
STRUCTURE AND SURFACE FEATURES 415 
to collapse, and thus in time the lines of drainage become 
modified. A river which at first followed a synclinal trough 
tends gradually to shift its course as the contiguous anticlines 
are reduced, and ere long the syncline is abandoned in whole 
or in part. Even amongst folded mountains of relatively | 
recent age, the longitudinal or strike-valleys often do not 
coincide with synclinal troughs—they have forsaken these, 
and now flow in true valleys of erosion. 
2. SUBSEQUENT OR EROSION :- VALLEYS.—No hard-arid- 
fast line can be drawn between Tectonic valleys and Erosion 
valleys, for many valleys are partly of original, partly of 
subsequent origin, as is well seen in regions of recent 
mountain-uplift. In the vast majority of cases, however, 
the valleys through which rivers run are hollows of erosion. 
The main lines of drainage have doubtless been determined 
by the original inclination of the surface—but the actual 
formation of the valleys themselves is the result of epigene 
maction alone. Let us picture to ourselves some -extensive 
land-area just newly raised above the level of the sea. We 
shall suppose that the surface is very gently undulating, 
and that it rises gradually from the sea-coast, and culminates 
in a more or less abrupt tract of high ground which repre- 
sents, let us say, the cores or stumps of some ancient reduced 
mountain chain—mere torso-mountains overlooking a broad 
tableland that sinks gradually seawards. It is obvious that 
the new-born rivers would necessarily follow the slope of the 
ground; their direction would be determined by the con- 
figuration of the surface. At first these primeval rivers might 
have few, if any, tributaries. As time went on, however, 
many lateral brooks and streams would come into existence. 
The land, we shall suppose, consists of many different kinds 
of rocks arranged in many different ways. Consequently 
these would yield very unequally—gradually relict hills would 
come into existence, owing to the reduction of the less 
resistant rocks and rock-structures in their vicinity. In a 
word, the undulating land would tend in time to show a 
more diversified surface—heights and hollows would become 
more pronounced. Meanwhile, the main rivers have been 
continually widening and deepening their courses. Where 
they traverse relatively hard rocks, the valleys are narrow, 
