks exposed near the surface trace of the axial plane, and the rocks get progressively older as you move away from the axial plane in either direction. Examine Figure 9.2.5 to confirm this: the youngest rock in the diagram (labeled ‘6’) is exposed in the centre of the syncline, whereas the oldest rock visible in plan view is exposed in the centre of the anticline (labeled as ‘4’).

Figure 10.2.5: Plan view (top) and cross-section (bottom) of a portion of Figure 10.2.4. Numbers 1 to 6 refer to the relative ages of the layers, where 1 is the oldest and 6 is the youngest. The surface trace of the axial plane are shown for both the anticline and the syncline (red lines with arrows).
Figure 9.2.5: Plan view (top) and cross-section (bottom) of a portion of Figure 9.2.4. Numbers 1 to 6 refer to the relative ages of the layers, where 1 is the oldest and 6 is the youngest. The surface traces of the axial planes are shown for both the anticline and the syncline (red lines with arrows).

 

Practice Exercise 9.1 Folding style

Figure 9.2.6 shows folding near near Golden, B.C. in the Rocky Mountains.  Describe the types of folds using the appropriate terms from above (symmetrical, asymmetrical, isoclinal, overturned, recumbent etc.).  You might find it useful to first sketch the outcrop by tracing one or two beds to get a better idea of the shapes of the folds, then sketch in the axial planes.

Figure 9.2.6

Media Attributions

  • Figures 9.2.1, 9.2.2, 9.2.3, 9.2.4, 9.2.6: © Steven Earle. CC-BY-4.0.
  • Figure 9.2.5: © Siobhan McGoldrick. Derivative of Figure 9.2.4 by Steven Earle. CC-BY-4.0.

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OLD - A Practical Guide to Introductory Geology (2023-2024 Edition) Copyright © 2022 by Matthew Minnett and Benjamin Daniels is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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