prevents correct volume analyses. A similar method of topographical
reconstruction has been presented by Oppikofer
(2009).
The 35 debris rockslides mapped were predominantly
controlled by bedding planes. Cataclinal dip slope debris rockslides
sensu Cruden (2000) are widespread in the study area,
where slope angles are close to the bedding plane dip angle as it
was observed by Cruden (1976) studies in Kananaskis Country in
the southern Rockies. Debris rockslides are also common near
resistant geological contacts, usually massive cliff-forming sandstone
units or thick, resistant clastic units overlie recessive (less
competent) clastic rocks. The bedrock geology of the rock slopes
are sandstones and lutites of Ordovician and Silurian-Devonian
age. All these rocks are strongly folded and faulted. In all the
studied debris rockslides the movement was a typical rotational
block sliding sensu Skempton and Hutchinson (1969) with a ratio
Dr/Lr, between 0.1 and 0.3, and a slide body consisting of debris
and rocks mass blocks.
The observed distribution of the debris rockslides, the frequency
ratio and logistic regression coefficients are shown in Table 2. The
variables were converted to a raster grid with 30 30 m cells. The
area grid was 562 rows by 637 columns (i.e., total number is
355,885) and 2422 cells represent the rupture zone. The variables
chosen and the system constructed were found to be valid; with
84% of the pixels used being correctly predicted (97% of the pixels of
the debris-rockslides and 71% of non-slides). The test showed that
the goodness of fit of the equation could be accepted because the
values of Cox and Snell R2 (0.433) and Nagelkerke R2 (0.577) are
greater than 0.2 (Clark and Hosking, 1986). The TOL, VIF values, and
Pearson correlations in this study are shown in Table 3. There is no
multicolinearity between any of the variables, which are weakly
correlated with each other. The highest correlation was found between
lithology and slope (0.606) but its value is below the risk
level of 0.7 (Clark and Hosking, 1986).