Geology Reference
In-Depth Information
Figure 1.15.
Transverse stream patterns on the Atlantic slope of the northwestern Iberian Peninsula
(Adapted from Parga Pondal et al., 1982).
feedback or reinforcement mechanisms have together enabled it to maintain its course across the
strike ridges of the Witwatersrand system, as well as the structures of the granitic core at the local
scale, where straight stream sectors reflect fracture control. Superimposed, like antecedent, rivers
imply unequal activity and the capacity of rivers to maintain their courses across transverse structures.
The Western Iberian Peninsula drainage provides especially clear examples. Structurally the
region is dominated by Variscan orogenic trends running NNW-SSE (Iglesias and Choukrounne,
1980; Parga, Parga, Vegas et al., 1982). The many granite batholiths of the area intruded the pre-
vious sedimentary and metamorphic terranes which are more susceptible to weathering and erosion
than the granitic rocks. Nevertheless, several of the rivers flowing to the Atlantic, and notably the
Xallas, Tambre, Ulla, Umia, Miño, Lima, Cávado and Douro, flow across the structural grain and
across some granite emplacements (Fig. 1.15 and see Richthofen, 1886). These rivers drain the inner
plains of Iberia inherited from the Pangaean stage. In the northwestern Iberia there are preserved
