CHAPTER VIII 



LAKES AND TARNS 



The making of lakes Lake plants Fauna of lakes Tarns and 

 their life Tarn plants Adaptations of water plants in general 

 Hydra The tarn in winter Dragon Flies The Heron The 

 Dabchick. 



FROM the top of the hill we saw many sheets of 

 water two great lakes, many little lakes or lochans, 

 and innumerable tarns among the Heather and on the 

 moor. The question rises : How are these formed ? 

 The geologists give three answers. 



The formation of a lake presumes a hollow which 

 gets filled with water unless there is some subter- 

 ranean escape. The hollow will remain full if the 

 original supply of water e.g., by rainfall is kept up, 

 and if the hollow is deep enough. For if it be very 

 shallow, as in a pond, the evaporation may be greater 

 than the supply. Everyone knows how pools dry up. 



The first way in which lakes may be formed is by 

 the unequal deposition of material which results in 

 making a dam. We illustrate this for amusement 

 when we make a temporary dam in a little stream. A 

 lake is formed, but when the body of water begins to 

 exert a pressure greater than the dam can bear the 

 dam gives way and there is a flood. Man illustrates 

 this on a large scale when he makes a reservoir by 

 building a great dam across a valley. The deposits 



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