STABLE AND MIGRATORY PLANT FORMATIONS 75 



Plain, and the woodlands were forced to retreat towards the 

 Atlantic border and on to the slopes of the mountains. Marr 

 has pointed out the significance of the presence of fossil 

 steppe mammalia, and of living steppe plants, in the sandy 

 tracts of the S. E. of England. He suggests that these plants 

 may be relics of a former steppe period in which the steppe 

 had a western extension as far as the S.E. of England (3). 



The heathland tract has perhaps resulted from the altera- 

 tion of markedly different climatic influences within this 

 district. During the Ice Age, the N. German Plain was 

 widely covered by sandy drift, which was afterwards subjected 

 to the sifting action of wind. Sand covers much of the 

 northern tract, while the fine dust or loess has accumulated 

 along its southern borders In Late Glacial times an advance 

 of the steppe covered wide areas in loess and entombed many 

 steppe mammalia, as described by Nehring (32). With 

 later advances the area would be further subjected to the 

 sifting influence of wind. With the late return of a moor- 

 land insular climatic phase, the more fertile sands and loess 

 would be covered by forest, but certain sandy areas would 

 be rapidly leached of their soil-nutriment and only heath or 

 moorland would obtain a footing. With prolonged leaching 

 of the surface of the sandy tracts, the heathland would 

 advance at the expense of the forest, as shown by Graebner. 



If the steppe plants in the heaths of the S.E. of England 

 are relics of a former steppe phase, as Marr suggests, they 

 have probably managed to persist in the sand-dune areas of 

 the S.E. coasts, and have reinvaded the heath, when the 

 moorland climatic phase passed away. It is, however, 

 possible that these steppe types were reintroduced into the 

 S.E. counties from the Baltic coast in early historic times. 



All the stable types of vegetation have their homologues 

 in migratory formations. The moorland is represented by 

 fenland. Migratory woodlands are represented by alder 

 and sallow swamps, and a kind of open heath is frequent, 

 especially within the moorland region, on leached sandy and 

 gravelly alluvia. It is, however, with the grasslands that we 

 find the predominance is on the side of the migratory 

 formations. Perhaps the only truly stable grassland in this 

 country is that of the chalk downs. 



