Along convergent plate boundaries, rock strata are often bent into a series of wavelike undulations called folds. Folds in flat-lying strata are much like those that would form if you were to hold the ends of a sheet of paper on a flat surface and then push them together. Such folds come in a wide variety of sizes and configurations. Some are broad flexures in which strata hundreds of meters thick have been slightly warped. Others are very tight, even microscopic structures found in metamorphic rocks. Most folds result from compressional stresses that result in a lateral shortening and vertical thickening of the crust.

Folds are geologic structures that result when originally planar (“flat”) surfaces, such as sedimentary strata, are permanently bent as a result of deformation. Each layer is bent around an imaginary axis called a hinge line, or simply a hinge (Figure 1).

Folds are also described by their axial plane, which is a surface that connects all the hinge lines of the folded strata. In simple folds, the axial plane is vertical and divides the fold into two roughly symmetrical limbs (refer to Figure 1). However, the axial plane often leans to one side so that one limb is steeper than the other.

Anticlines and Synclines

The two most common types of folds are anticlines and synclines (Figure 2). Anticlines usually form by the upfolding, or arching, of sedimentary layers. Typically found in association with anticlines are downfolds, or troughs, called synclines. Notice in Figure 2 that the limb of an anticline is also a limb of an adjacent syncline.

Depending on their orientation, these basic folds are described as symmetrical when the limbs are mirror images of each other and asymmetrical when they are not. The limbs of a symmetrical fold dip at the same angle, resulting in a vertical axial plane. By contrast, the limbs of an asymmetrical fold dip at different angles, which results in an inclined axial plane. An asymmetrical fold is said to be overturned if both limbs dip in the same direction, with one limb tilted beyond the vertical (refer to Figure 2). An overturned fold can also “lie on its side” so that the axial plane is horizontal. These recumbent folds are common in highly deformed mountain belts, such as the Alps.

Folds can also be tilted by tectonic forces so their hinge lines slope downward (Figure 3A). Folds of this type are said to plunge because the hinge lines penetrate Earth’s surface. Sheep Mountain, Wyoming, is an example of a plunging anticline (Figure 3C). Notice in Figure 3B that a V-shaped pattern is produced when erosion removes the upper layers of a plunging fold and exposes its interior. In the case of an anticline, such as Sheep Mountain, the tip of the V points in the direction of plunge (Figure 3C); the opposite is true for a syncline.

A good example of the topography that can result when erosional forces attack folded sedimentary strata is found in the Valley and Ridge Province of the Appalachians (refer to Figure 4). It is important to realize that anticlines typically do not show up as ridges, nor synclines as valleys. Rather, ridges and valleys result from differential weathering and erosion. For example, in the Valley and Ridge Province, resistant sandstone beds remain as imposing ridges separated by valleys that are cut into more easily eroded shale or limestone beds.

Domes and Basins

When a broad upwarping of basement rock deforms the overlying cover of sedimentary strata to produce a circular or slightly elongated bulge, the feature is called a dome (Figure 5A). The Black Hills of western South Dakota represent a large structural dome generated by upwarping. Here, erosion has stripped away the highest portions of the overlying sedimentary beds, exposing older igneous and metamorphic rocks in the center (Figure 6).

Structural domes can also be formed by the intrusion of magma (laccoliths), as shown in Figure 7. In addition, the upward migration of buried salt deposits can produce salt domes like those beneath the Gulf of America (Gulf of Mexico). Salt domes are economically important rock structures because when salt migrates upward, the surrounding oil-bearing sedimentary strata deform to form oil reservoirs (refer to Figure 8).

The inverse of a dome is a downwarped structure termed a basin (Figure 5B). Several large structural basins exist in the United States (Figure 9). The basins of Michigan and Illinois have gently sloping beds similar to saucers. These basins are thought to have resulted from large accumulations of sediment, whose weight caused the crust to subside. A few structural basins may have resulted from giant meteorite impacts.

Because the sedimentary beds in structural basins usually slope at low angles, basins are identified mainly by the age sequence of their strata. In a basin, the youngest rocks are at the center and the oldest on the flanks. In a dome, such as the Black Hills, the reverse is true: The oldest rocks form the core.

Monoclines

Although we have discussed folds and faults separately, they may occur together as a result of the same tectonic stresses. One example can be found in broad regional features called monoclines. Particularly prominent features of the Colorado Plateau, monoclines (mono = one, kleinen = incline) are large, steplike folds in otherwise horizontal sedimentary strata (Figure 10). These folds appear to have resulted from the reactivation of ancient, steep-dipping reverse faults located in basement rocks beneath the plateau. As large blocks of basement rock were displaced upward, the comparatively ductile sedimentary strata above responded by draping over the fault like clothes hanging over a bench. Displacement along these reactivated faults can exceed  kilometer (0.6 miles).

• Folds are wavelike undulations in layered rocks that develop through ductile deformation caused by compressional stresses.

• Anticlines usually arise by upfolding, or arching, of sedimentary layers, whereas synclines are downfolds, or troughs. Anticlines and synclines may be symmetrical, asymmetrical, overturned, or recumbent.

• When folded rocks erode to form a series of ridges and valleys, the ridges represent resistant beds (not anticlines), and the valleys represent softer beds (not synclines). A fold is said to plunge when its axis penetrates the ground at an angle. This results in a V-shaped outcrop pattern.

• Domes and basins are large bowl- or saucer-shaped folds that produce roughly circular outcrop patterns. When eroded, a dome has the oldest beds in the middle, and a basin has the oldest beds around the margin.

• Monoclines are large steplike folds in otherwise horizontal strata that develop when beds drape over a vertical offset produced by subsurface faulting.

• anticlines: Folds in sedimentary strata that resemble an arch; the opposite of synclines.

• basin: A circular downfolded geologic structure. The youngest rocks are in the center of a basin structure.

• dome: A roughly circular upfolded geologic structure. The oldest rocks are in the center of a dome structure.

• folds: Rock layers, or series of layers, that were originally horizontal and subsequently deformed.

• monoclines: One-limbed flexures in strata. The strata are unusually flat-lying or very gently dipping on both sides of the monocline.

• syncline: Linear downfolds in sedimentary strata; the opposite of anticlines.