Vegetation of Northern Cape Breton. 317 



Equisetum arvense Viola pallens 

 Calamagrostis canadensis Epilobium angustifolium 



Agropyron repens Epilohium adenocaulon 



Pea pratensis Apocynnm cannabinnm 



Carex torta Eupatoriiim purpureiim 



Ranunculus repens Solidago canadensis 



Fragaria virginiana Aster radula 



Viola cucullata Aster puniceus 



e. THE ASSOCIATION-COMPLEXES OF FLOOD PLAINS 



Transition from boulder plain to flood plain. — It commonly 

 happens, sooner or later, that the stream shifts its course or that 

 the current is deflected by some sort of an obstruction, so that an 

 area occupied by a boulder plain becomes protected in a measure 

 from the erosive activity of the stream. If the protection is 

 sufficient, degradation may become largely superseded by 

 aggradation, and a flood plain may gradually be built up on top 

 of the former boulder plain (Fig. 21). Eventually, even along 

 swift stretches of the stream, such flood plains may attain a 

 height of five or six feet above low water level. At first com- 

 posed of coarse gravel and cobbles, as the surface is raised 

 higher the successive deposits become finer, and finally the soil 

 comes to consist of coarse sand. Only in exceptionally favorable 

 situations, however, does the soil approximate the fine alluvium 

 of old-age rivers. 



The succession of plant associations outlined. — In the familiar 

 type of flood-plain succession (to be discussed later), the pioneer 

 stages of the series are usually hydrophytic : in other words, the 

 succession is hydrarch. In the boulder plain-flood plain succes- 

 sion, on the other hand, the pioneer stages, as a rule, are relatively 

 xerophytic : that is, the succession is xerarch. Three more or 

 less distinct stages in the succession may be distinguished : the 

 gravel bar stage, the pioneer tree stage, and the edaphic climax 

 forest. 



The gravel bar association-type. — The pioneer association-type 

 of gravel bars consists largely of the shrubs and herbaceous 

 perennials listed as characteristic of boulder plains, most of which 

 grow in greater profusion here than there. It also may include 

 many species which are not prominent on boulder plains : such, 

 for example, as Alnus mollis and Diervilla Loniccra; Campanula 



