QUATERNARY DEPOSITS OF BRITISH ISLES BROOKS. 345 



Sub-Atlantic, wet. 

 Subboreal, dry. 

 Atlantic, wet. 

 Boreal, dry. 

 Subarctic and arctic. 



These periods are adopted by Sernander (108). He correlates the 

 boreal period with the Ancylus lake and the Atlantic, subboreal, 

 and sub-Atlantic periods with the Litorina period. While the ice 

 was retreating from south of Scania to the F jails (the mountains 

 about latitude 64° ) it was followed by an arctic flora of xerophilous 

 type, which Sernander regards as the equivalent of the Yoldia 

 period, but as soon as the ice edge passed the Maler-Hjalmar Valley 

 this flora for the most part disappeared, and the ice — which at this 

 period was melting rapidly — was followed directly by Sphagnum 

 peat. The transition period was marked by BetuZa odorctta and 

 Populus tremula, but the characteristic tree associated with the peat 

 of early Ancylus time is the pine. The late Ancylus period was 

 marked by widespread forests of birch (B. alba) which have left 

 a well-marked layer of stools in the peat bogs. To the evidence of 

 the tree stools Sernander adds that of a snail, Helix adela (II. tenu- 

 ilabris), found in sand of "boreal" age in a boring at Ystad, but 

 not now living nearer than southeast Europe. 



Resting on the tree stools of the birch forest is generally another 

 layer of Phragmites peat, often giving place to lacustrine marls. This 

 is the peat of the Atlantic period. It is followed by another layer of 

 stools, this time of Pinus silvestrls, forming the subboreal period. 

 This is finally replaced by Cladium or Sphagnum peat, which is still 

 forming. In the lake basins the stools of Pinus silvestris often occur 

 on lacustrine marl, and are normally covered by the present waters 

 of the lakes, being visible only in very dry seasons. 



Sernander considers that the warm climate of the boreal period 

 extended into the sub- Atlantic and subboreal times, since subboreal 

 peats, comparatively late and near sea level, contain plants of a 

 more southern type than any now living in the district, and hazel- 

 nuts occur in peat far north of the present limit of the hazel, while 

 L. von Post (109) has found the pine in beds of Atlantic and sub- 

 boreal age above its present upper limit in the mountains of southern 

 Sweden. 



The researches of Gunnar Andersson (110) in part confirm Blytt's 

 sequence, but as regards the latter part of the postglacial period his 

 conclusions are very different. He finds that the melting of the ice 

 began in a high arctic climate, the Mollusca of the Yoldia. sea, which 

 is its equivalent, indicating a mean annual temperature of — 8° to 

 — 9° C, but the conditions rapidly improved, the mean temperature 

 at the beginning of the Ancylus period being at least 2° C. The 

 early Ancylus period was marked by a forest of pines, which as the 



