3l6 ECOLOGY. 



480. Filling of ponds by plants. — Not only are plants im- 

 portant agencies in the formation of soil in rocky regions, they 

 are slowly but surely playing a part in the changes of soil and 

 in the topography of certain regions. This is very well marked 

 in the region of small ponds, where the bottom slopes gradually 

 out to the deeper water in the centre. Striking examples are 

 sometimes found where the surface of the country is very 

 broken or hilly with shallow basins intervening. In what are 

 termed morainic regions, the scene of the activity of ancient 

 glaciers, or in the mountainous districts, we have opportunities 

 for studying plant formations, which slowly, to be sure, but 

 nevertheless certainly, fill in partly or completely these basins, 

 so that '.the water is confined to narrow limits, or is entirely 

 replaced by plant remains in various stages of disintegration, 

 upon which a characteristic flora appears. 



481. A plant atoll. — In the morainic regions of central New 

 York there are some interesting and striking examples of the 

 effects of plants on the topography of small and shallow basins. 

 These formations sometimes take the shape of " atolls," though 

 plants, and not corals, are the chief agencies in their gradual 

 evolution. Fig. 263 is from a photograph of one of these plant 

 atolls about 15 miles from Ithaca, N. Y. , along the line of the 

 E. C. & N. R. R. near a former flag station known as Chicago. 

 The basin here shown is surrounded by three hills, and is 

 formed by the union of their bases, thus forming a pond with 

 no outlet. 



482. Topography of the atoll moor. — The entire basin was 

 once a large pond, which has become nearly filled by the 

 growth of a vegetation characteristic of such regions. Now 

 only a small, nearly circular, central pond remains, while 

 entirely around the edge of the earlier basin is a ditch, in many 

 places with from $o-6ocm. of water. There is a broad zone of 

 land then lying between the central pond and the marginal 

 ditch. Just inside of the ring formed by the ditch is an elevated 

 ring extending all around, which is higher than any other part 



