CHAPTER IX 



INTERDUNAL PONDS AND TAMARACK SWAJ^IPS 



S THE dunes are a series of sand ridges, 

 more or less parallel to the lake shore, so 

 between them there lies a corresponding 

 series of valleys, often steep sided with 

 interdunal ponds, swales, or swamps 

 (Fig. 175) in their bottoms according 

 as these are low enough to reach or 

 i^ nearly reach the water level. Since the 

 newer dunes are close to the lake, so also are the new interdunal 

 ponds, while those farther and farther back from the lake are 

 of greater and greater age. 



Now it is the fate of the permanent pond among the hills 

 to fill up and disappear. Rains wash soils down into it, plants 

 grow in its waters and along its margins, and their accumulating 

 remains fill it with vegetable debris that does not have time to 

 decompose before the next season's luxuriant growth is piled 

 upon it. The black muck at the bottom of the pond rises ever 

 nearer the surface; its lower layers transform to peat. Plants 

 that grow on sandy bottoms are replaced by those that grow 

 on muddy bottoms. Deep-water plants give rise to shallow- 

 water forms. Those plants characteristic of the margins 

 advance toward the center. So the pond transforms to marsh 

 and the marsh to low prairie or wet woodland. (See chap, xi 

 also.) These stages gradually ensuing with the characteristic 

 accompanying plants and animals may each be studied in 

 these interdunal ponds, beginning with those near the shore 

 and working back to the later stages. 



Ponds near the lake shore are in a condition similar to the 

 central portions of shallow lakes and ponds farther inland, which 



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