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bonds are broken, and significant reorientation of the soil 
particles is possible. 
The physical features of a Leda clay landslide are indica 
ted in FIGURE 2. The crater is bowl - shaped and concave, while 
the debris material spreads out in a fan or apron shape. If the 
landslide retrogresses, due to secondary failure planes develop 
ing, then the back scarp of the crater will have a scalloped arc 
and the debris remaining in the crater will show the form of 
ridges and troughs. 
LANDSLIDE INVENTORY 
After familiarization with the geotechnical and engineering 
properties of Leda clay, and investigation of several landslides, 
in both theory and practice, an inventory was conducted of 
landslides in eastern Ontario by airphoto interpretation. 
Thousands of available black and white photos were examined 
during the inventory, and the following observations made: 
1. Stereo viewing is essential, since most of the recog 
nition features are identifiable only in three dimen 
sions . 
2. Black and white photography is sufficient since land 
slide detection is a pattern recognition exercise, 
not spectral analysis. 
3. Old photography is very useful to cover those areas 
which have been altered due to urban expansion, vege 
tation overgrowth, or erosion. 
4. Photography taken in the fall or spring, when the 
leaves are off the trees will permit the best view of 
the ground. 
5. Photography scales of 1:20,000 to 1:10,000 are 
necessary for feature identification, although smaller 
scale imagery should also be used for the overall look 
at the vegetation, surficial geology, drainage and 
land use of the area. 
The features of identification are primarily those physical 
shapes shown on FIGURE 2. It is rare, however, to find all the 
attributes of any one landslide. In most cases, just one or two 
features are detectable. In many cases, it is a considerabel help 
to identification, if the ridge - like pattern of debris, left in 
the crater is recognized. If there is no vegetation, the ridges, 
which are topographically higher, and therefore better drained, 
will have a light tone, and the troughs between them, which hold 
moisture well, will show in a darker tone. When the crater is 
over - vegetated, the vegetation will tend to take on a growth