HISTORY OF LANDSCAPE 183 



700 feet level. Numerous other instances might be given where 

 traces of the old base-level may be detected in such a position as 

 to show that the work of making the present landscape was 

 started on it. 



In porous and pervious rock a good deal of the denuding work 

 is carried on underground. Thus in the case of limestone rocks, 

 systems of caves and underground channels are dissolved out 

 along the widened joints of the rock. The erosive work is carried 

 on underground without the formation of valleys on whose sides 

 the lateral agencies can work. When these underground channels 

 become very much widened they must eventually collapse, giving 

 rise to steep-sided, youthful topography until the lateral agencies 

 which then come into play can smooth down the abrupt slopes. 



The travel of underground water is worthy of further study. 

 Of the rainfall, an average of about one-third is evaporated, a 

 second third flows over the surface as streams, and the last third 

 sinks underground into porous rocks. The relative amounts 

 vary according to the absorptive character of the rocks. Highly 

 absorbent and porous ones, such as chalk or open sandstone or 

 conglomerate, absorb a much larger proportion, while non-porous 

 rocks like clay absorb very little, and the ground is wet and the 

 streams are full. Even when a rock is not porous between its 

 grains, like compact limestones and igneous rocks, water may still 

 travel underground along fissures. Gravitation will carry the water 

 downward until it meets either with rock already saturated with 

 as much water as it can hold or with impervious rock. Its direct 

 downward journey is then stopped, but it may travel obliquely 

 downward if the surface of saturation in the rock or the imper- 

 vious layer slopes in any direction. This travel may bring the 

 water out again if the surface in question outcrops at a lower level. 

 The escape of this water thus forms a spring, and if the travel of 

 the water has been continuously downward it is called a surface 

 spring (Fig. 72). The place where water is absorbed, i.e. the 

 higher outcrop of the pervious rock, is called the gathering ground 

 or drainage area, and water will be delivered by the spring in pro- 

 portion to the area of gathering ground, the distance the water has 

 to travel, the slope down which it runs, and the resistance offered 

 to its passage by the closeness of grain or fissures of the rock 



