QUANTITATIVE PHOTOGRAPHY— A GEOLOGIC RESEARCH TOOL 
72 
  
e4r 
sor 
82} 4 
er 
44} 
density 
40} 
32r 
Drainage 
2. F Slcte-phyllite 
ier Sandstone -siltstone 
12 + Sandstone 
de — 
Granite 
o 10 20 30 40 so ‘80 70 
  
  
  
Scale, in thousands 
Frc. 1. Graph showing relation of scale of photo- 
graphs to drainage density of several different rock 
types. 
Shale, Utah 
Shale, South Dakota 
One Mile 
Scale 
aerial photographs. Preliminary data are too 
meager to permit an analysis of geologic 
significance of the drainage density dif- 
ferences, but several interesting observations 
are apparent. The coarse-grained intrusive 
rocks show low drainage densities, despite the 
different geographic locations of the samples. 
By contrast the fine-grained clastic sedimen- 
tary rocks show relatively high drainage den- 
sities. Not all factors affecting drainage den- 
sity have yet been assessed (i.e., climate, 
stage of the geomorphic cycle), but it is clear 
that permeability has a fundamental influence 
on drainage density, with high permeability 
resulting in low drainage density. The per- 
meability may be primary or secondary. For 
example, the low drainage density in granites 
is due in part to the high secondary permea- 
bility that results from fracturing. 
It is the hope that a photo interpretation 
study of drainage density, along with other 
criteria, will permit inferences as to rock 
types or groups of rocks being viewed. Quan- 
titative measurements of drainage density 
and other geomorphic features will provide a 
sound basis for comparing and evaluating 
  
Phyllite, Alabama 
Fic. 2. Sketch of drainage nets for several different rock types in different areas 
giving visual comparison of drainage densities.