76
the line of sight is slanted upward toward a cliff face,
the upper edge of the frame has a smaller scale than
the lower edge. Upper and lower edges cannot both
be exactly overlapped, and some compromise match
must be achieved. All these perspective problems
can be reduced by moving further away from the
outcrop and taking the pictures through a telephoto
lens.
Use of advanced cameras with adjustable bodies
can eliminate some of the regular, geometric distortions. Wizevich (1992a) provided a useful discussion
of equipment and techniques.
An alternative or supplement to photographic
methods is to construct a base map of the outcrop
using surveying techniques. Kocurek et a!. (1991)
carried out a very detailed architectural study of
eolian sandstones using a laser theodolite. ·This
method is, of course, complex and time-consuming.
It may be the only approach that can be used when
the topography of an outcrop is very complex. The
researcher should carefully assess, before beginning
such a survey, how accurate the outcrop base map
needs to be. Precision in the placement of bounding
surfaces to within a few decimeters. is claimed by
Kocurek et al. ( 1991). While this is impressive, it may
not be necessary. Much depends on the measurements that are to be made from the profile. Bearing
in mind that most fluvial depositional elements have
very irregular shapes, and change their thickness by
several meters within a few tens of meters laterally,
striving for a high degree of base-map precision may
be largely a wasted effort. A simple photographic
mosaic, with all its shape and scale distortions, is
ideal for plotting data, and provides a powerful basis
for visualizing and interpreting the geometry of the
architectural elements. Vertical profiles do not need
to be documented directly in the field, but can be
measured from the completed profile if required, for
example, to provide comparisons with subsurface
records.
In most countries, development of field film
can now be done very quickly. ((One-hour" commercial photographic laboratories are available in
many small towns in North America and Europe,
especially in tourist areas, and even in many ThirdWorld countries. Unless the services of a professional photographer are available it is recommended
that the mosaic be made from color-print fUm , as
this is the most popular type of film and the one
which most commercial tourist laboratories are
equipped to deal with. Instant cameras, such as
Polaroid, may also be usable, although print quality
does not tend to be as good. Commerical photoMethods of Architectural-Element Analysis
graphic laboratories are equipped to produce standard prints (such as those of 6 x4 inches, in the
United States) very quickly. Enlargements may take
a few days. In any case, it is now possible to make a
reconnaissance of an outcrop, and take the necessary photographs, and then return with the results to
carry out ground measurements within a few hours
or days.
The next step is to mount the photographic mosaic on a lightweight board suitable to be carried into
the field (thin plywood or Bristol board), and to
attach an overlay of tracing paper or drafting film.
The geologist should then spend a considerable
amount of time poring over the mosaic and drawing
in every visible bedding plane. For a large outcrop
this may take several hours of careful work. The first
round of drawing may be done in the office, but it is
essential that the placement of these lines on the
overlay be checked against the actual outcrop. Commonly, a complex corner of an outcrop, when reduced to the two dimensions of a photograph, can
yield confusing and conflicting bedding traces,
which can only be resolved by going back to the point
from which the picture was taken. For very large or
inaccessible outcrops, Dueholm and Olsen (1993)
and Olsen (1993) recommended the use of photogrammetric techniques to analyze the outcrop photographs. The construction of steroscopic overlaps
between the photographs should be done to facilitate
this. This technique will permit an analysis of the
channel-scale lithosomes, but interpretation of the
smaller-scale features, such as individual macroforms, cannot be carried out reliably without ground
checking.
In the rocks, a bedding plane is a continuous
surface, but it may not be equally visible everywhere
on an outcrop face, because of variable weathering
characteristics. For example, even major channel
scours may be virtually invisible where the basal
channel lag has beeri removed by erosion, and identical sandstones ·are amalgamated across a clean
bounding surface. Inexperienced interpreters tend
to avoid tracing bounding surfaces through such
areas of faint bedding traces, leaving the lines "hanging". It is an essential part of the interpretive process
to eventually complete and join up all these hanging
lines. It may be necessary, however, to leave the
completion of this process until lithofacies and
paleocurrent observations are underway, so that
where there is ambiguity, alternative correlations
can be evaluated in the light of the alternative fluvial
architectures they would create. Architectural-ele- ·
ment analysis necessarily includes this type of ongo-
the line of sight is slanted upward toward a cliff face,
the upper edge of the frame has a smaller scale than
the lower edge. Upper and lower edges cannot both
be exactly overlapped, and some compromise match
must be achieved. All these perspective problems
can be reduced by moving further away from the
outcrop and taking the pictures through a telephoto
lens.
