PHOTOGRAMMETRIC ENGINEERING 
limits are in man’s mind, for he cannot easily 
evaluate things that he does not realize exist. 
The second point, although philosophical, 
is also appropriate as an introduction to a de- 
tailed discussion of micro-features. It is 
worthwhile to consider the thought that bed- 
rock exposures appearing in an air-photo show 
the same tendencies in physical form that are 
found in weathered hand specimens. This says, 
in effect, that in spite of the difference in size 
between a hand specimen and a land mass of 
the same rock, they will have similar physical 
characteristics. Consequently, when a land 
mass is reduced in size, as it is when it appears 
on an air photograph, the comparison be- 
comes nearly direct. The comparison cannot 
be made without some thought and study, 
but for experimental purposes much can be 
learned by applying this initially to granite, 
gneiss, schist, lavas, basalts, limestone and 
shales. 
Micro-relief is the most obvious and the 
most important phase of micro-features. It 
applies especially to bedrock identification. It 
consists of minor irregularities of surface or 
outcrop exposure that are related to the mode 
of formation, segregated mineral constituents, 
bedding plains, joint patterns, and other in- 
fluences that establish opportunities for dif- 
ferential weathering on the surface of the 
rock mass. 
A micro-feature seldom occurs only once; 
it is usually repeated several times within a 
local area. If this were not true, it is doubtful 
that it would be detected. Repetition of these 
features in the same local area take it out of 
the class of accidental occurrences and bring 
it up to a recognizable level. Like a noise sig- 
nal below the audible level that can be raised 
to audibility by repetition, these tiny features 
on an outcrop repeat, or connect, or associate 
to form an identifiable pattern. Knowing 
what it is as well as what it looks like on the 
photo and on the ground, raises it to a level of 
significance that often makes it vital to prop- 
er identification. 
Just as schist weathers more rapidly than 
gneiss, certain constitutional masses within a 
granite will weather differentially to produce 
an over-all surface characteristic that falls into 
the macro-relief category. Superimposed on 
this in some granites, residual boulders rang- 
ing from two feet to twenty feet in diameter 
form rounded outcrops. They are concen- 
trated in gullies by erosion and gravity, and 
can be detected by careful observation in air- 
photos, but are never noted on a topographic 
sheet. Fractures in granites are micro-features 
combining relief and vegetation and tone that 
in turn is related to soil and moisture. 
The “condition” of a fracture is reflected 
in its micro-relief. If it is old and weathered 
the edges have crumbled and the line widens 
and narrows along its course. This presents a 
distinct contrast to a recent fracture that of- 
ten seems to disappear on bold outcrops be- 
cause of its tightness. The differentiation is 
especially worthwhile in ground water work. 
Along with relief, tone is also indicative of 
weathering and the presence of soil. Granites 
having a deep soil mantle as in tropical areas 
will retain a faint streak, coincident with the 
fracture, that is slightly darker than the gen- 
eral soil color. 
Another example of micro-relief is to be 
found in relation to basaltic type rocks. In 
these rocks that form a columnar structure, 
the weathered edge of a flow presents in fine 
detail a ''saw-tooth" appearance resulting 
from the falling away of the columns. This 
process also forms a unique talus below the 
outcrop. One does not necessarily see the 
minute indentation left by the removal of an 
individual basalt column, but by groups of 
columns that are undercut by subsurface ero- 
sion. The observer should also understand 
that this may occur only at two or three 
points along an extended line of outcrop. The 
balance of the outcrop may be ''incriminated" 
by association, continuity, etc. 
On the surface of basalt flows, sills and re- 
lated forms other than dikes, it is often pos. 
sible to see flow patterns although they are 
measurable in inches of depth. Near the edge 
of drainage-ways where soil has been removed 
by erosion, and perhaps wind has scoured the 
surface, multiples of the polygon shrinkage 
pattern can be seen on the surface. In fact, on 
all eroded surfaces it is natural to expect the 
best degree of development of micro-forms of 
relief and color tone. 
Where dikes have been exposed, weather- 
ing attacks in a conventional manner that 
makes it obviously a rock solidified by cool- 
ing. The temperature gradient prevailing be- 
tween the contact faces and the interior oí 
the dike results in differential structural con- 
ditions that respond accordingly to weather- 
ing. The result is a hard core with rotten, fri- 
able rock crumbling away at the edge leaving, 
in thin dikes, a sharp convex ridge. In thick 
dikes the tendency is to minimize this edge ef- 
fect of cooling and give it a blocky structure 
along the exposed edges. 
In recent flows the micro-relief is most obvi- 
ous. The same principle applies to the ''dat- 
ing" of adjacent flows. Age modifies the flow 
patterns in ways that are both obvious and in- 
teresting. When the molten lava comes into 
contact with other rock, it cools and builds up 
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