LAKES 171 



may occupy the upper two thirds of a gentle slope and the bare 

 surface of the pan with the drainage holes, the lower one third. 



Allow the spray from 

 the spoon to play over the 

 layers in the dish for some 

 time. Tiny rivulets will 

 grow in the layered sur- 

 face, gradually deepening 

 and extending their valleys, FIGURE 80 



and more and more thor- 

 oughly dissecting the surface. Deltas will be formed in the still 

 water in the lower part of the pan, and many of the erosion 

 phenomena of a stratified, slightly elevated region will appear. 

 (Figure 80.) 



Run-off. The rain that falls upon the land and neither 

 evaporates nor sinks into the surface runs off as fast as it 

 can toward the sea. It is joined sooner or later by the 

 water from the springs and by the rest of the underground 

 drainage. Sometimes the journey is long and there are many 

 stops and delays in lakes and pools; sometimes the course 

 is quite direct and quickly traveled. The run-off most 

 profoundly affects the earth's surface. Gullies and valleys 

 are cut, depressions are filled ; in fact, running water is the 

 chief tool which has carved the features of the earth. It has 

 had a long time to act and it has kept unremittingly busy, 

 so that the results of its action appear now in our varied 

 landscape. 



Lakes. The water which runs off the surface first fills 

 the depressions. As soon as these are filled, it runs over the 

 lowest part of their rims and starts again on its course to 

 the greatest of all depressions, the sea. If depressions of 

 considerable size become filled with water, we call them 

 lakes. 



