Vegetation of Northern Cape Breton. 415 



The association-types of open valleys. — While practically all 

 the larger streams on their way to the sea run for long distances 

 through deep gorges and ravines, on the plateau itself most of 

 the streams flow through broad, shallow, characteristically flat- 

 floored valleys, but little below the general level of the surround- 

 ing country. The slopes which flank these valleys may be covered 

 with low woodland, or in the barrens by Krummholz. Their 

 floors are commonly occupied by "hay marshes," alder thickets, 

 and swampy woodland, which will be discussed under hydrarch 

 successions. 



C. Formations of the H\t)rarch Series 

 I. The Formation-types of Lakes and Ponds 



a. INTRODUCTORY 



Small lakes and ponds of all sizes, but mostly quite shallow, 

 are freely interspersed among the countless low hills which go 

 to make up the surface of the plateau and occur scattered here 

 and there along the seaward slopes of the highland. Many of 

 them lie at the sources or along the courses of the innumerable 

 streams which originate in the barrens, but a large proportion 

 are devoid of any definite outlet. Ponds of the latter type are 

 especially common on the plateau where, owing to the abundant 

 precipitation and the impermeable nature of the rock floor, more 

 or less permanent bodies of water tend to collect in basins of 

 any description. On an undulating, rock-floored table-land such 

 as this, the number of depressions suitable to pond formation is 

 naturally very great, but the number of ponds actually present 

 is even greater. This is due to the fact that, in addition to those 

 whose presence is conditioned by the character of the topography, 

 there are numerous ponds which bear no relation whatever to 

 the topography, whose presence is attributable primarily to the 

 activity of vegetation. The manner in which ponds of this latter 

 sort arise will be discussed in some detail in later paragraphs 

 (p. 449 et seq.). On the highland, as in the lowland (while there 

 are plenty of apparent exceptions, particularly in the case of well- 

 drained water bodies) there is a general tendency for lakes and 

 ponds to become clogged up through vegetative activity, and in 

 this way many basins formerly occupied by ponds have become 

 more or less completely filled in. 