Use of advanced cameras with adjustable bodies
can eliminate some of the regular, geometric distortions. Wizevich (1992a) provided a useful discussion
of equipment and techniques.
An alternative or supplement to photographic
methods is to construct a base map of the outcrop
using surveying techniques. Kocurek et a!. (1991)
carried out a very detailed architectural study of
eolian sandstones using a laser theodolite. ·This
method is, of course, complex and time-consuming.
It may be the only approach that can be used when
the topography of an outcrop is very complex. The
researcher should carefully assess, before beginning
such a survey, how accurate the outcrop base map
needs to be. Precision in the placement of bounding
surfaces to within a few decimeters. is claimed by
Kocurek et al. ( 1991). While this is impressive, it may
not be necessary. Much depends on the measurements that are to be made from the profile. Bearing
in mind that most fluvial depositional elements have
very irregular shapes, and change their thickness by
several meters within a few tens of meters laterally,
striving for a high degree of base-map precision may
be largely a wasted effort. A simple photographic
mosaic, with all its shape and scale distortions, is
ideal for plotting data, and provides a powerful basis
for visualizing and interpreting the geometry of the
architectural elements. Vertical profiles do not need
to be documented directly in the field, but can be
measured from the completed profile if required, for
example, to provide comparisons with subsurface
records.
In most countries, development of field film
can now be done very quickly. ((One-hour" commercial photographic laboratories are available in
many small towns in North America and Europe,
especially in tourist areas, and even in many ThirdWorld countries. Unless the services of a professional photographer are available it is recommended
that the mosaic be made from color-print fUm , as
this is the most popular type of film and the one
which most commercial tourist laboratories are
equipped to deal with. Instant cameras, such as
Polaroid, may also be usable, although print quality
does not tend to be as good. Commerical photoMethods of Architectural-Element Analysis
graphic laboratories are equipped to produce standard prints (such as those of 6 x4 inches, in the
United States) very quickly. Enlargements may take
a few days. In any case, it is now possible to make a
reconnaissance of an outcrop, and take the necessary photographs, and then return with the results to
carry out ground measurements within a few hours
or days.
The next step is to mount the photographic mosaic on a lightweight board suitable to be carried into
the field (thin plywood or Bristol board), and to
attach an overlay of tracing paper or drafting film.
The geologist should then spend a considerable
amount of time poring over the mosaic and drawing
in every visible bedding plane. For a large outcrop
this may take several hours of careful work. The first
round of drawing may be done in the office, but it is
essential that the placement of these lines on the
overlay be checked against the actual outcrop. Commonly, a complex corner of an outcrop, when reduced to the two dimensions of a photograph, can
yield confusing and conflicting bedding traces,
which can only be resolved by going back to the point
from which the picture was taken. For very large or
inaccessible outcrops, Dueholm and Olsen (1993)
and Olsen (1993) recommended the use of photogrammetric techniques to analyze the outcrop photographs. The construction of steroscopic overlaps
between the photographs should be done to facilitate
this. This technique will permit an analysis of the
channel-scale lithosomes, but interpretation of the
smaller-scale features, such as individual macroforms, cannot be carried out reliably without ground
checking.
In the rocks, a bedding plane is a continuous
surface, but it may not be equally visible everywhere
on an outcrop face, because of variable weathering
characteristics. For example, even major channel
scours may be virtually invisible where the basal
channel lag has beeri removed by erosion, and identical sandstones ·are amalgamated across a clean
bounding surface. Inexperienced interpreters tend
to avoid tracing bounding surfaces through such
areas of faint bedding traces, leaving the lines "hanging". It is an essential part of the interpretive process
to eventually complete and join up all these hanging
lines. It may be necessary, however, to leave the
completion of this process until lithofacies and
paleocurrent observations are underway, so that
where there is ambiguity, alternative correlations
can be evaluated in the light of the alternative fluvial
architectures they would create. Architectural-ele- ·
ment analysis necessarily includes this type of ongo-