254 SUCCESSION IN EURASIA. 



III. Plant formations zonal to the drainage system — Contin/ued. 



A. Formations consequent on deposition. 



(1) Subformations of the river flood gravels. 



(2) Subformations of the alluvial terraces. 



(3) Subformations of oxbows and backwaters. 



B. Formations consequent on recent erosion. 

 Plant formations of the rocky stream-channel. 



C. Formations of the deserted banks of erosion limiting post-glacial stream migration. 



1. Formation of the banks of sandy boulder clay and metamorphic rocks. 



(1) Birchwood association of damp banks. 



(2) Birch-scrub-heath association of dry rocky banks. 



2. Formation of the banks of calcareous shelly boulder clay. 



(1) Birchwood association of damp banks. 



(2) Grass association (probably largely artificial). 



(3) Hazel-wood association of dry rocky banks. 



IV. Plant formations of the coastal belt. 



A. Marine algal formations. 



B. Plant formations dependent on marine littoral deposit. 



I. Formations of sea-beaches. 



1. Open associations of storm beaches. 



2. Subsequent transitional, or vagrant associations of deserted beaches. 



II. Formations of coastal sands. 



1. Littoral shallow sand accumulations, behind rock-reefs, or on deserted beaches. 



2. Coastal sand-dune formation. 



(1) White dunes. 



(2) Fixed dunes. 



3. Sand-flat formation, dependent on level of ground water. 



4. Lichen-heath association, of old sand-flats, dependent on accumulation of humus 



and surface fixation. 



III. Salt-marsh formations. 



1. Subformation of flagstone rock-ledges: habitat dependent on the nature of the 



cliffs and sphere of surf-action. 



2. Subformation of banks of estuaries: habitat dependent on tidal condition of the 



rivers. 



C. Plant formations directly consequent on marine erosion. 



D. Subsequent plant formations, due indirectly to sea erosion. 



Formations of the seaward contoiir other than rock cliffs. 



Priestley (1911) has described the vegetation of the left bank of the Severn 

 Estuary, in which he recognizes five serai zones, namely: 



(1) Bare mud swept by tides; (2) SaKcomia zone; (3) Sderochloa zone; 

 (4) Festuca zone; (5) reclaimed pasture. The minimum salt-content for the 

 lower zones is uniformly higher than the minimum content for the higher 

 zones. The reclaimed pastures owe their existence to an ahnost uniformly 

 low saline content due to two factors, viz, infrequent tidal immersion, and 

 improved surface drainage due to the accumulation of hiunus. 



Tansley (1911) and his associates have distinguished 14 formations in 

 Britain. From their concept of the formation, these usually represent two 

 or more of the later stages of succession, while, on the other hand, initial and 

 medial stages have received less attention, except in areas topographically 

 active: 



In the Qvercetum roburis, lumbering and grazing first destroy the trees, 

 producing a scrub of Crataegus, Prunus, Rvbus, Rosa, and Ulex, the destruction 

 of which in turn permits the appearance of grassland of Lolium, Cynosurus, 



