Laboratory Manual for Introductory Geology 4e

10.3.2 Block Diagrams of Simple Folds
When rocks are deformed and geologic structures develop, the geometry of layers
depicted on a block diagram becomes more complicated (FIG. 10.5). If deformation
causes rock layers to bend and have a curve, we say that a fold has developed.
Geologists distinguish between two general shapes of folds: an anticline is an archlike
fold whose layers dip away from the crest, whereas a syncline is a trough-like fold
whose layers dip toward the base of the trough (Fig. 10.5b). Anticlines arch layers of
rocks upward; synclines do the opposite—their strata bow downward. For the sake of
discussion, the side of a fold is a fold limb, and the line that separates the two limbs
(i.e., the line along which curvature is greatest) is the fold hinge. We can represent the
hinge with a dashed line and associated arrows on the map: the arrows point outward
from the hinge on an anticline and inward toward the hinge on a syncline. On a block
diagram of the folds, we see several layers exposed (Fig. 10.5c). Note that the same set
of layers appears on both sides of the hinge. (Exercise 10.5 allows you to discover that
the age relations of layers, as seen on the map view, indicate whether a given fold is an
anticline or syncline.) If the folded strata include a bed that is resistant to erosion, the
bed may form topographic ridges at the ground surface (Fig. 10.5d). Note that the
layers are repeated symmetrically, in mirror image across the fold hinges.
The hinge of a fold may be horizontal, producing a nonplunging fold (FIG. 10.6a),
or it may have a tilt, or “plunge,” producing a plunging fold (FIG. 10.6b); an
arrowhead on the hinge line in Fig. 10.6 indicates the direction of plunge. Note
that if the fold is nonplunging, the contacts are parallel to the hinge trace, whereas
if the fold is plunging, the contacts curve around the hinge—this portion of a fold
on the map surface is informally called the fold “nose.” Anticlines plunge toward
their noses, synclines away from their noses. Curving ridges may form if one or
more of the beds that occur in the folded sequence are resistant to erosion.
In some situations, the hinge of a fold is itself curved, so that the plunge direction
of a fold changes along its length. In the extreme, a fold can be as wide as it is long.
In the case of downwarped beds, the result is a basin, a bowl-shaped structure, and
in the case of upwarped beds, the result is a dome, shaped like an overturned bowl.
Try Exercise 10.6 to see the differences between basins and domes.
FIGURE 10.5 The basic types of folds.
Fold limbs
Hinge of anticline
Hinge of syncline
3
2
1
(a) Undeformed, horizontal strata.
Anticline
(b) After deformation, folds have
formed. This figure shows the fold
shape before erosion.
3
2
1
Syncline
Fold limb
Future
erosion level
Hinge trace
Rocks younger than unit 3
Resistant bed
3 2 1
2 3 3 2 1
Rocks older than unit 1
(c) After erosion, the map surface of the block exposes
several different layers of strata.
(d) Topographic ridges may form if one of the beds of
the folded strata is resistant to erosion.