820 MR FRANCIS J. LEWTS 



by forest. This succession might continue as long as the conditions favoured the 

 existence of forest. 



The causes of the existence and of the disappearance of the two forest beds may 

 now be briefly considered. 



The Lower Forest. — This lies near the base of the peat, immediately over the First 

 Arctic Bed. A gradual transition can be traced in Shetland from the basal true arctic 

 plants through moorland conditions where Empetrum nigrum and Potentilla Comarum 

 were dominant, to the layer which, on account of the closely crowded roots, trunks, and 

 twigs, may be designated as forest. This layer occurs in S. Scotland (8) in positions 

 which are well within the present range of trees, but in Shetland it is present in a 

 district in which no trees are to be found growing wild, and where only in a few most 

 sheltered spots are planted trees to be found. It may be contended that the afforesta- 

 tion of a large tract of country might so break the force of the wind and afford shelter 

 that trees might grow under conditions which would prevent them coming to maturity 

 in small copses. While this may be true, it does not explain how this forest spread 

 over Shetland at the close of the first arctic stage, or how it established itself in a wind- 

 swept island like Foula. 



It is well known that high moor may give rise to a totally different type of 

 vegetation such as dwarf shrub heath, when the upward growth of the peat causes a fall 

 in the soil water-level. Under favourable conditions this may give rise to forest. The 

 developmental stages of the vegetation clearly preclude such edaphic causes for the 

 growth of this forest, as an arctic vegetation gives place to Betula glutinosa. Fries., 

 Alnus glutinosa, etc., together with many plants having a wide distribution and 

 others of a distinctly southern aspect. This, then, clearly marks amelioration of the 

 climate following on the arctic period at the close of the glaciation. The plant remains 

 make it clear that the Lower Forest was not a dry period, for all the plants (with the 

 exception of one or two) are common in temperate European regions. Similarly, it is 

 not clearly shown that this stage was distinctly warmer than the present. Certain 

 plants, such as Carex paniculata, might suggest this, but their absence in Shetland now 

 might well be due to their having been driven southward during the second arctic stage 

 and their failure to reinvade the area. On the whole, the bed suggests present 

 temperamental conditions. But the facts are overwhelming in favour of its not being 

 the product of present meteorological conditions. If trees planted in the most sheltered 

 spots and nursed with the most cunning artificial devices fail to come to maturity, we 

 should not expect the growth of forest on elevated hill-slopes and in a small spray- 

 swept island like Foula. 



It seems probable that wind is the chief factor that prevents the growth of extensive 

 woodlands in Shetland at the present time. Although the Mainland is 54 miles from 

 north to south, no place is more than 4 miles from the sea. According to a chart 

 prepared by II. II. 0. Dunwoody (13), the area embracing Shetland, Fseroe, ami 

 north-westward to the coast of Norway has an annual storm frequency of over 20 per 



