RULE OF V'S 17
RULE OF V'S As the valley gradient and structural dip determine
whether the pattern will V up- or downstream, the
In areas of sloping terrain, additional factors boundary case occurs when the two are parallel
are involved in determining the character of (Fig. 3.4e).
the outcrop pattern, including slope angle and
direction with respect to the attitude, and on These examples illustrate clearly the general
variations in slope angle and direction. In principle that map patterns depend on the
other words, in addition to thickness and relation between slope and dip and not
angle of dip, map pattern also depends on the directly on the actual slope and dip angles,
details of topography. The relationships be- which are measured with respect to horizontal.
tween dip and topography have been for- Another and opposite set of rules could be
malized into a series of rules, called the Rule formulated for traces crossing ridges. How-
of V's, by which the direction of dip of ever, such ridges tend to have greater variety
structural planes can be estimated directly of shapes (sharp, broadly rounded, flat, etc.),
from outcrop pattern. Wherever the trace of depending on the area's geomorphic history,
a plan crosses a valley the resulting outcrop and the patterns are correspondingly variable.
pattern is characteristic of the attitude, Stream valleys on the whole tend to produce
especially dip. There are several distinct type more uniform patterns. In the stated rules the
of patterns (Fig. 3.4). strike was assumed to cross the valley at
approximately 90°; the resulting V-patterns
1. Horizontal planes: Topographic contour lines are more or less symmetrical. With other
can be considered to be the surface traces of strike directions, asymmetrical V's result,
imaginary horizontal planes. The traces of real with the limiting case occurring when the
horizontal planes therefore exactly follow the valley axis and strike are parallel.
topographic contours. Such patterns are completely
controlled by topography; the outcrop trace faith-
A simple, easily remembered statement that
fully reflects the contour lines in every detail. summarizes all these relationships can be
Therefore, the outcrop pattern V's upstream, just as given: the outcrop V points in the direction in
the contour lines do (Fig. 3.4a). which the formation underlies the stream
2. Planes inclined upstream: As the attitude de- (Screven, 1963). Better yet, however, is to
parts from horizontal, with the dip direction visualize the geometrical relationship between
upstream, the pattern made by the horizontal plane topography and planar structures in three
is progressively modified into a blunter V, still dimensions. In an area of topographic relief
pointing upstream (Fig. 3.4b ). With steepening dip, the outcrop pattern of dipping beds viewed
the outcrop trace is an increasingly subdued reflec- obliquely from an airplane down the line of
tion of topographic detail.
dip appears in edge view (the block diagram
3. Vertical planes: In this special case of a 90° dip,
the outcrop trace is straight and parallel to the
of Fig. 3.4b is very nearly in this position).
strike, regardless of the topography. There is no V The traces of the inclined layers are straight
at all, and thus no control by the topography on and thickness appears directly. From this
the map pattern (Fig. 3.4c). position the topographic irregularities have no
4. Planes inclined downstream: There are three effect on the pattern, and in this sense it is
subcases, depending on the relationship between the same as the case of the vertical dip in map
the dip angle and the valley gradient. view. In other words, the pattern is complex
a. With dip greater than valley gradient, the from every other view point, but simple when
pattern V's downstream (Fig. 3.4d). viewed down-dip. The relationship would
b. If dip and gradient are exactly equal, the hold, of course, for a scaled topographic relief
outcrop trace does not cross the valley and
model with outcrop pattern included. By
there is no V (Fig. 3.4e). However, stream
gradients steepen headward, continuous planar
learning to perceive the surface depicted by
structure must therefore cross somewhere up- contours on a map as a relief model, the
stream. mind's eye can accommodate the influence of
c. If the dip is less than the valley gradient, the topography of the outcrop pattern. This is
but still in a downstream direction, the pattern not always easy to learn; practice is the key.
will V upstream (Fig. 3.4f). Once the ability is attained, however, it is a
18 PLANES AND TOPOGRAPHY
powerful aid in map interpretation, for even can be viewed down-dip directly on the map
in areas of considerable and varied relief, and with a great simplification.
therefore irregular map patterns, the structure
(a) (b) (c)
(d) (e) (f)
FIGURE 3.4 Outcrop patterns illustrating the rule of V's: (a) horizontal layer, (b)
layer dipping upstream, (c) vertical layer, (d) layer dipping downstream, (e) layer and
valley with equal inclinations, (f) layer dipping downstream at an angle at less than
valley gradient.
