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1. Land masses undercut by streams; 
2. Steep slopes having large masses of loose soil and rock; 
3. Sharp line of break at the scarp (head end) or presence of 
tension cracks or both; 
4. Hummocky surface of the sliding mass below the scarp; 
5. Unnatural topography, such as spoon-shaped trough in the 
terrain; 
6. Seepage zones; 
7. Elongated undrained depressions in the area; 
8. Closely spaced drainage channels; 
9. Accumulation of debris in drainage channels or valleys; 
10. Appearance of light tones where vegetation and drainage have 
not been reestablished; 
11. Distinctive change in photograph tones from lighter to 
darker, the darker tones indicating higher moisture content; 
12. Distinctive changes in vegetation indicative of changes in 
moisture; and ; 
13. Inclined trees and displaced fences or walls due to creep. 
Many slides are too small to be detected readily in photography at the 
scales normally available (i.e., 1:15 000 to 1:40 000). Consequently, 
the photographs should be closely examined for signs that indirectly 
indicate the presence of slides or, if signs are not visible, for the 
vulnerable locations where slides usually occur. For example, where a 
highway is built on unstable soil, the irregular outlines and non- 
uniform tonal patterns of broken or patched pavements are often visible. 
Typical vulnerable locations evaluated include areas of steep slopes, 
cliffs or banks being undercut by stream or wave action, areas of 
drainage concentration, seepage zones, areas of hummocky ground, and 
areas of fracture and fault concentrations. 
Aerial photographs are valuable aids in identifying the vulnerable 
locations. The shape and slope of the terrain are readily discernible 
from the stereoscopic examination of the photographs. In fact, the 
vertical appearance of the terrain is exaggerated when viewed with a 
lens stereoscope. Moderate slopes appear steep, and steep slopes 
appear almost vertical, making them easier to delineate. In addition, 
the slopes can be measured on the photographs by using simple measuring 
devices, such as an engineer's scale or a parallax bar, and by applying 
photogrammetric principles. Details of this technique are described by 
Ray (8). 
The presence of drainageways, seepage zones, fractures, and fault zones 
is also readily evident on aerial photographs. By means of stereoscopic 
examination, the complete drainage network can be mapped, including the 
intermittent streams and small gullies. The presence of wet zones or 
seepage areas is evidenced by darker tones caused by the higher moisture 
content in the soils or by a more luxuriant vegetative growth over the 
wet areas. Areas of drainage or water concentration above a slope 
should be closely examined because they are vulnerable locations. Sub- 
surface seepage from these areas can lead to slope failures. Fracture 
and fault zonés are indicated on the photographs by dark linear or