Any inclined plane can be drawn on a equalangle stereonet (Wulff net) as a great circle. As well as the great circle, a straight line connecting two centers of the stereonet and the great circle represents both angle and direction of dip of the plane, although length of the line become shorter when the dip angle increases. This line, hereafter termed as Angle Line, is useful as a tool of analyzing rock slides.
A slope which bedrock is constructed of two geological separations is defined as the wedge type bedrock slope, and those two separations as the wedge type geological separations. Then, if the plunge of the line of intersection made of the wedge type geological separations is larger than the, angle of sliding friction, and if the line plunges toward the downslope but not the upslope side, failure by sliding should occur in association with opening of the geological separations. When each of those two discrete surfaces forming a wedge type bedrock slope is termed as the geological separations I and II, respectively, plunge and trend of the intersecting line can be calculated with the following equations:
where i is plunge of the line of intersection, β1 is dip of the geological separation I.β2 is one of the geological separation II, and λ, is angle between strikes of the geological separation I and II.And,
where γ is angle between the trend of above intersecting line and the dip of the geological separation I.
Following four examples of small-scale rock sliding failure found within Iwate Prefecture, in which failures by wedge type rock sliding is included together with those by plane (plate) type rock sliding, were invetigated;
(1) Failure of bedrock slope at the back hill of Iwate prefectural baseball ground.
(2) Rock sliding failure along Route 396 at Sahinai Shiwa town.
(3) Rock sliding failure at the back hill of Hotel Komyoso, Dai Spa, Hanamaki city.
(4) Rock sliding failure at a woodland path in Yokota-cho, Rikuzen-Takada city.

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