Geology Reference
In-Depth Information
In a conical fold the plunge amount and direction changes with location. A cross
section perpendicular to the crestal line will be closely equivalent to a normal section
in slightly conical structures. Substitute the plunge of the crestal line in Eqs. 6.17 and
6.18 to approximate a normal section. The shorter the projection distance, the better
the approximation.
6.6.1.3
Graphical Projection
The concept of projection along plunge is the basis of the graphical cross-section con-
struction technique of Stockwell (1950). This method makes it possible to project data
onto cross sections that have steep dips, such as vertical sections or sections normal to
gently dipping fold axes. The graphical method is given by the following steps. Refer
to Fig. 6.42 for the geometry.
1. Create the graph on which the cross section will be constructed at the same scale as
the map and align it perpendicular to the plunge direction. Draw the line AC par-
allel to the plunge direction at the edge of the map. AC represents a horizontal line
on the plunge projection. The plunge projection will be constructed from this line.
2. Begin the projection with the point that is to be projected the farthest (P
1
). Project
this point along the dotted line P
1
A, perpendicular to the line AC to point A. From
point A, draw the line AB at the plunge angle,
, from AC.
3. Draw the orientation of the plane of the cross section (line CB) at the desired ori-
entation to the vertical (angle ACB). In Fig. 6.42 the plane of the cross section has
been chosen to be perpendicular to the plunge (angle ABC = 90°). The plane of ABC
represents a vertical cross section in the plunge direction through point P
1
.
4. The length CB is the distance in the plane of the east-west section from the map
elevation of P
1
to its location P
1
' on the cross section. Draw a vertical construction
line (dotted line P
1
C') through P
1
onto the line of section and measure the length C'B'
(solid line) = CB down from the map elevation of the point to find P
1
'.
5. Repeat step 4 to project all other points, for example P
2
is projected to P
2
'.
φ
Fig. 6.42.
Projection of map data onto a
cross section normal to plunge.
The plunge is
to the south.
Numbers
on the map (
square
box
) are the topographic eleva-
tions of the points to be pro-
jected
φ
