Geology Reference
In-Depth Information
Box 5.1
DIP, STRIKE, AND PLUNGE
In tilted beds, the bedding planes are said to dip . The
dip or true dip of a bed is given as the maximum angle
between the bed and the horizontal (Figure 5.14a).
The strike is the direction at right angles to the dip
measured as an azimuth (compass direction) in the
horizontal plane.
An anticlinal axis that is tilted is said to pitch or
plunge (Figure 5.14b). The angle of plunge is the
angle between the anticlinal axis and a horizontal plane.
Plunging anticlines can be thought of as elongated
domes. Synclinal axes may also plunge.
()
a
()
b
Plunge
Angle
of dip
Figure 5.14 Terms relating to sedimentary structures. (a) Dip and strike. (b) Plunge.
each side tend to be archetypal dip and scarp slopes, with
a typical drainage pattern, and between the streams that
cross the strike the dipping strata have the characteris-
tic forms of flatirons , which are triangular facets with
their bases parallel to the strike and their apices pointing
up the dip of the rock. The strike ridges are very long
where the folds are horizontal, but they form concentric
rings where the folds form a dome. The scarp and vale
sequence of the Kentish Weald, England, is a classic case
of a breached anticline (Figure 5.15). Strike ridges may
surround structural basins, with the flatirons pointing in
the opposite direction.
Where strata of differing resistance are inclined over
a broad area, several landforms develop according to
the dip of the beds (Figure 5.16). Cuestas form in
beds dipping gently, perhaps up to 5 degrees. They
are asymmetrical forms characterized by an escarpment
or scarp , which normally forms steep slopes of cliffs,
crowned by more resistant beds, and a dip slope , which
runs along the dip of the strata. Homoclinal ridges ,or
strike ridges , are only just asymmetrical and develop
in more steeply tilted strata with a dip between 10
and 30 degrees. Hogbacks are symmetrical forms that
develop where the strata dip very steeply at 40 degrees
plus. They are named after the Hog's Back, a ridge of
almost vertically dipping chalk in the North Downs,
England.
On a larger scale, large warps in the ground surface
form major swells about 1,000 km across. In Africa,
raised rims and major faults separate eleven basins,
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