Vegetation of Fleam Dyke 



179 



covered by chalky boulder clay. These earthworks cross, approximately 

 at right angles, the line of the early pre-Boman road called Icknield 

 Way, which follows the line of the chalk outcrop between the boulder 

 clay country (which was no doubt once covered with thick forest) to the 

 south-east, and the (originally) impassable Fenland to the north-west ; 

 and they are supposed to have been thrown up to facilitate the obstruction 

 of the road to the passage of hostile forces. 



Fleam Dyke, in its least altered portions, is a narrow and steep-sided 

 bank of solid chalk, its summit lying about 12 feet above the general 

 level of the surrounding country. On its south-west side is the ditch 

 from which the chalk was dug. The surrounding country is almost 

 entirely arable, but for a length of about two miles the dyke has been 

 left almost untouched, though here and there pines and birches have 

 been planted. The south-west side of the dyke bears a typical chalk 

 grassland association ; on the north-west side a good deal of bare chalk 

 is exposed and the vegetation is taller though composed of few individual 

 plants. At the bottom of the ditch are numerous rabbit burrows. 



The following is a list of the species which occur in the chalk grass- 

 land association of the dyke. 



Festuca ovina d. 



Thalictrum minus 



[Mahouia vulgaris] 



[Papaver Ehasas] 



[P. hybridum] 



[Fumaria officinalis] 



[Brassica nigra] 



Beseda lutea 



Helianthemum Chamse- 

 cistus 



Viola hirta 



Polygala vulgaris 



[Lychnis alba] 



Silenelatifolia(Cucubalus) o 



Cerastium vulgatum 



[Stellaria media] 



Linum catharticum 



Bhamnus oathartious 



[Acer Pseudoplatanus] 



Ulex europseus 



Ononis spinosa 



Medieago lupulina 



Trifoliura procumbens 



T. pratense 



12—2 



