I40 THE SCOTTISH BOTANICAL REVIEW 



and silts. Mud-flats occur where the delivery of silt by the 

 rivers is greater than the marine currents can dispose of or 

 where the physiography prevents its dispersal. Mud accumu- 

 lates in the protected estuaries of big rivers where the coast- 

 line is indented and behind sand-bars or shingle spits in the 

 rather more exposed positions, the barrier in such cases 

 forming the forefront of a delta, or extending across the 

 mouths of bays where smaller rivers debouch. Through 

 changes in the position of the barriers, the areas forming 

 such bays have sometimes been alternately occupied by the 

 sea with mud-flats and salt-marshes, and by brackish and 

 fresh water, marsh, reed-swamp, and fen, fully demonstrating 

 the migratory and unstable nature of such plant formations. 



Parts of what were originally extensive marshes bordering 

 on exposed coast-lines, like the Romney Marsh and Pevensey 

 Levels in the south of England, are at present somewhat 

 lower than the high-water mark of spring tides, but are 

 protected from the sea by a barrier of shingle extending 

 across the coastal margin of the marsh. The history of the 

 evolution of the Pevensey Levels has been much obscured 

 through the prolonged interference by man, but from the 

 evidence of borings (42), and from that of the submerged 

 forests exposed at low water along the coasts, there is some 

 reason for thinking that the areas of these levels, during one 

 stage of post-glacial time, were largely covered by forest, 

 the sea level being then lower than at present. With a 

 gradual advance of the sea and, perhaps, a deterioration of 

 climate marking a phase of moorland extension, the forest 

 seems to have been locally swamped and buried in fenland 

 peat, a great part of the area at length becoming submerged 

 by the sea and covered with sand and silts. It would be at 

 about this time, no doubt, that the great shingle accumula- 

 tion of "The Crumbles," near Eastbourne, with its numerous 

 parallel "fulls" or ridges and hollows, began to form, and 

 a bank of this shingle probably prevented the sea from enter- 

 ing the smaller Bourne Level, which is separated from the 

 Pevensey Level by a ridge of higher ground. Pevensey is 

 supposed to have still been reached by the sea in historic 

 times, and previous to, and since that time, the Pevensey 

 Level must have been successively mud-flats, salt-marsh, and 



