

416 STRUCTURAL AND FIELD GEOLOGY 
forming, it may be, ravines and gorges; where only soft 
rocks and yielding structures have been encountered, the 
valleys are relatively wide. In short, the rate of erosion 
will vary in the valleys, just as it does over the surface of 
the land generally. Thus it will come to pass that the 
ravines and gorges of the rivers will mark the outcrops of 
those hard rocks which, outside of the valleys, form hills, 
ridges, or escarpments, while the more open reaches of the 
valleys will coincide with the outcrops of the relatively soft 
rocks, which throughout the region have determined the 
position of the lower grounds. The gradual development 
of surface features implies, of course, the growth of a secondary 
drainage system. Here and there the trunk rivers are joined 
by tributaries, formed by surface-waters making their way down 
the slopes of the land and converging in those depressions that 
open directly upon the wider reaches of the river-valleys. As 
the main rivers continue to deepen and widen their valleys, 
their tributaries will be correspondingly active, and _ still 
younger brooks will begin to appear further inland, as the 
lateral streams cut their way back into the heart of the 
country. Eventually, when the whole drainage system has 
reached maturity, the catchment area of each large river 
will show a more or less complex network of tributaries large 
and small. The whole surface of the tableland will now be 
broken up to such an extent that it may be hard to realise 
its primeval configuration. Nevertheless, the general inclina- 
tion of the original surface will be indicated by the trend of 
the chief rivers. That trend being quite independentwer 
the geological structure, the rivers have cut their courses 
across soft and hard rocks alike—they seem to ignore all 
obstacles, traversing hill-ranges and escarpments just as if 
they had followed lines of gaping faults or fissures. All 
these apparent obstacles had no existence, however, when 
the rivers began to flow. They have been slowly developed — 
during the gradual denudation or lowering of the surface, 
and while the intersecting valleys were at the same time 
being deepened and widened. In a word, the formation of 
river-gorges, hill-ranges, and escarpments has proceeded 
contemporaneously. The direction of the main lines of 
drainage has thus been determined by the original slope 


