VEGETATION SURVEY OF NORFOLK. 
755 
Marsh and Water. 
Low angle of rest of unconsolidated strata, down wash of 
fine silt, and accumulation of vegetable debris may be cited as 
contributary causes in the formation of marsh in valley bottoms. 
Artificial drainage and grazing improve these swamps into 
alluvial pasture ; but where spring heads occur, and especially 
where both soil and water are deficient in food salts, the natural 
balance of species is retained. Two distinct types of swamp 
occur in the area, one associated with Galluna heath, and the 
other related to fen. The former is restricted, with few 
exceptions, to the heads of valleys, the water which reaches 
it percolating non-calcareous beds. Putrescent mud and 
calcareous tufa, which is often deposited near springs, are 
absent. As the main water supply of the county is calcareous, 
swamps associated with the principal streams and those on 
boulder clay are of fen type. An interesting exception to this 
is the frequent occurrence of heath swamp species in fen 
swamps near the normal marsh level, attributable to two 
distinct sources of water supply, the swamp being inundated 
by a calcareous stream, while a more local drainage from 
gravel supports heath swamp plants at a level slightly above 
that of the marsh floor. Costessey and Great Witchingham 
Commons in the Wensum Valley show this heath swamp zone 
on the slopes of gravel banks. Its occurrence is not rare, and 
taking Sphagnum as the indicator, it is possible to judge with 
some degree of accuracy, from the contour and general 
vegetation, where it may be expected. The main portion of 
the marsh at Witchingham is lower than the river Wensum, 
which is here banked, and the vegetation varies from a pure 
reed bed in the lowest part to heath pasture on the gravel 
slopes. 
A tabulated list of species common on acid peat but absent 
from fen, and common to both acid peat and fen, will show at 
a glance the essential differences in the flora of the two types 
of habitat. 
