Geology Reference
In-Depth Information
Currie et al. 1962). A short-wavelength structural lithic unit may form inside the bound-
ary of a larger-wavelength unit and be folded by it, in which case the longer-wave-
length unit is termed the dominant structural-lithic unit and the shorter-wavelength
unit is a conforming structural-lithic unit (Currie et al. 1962).
The dip changes within structural-lithic units may obscure the map-scale geometry.
For example, the regional or map-scale dip in Figure 1.15 is horizontal, although few
dips of this attitude could be measured. Where small-scale folds exist, the map-scale
geometry may be better described by the orientation of the median surface or the
enveloping surface (Fig. 1.16; Turner and Weiss 1963; Ramsay 1967). The median sur-
face (median line in two dimensions) is the surface connecting the inflection points of
a folded layer. The inflection points are located in the central region of the fold limbs
where the fold curvature changes from anticlinal to synclinal. An enveloping surface
is the surface that bounds the crests (high points) of the upper surface or the troughs
(low points) of the lower surface on a single unit. Figure 1.16 shows that the dip ob-
served at a single location need not correspond to the dip of either the median surface,
the enveloping surface, or to the trace of a formation boundary. The dip of the median
surface may be more representative of the dip required for map-scale interpretation
than the locally observed bedding dips.
Fig. 1.16.
Terminology for folded surfaces
Fig. 1.17.
Fold symmetry.
a
Symmetri-
cal, upright.
b
Asymmetrical,
overturned.
c
Symmetrical,
overturned.
d
Asymmetrical,
upright.
Arrow
gives direction
of vergence
