QUATERNARY DEPOSITS OP BRITISH ISLES — BROOKS. 297 



with a complete absence of high arctic species. It appears to cor- 

 respond to the less arctic part of the upper Dryas period, with a 

 mean temperature of 8° to 12° C. ; it passes upward into the alluvial 

 beds. 



The sequence of events in Denmark may now be summarized as 

 follows : 



First glaciation. — Bowlder clay underlying Eem beds, with Baltic 

 and Norwegian erratics. Land much higher than now at first, but 

 subsided toward the close of the period. 



First interglacial. — Represented by the Eem zone, during the for- 

 mation of which the land lay somevs^hat below its present level. 



Second glacial. — Bowlder clay with erratics from east Baltic and 

 southeast Sweden. This again was marked by elevation, for the 

 ground moraine of this glacial lies much below present sea level. 



Second interglacial, — Skaerumhede marine series. At first the land 

 lay about 100 meters above its present level; at the maximum of tem- 

 perate conditions it had sunk to 40 or 50 meters, and at the conclu- 

 sion of the interglacial to only about 10 meters above its present level. 

 This period closes with the Portlanul'm arctica or older Yoldia period. 



Third glacial. — Fluvio-glacial deposits and moraine with erratics 

 from south Norway. Glacial conditions afterward gave place to an 

 arctic vegetation, the older Dryas feriod. 



By the conclusion of this glaciation the land had risen to slightly 

 above its present level, for it is immediately followed by a shore sand 

 and gravel, but this elevation at once gave place to subsidence, during 

 which the younger Yoldia clay, with high arctic Mollusca, was 

 fonned. 



Allerod oscillation. — A decided amelioration of temperature, with 

 a July temperature of 12° to 15° C. As no marine deposits of this 

 period are known, the land probably lay above its present level. 



Younger Dryas period. — Recrudescence of arctic climate and arctic 

 vegetation. To its close belong the Zirphaea sands, with a fauna 

 somewhat less arctic than that of the younger Yoldia clay. 



The exact chronological position of the younger Yoldia clay and the 

 Allerod oscillation will be discussed in greater detail when the late 

 glacial history of the Baltic is considered. 



4. RUSSIA. 



The standard region for a study of the glaciation of European 

 Russia is the neighborhood of Moscow. In 1890 M. Krischtafowitsch 

 (38) described in Schernigow Province two very dissimilar types 

 of glacial deposits — bowlder clay below and fluvio-glacial sand and 

 gravel above — between which he found at Troitskoe near Moscow 

 lacustrine formations with Quercus pedunculata^ Alnus glutinosa, 



