THE PLEISTOCENE SERIES 647 



confirmed by the discovery of a steppe-fauna in the loess at 

 Tliiede in Brunswick and at Westeregeln in Prussia. This 

 assemblage includes some northern forms, such as Elephas primigeniut, 

 Cervus tarandus, and Ovibos moschatus, associated with species that 

 now live on the Siberian tundras such as Saiga tartarica, Myodes 

 torquatus (a Lemming), Arctomys bobac (a Marmot), Lagomys 



-rtlus (the tailless hare), Alactatus jaculus (a Jerboa), and several 

 species of Spermophilus. 



\\'illi regard to age it has been considered to be entirely post- 

 Glacial or late Glacial, but the evidence tends to favour the view 

 that its formation extended over a long period of time, that its 

 lower parts are what may be termed meso-Glacial and contain 

 flint implements of Mesolithic types, while its higher parts are 

 late Glacial and locally post-GlaciaL 



Deposits of the Baltic Region. During the final retreat 

 of the Scandinavian ice-sheet, the sea gradually reoccupied the 

 trough of the Baltic, together with parts of Southern Sweden, a 

 large part of Finland, and a small part of Russia between the Gulf 

 of Finland and the White Sea. The Swedish geologists Munthe 

 and de Geer assume that this was due to a concomitant subsidence, 

 but it is quite as likely that the ice-sheet had occupied the place 

 of the sea, which simply returned into the area as the ice retired. 



The deposits of this sea are laminated clays containing Yoldia 

 ardica, Y. hyperborea, Pecten islandicus, Sipho togatus, Trophon 

 truncatus, Natica affinis, and other boreal forms which are only 

 now found in the Arctic Ocean ; hence they have been called the 

 deposits of the Yoldia Sea. They do not occur in Germany, but 

 are found in the south of Sweden, and thicken northwards till 

 they attain a thickness of over 800 feet in Angermanland, so the 

 conditions must have lasted a considerable time. 



This geographical phase was brought to an end by an uplift of 

 the region which not only raised the area north of Lake Onega, 

 but also united Sweden to Denmark and converted the Baltic 

 basin into an enclosed sea which quickly became a freshwater lake. 

 Tliis has been called the "Ancylus Lake" because Ancylus 

 fluviatalis is a common fossil in its deposits, associated with 

 Limncea ovata, Planorbis contortus, Bithinia tentaculata, and other 

 t'n-shwater shells ; these deposits rest unconformably on the 

 Yoldia Beds. 



This phase was terminated by a subsidence which was greater 

 on the western than on the eastern side, allowing the waters of 

 the North Sea to flow freely into the Baltic area, so that the 

 freshwater lake was quickly converted into a gulf of the sea. 

 Moreover the opening must have been wider than it is at the 



