tect and may be confused with short linea-
ments unless studied carefully, both in-
dividually and in relation to other such
“lineaments.”
Lineaments shown by drainage and
marked topographic alignments are simple
to map. Lineaments due to soil tone and
subtle vegetation alignments are more dif-
ficult to map. It should be borne in mind
that the same lineament may be expressed
in one segment by drainage alignments and
straight stream courses, and along another
segment by soil tone, etc.
The obvious drainage and topographic
lineaments may be seen by placing the
mosaic on a table or wall, or by stapling
individual photographs on a Celotex
covered table or wall so as to form a
"staple" mosaic. Crude as it may sound,
the best method for plotting these linea-
ments consists of equipping three or four
geologists with ‘‘grease’’ pencils, and al-
lowing them to mark all the large linear
features they see. It has been found helpful
in studying very large mosaics to mount
them on a wall and periodically stand 15
to 25 feet away while viewing the mosaic
with the naked eye and through an or-
dinary two or three-power reducing lens.
Regional linear trends may often be seen
through the reducing lens which are not
obvious to an observer standing close to
the mosaic. If the mosaic is small, under
ten square feet, no particular advantage is
gained by the distant viewpoint. In either
event the observer or observers should
vary their position from very close to
several feet away from the mosaic period-
ically.
Many of the longer, more subtle tonal
lineaments (Olive, in press) are not readily
apparent when the line of sight is per-
pendicular to the mosaics. Many of these
lineaments are best seen by mounting the
mosaics on a large table and viewing with
line of sight at a low angle to the mosaic
surface. It has been found convenient
(and comfortable) to seat several geolo-
gists around the table in tilting chairs
mounted on casters. These geologists study
the mosaic which is on the table, from a
low angle and, by moving the chairs, vary
the direction of viewing, while plotting the
lineaments seen. The low angle of view
causes foreshortening of the lineament
parallel to the line of sight, and emphasizes
subtle tonal alignments as well as vegeta-
tion, drainage and topographic alignments.
A procedure that has been used with
MAPPING GEOLOGIC FRACTURE TRACES AND LINEAMENTS 575
success is to first place the mosaic, or
photographs, on a wall and allow one or
more geologists to mark with grease pencil
all of the topographic, vegetation and
tonal alignments more than one mile long.
Following this, the mosaic is viewed at a
low angle on a table as described above
and any additional lineaments seen are
plotted. If desired, the lineaments seen
only when viewed at a low angle may be
marked by different color or symbol. It
should be remembered that lineaments
may be discontinuously expressed on the
mosaic, and it may be desirable to mark
them with a solid line where visible and
use a dashed line between clearly ex-
pressed segments. Connecting of individual
segments to form a single lineament is a
matter of personal discretion, and it is
therefore useful to differentiate the clearly
expressed portions of the lineament from
the observer’s presumed connections.
Care must be taken when stapling pho-
tographs or mosaics to a table or wall to
obtain a flat surface, otherwise highlights
or shadows on the irregular surface may
confuse the observer. In addition, false
soil tonal lineaments may be due to varia-
tions in contrast of individual photographs
or flight lines, and other non-geologic fea-
tures (Olive, in press).
The mapped lineaments are transferred
to a transparent overlay or compiled from
the individual photographs by any of the
common procedures.
Some considerations with regard to
lineament mapping are listed below:
1) To make the study as objective as
possible it is advisable to have more than
one geologist study the mosaics simul-
taneously.
2) Lighting is important, particularly
when mapping lineaments due to soil tone
or vegetation. The lighting should be uni-
form and of medium to low intensity.
Mounting the mosaics on a light table and
viewing by transmitted light has been used
successfully.
3) The scale of the mosaics governs the
minimum size of the lineaments mapped.
Lineaments under about one-inch long on
the mosaic are not conveniently observed,
thus the mosaic should be at a scale of 1
to 63,360 or larger. It is of course perfectly
feasible to use smaller scales, in which case
only lineaments longer than one mile will
be easily seen. To a considerable extent the
best scale to use may be governed by the
purpose of the study.
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