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(b)
Figure 1.10.
(b) At the local scale, as exposed on the surface of Pildappa Rock, northwestern Eyre Peninsula,
South Australia.
Figure 1.11.
Orthogonal fracture system and stress directions (Davis, 1984).
1.7
FRACTURES AND DRAINAGE PATTERNS
Most drainage patterns are determined by the structural characteristics of the country rock and the
slope of the land surface (Zernitz, 1931). On gentle slopes or plains, structural control is minimal
and parallel, subparallel or dendritic patterns are developed. Thus, the regional drainage pattern on
Dartmoor, southwestern England, is radial, reflecting the domical topography of the region ( Fig.
1.12a) . Quite commonly in granitic terrains, systems of steeply inclined orthogonal or rhomboidal
fractures have been exploited by rivers to give straight, angular or rectangular stream patterns,
according to the precise geometry of the fracture systems (Figs 1.12b and 1.13a and b), and even
where slope is the dominant factor, angular patterns are developed locally in response to structure
(Fig. 1.12c). The reason for the coincidence between fracture and stream channel is that fractures
 
 
 
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