20 WALTER H. BUCHER 
material at the toe of the new steep slope exceeds the elastic limit, 
rupture along planes of shearing will take place sooner or later in 
brittle materials. Ductile substances, on the other hand, such as 
a 
OMtS.) 10) [See Zones: 
Fic. 18.—Diagrammatic contour map of the surface of a bed of shale buckled 
and cut by an inclined joint plane. Superimposed on it is the outline of a new stream 
bed cut into this structure after rejuvenation. 
clays, will flow, causing the lower part of the steep slope to bulge 
out and the bottom of the excavation to buckle up.t 
Van Horn has given a detailed description of this buckling at 
the base of a rock slide.2 Small and entirely local anticlines which 
=D. F. MacDonald, “Excavation Deformations,’ Congrés géologique interna- 
tional, Compte rendu de la XIIé session (Canada, 1913) p. 791, Fig. 3. 
2 Frank R. Van Horn, “Landslide Accompanied by Buckling, and Its Relation 
to Local Anticlinal Folds,” Bull. Geol. Soc. Amer., Vol. XX (1910), pp. 625-32. 
