QUATERNARY DEPOSITS OF BRITISH ISLES — BROOKS. 283 



rences of land and fresh-water Molliisca in glacial and interglacial 

 horizons in Germany. He finds that the varions molluscan faunas 

 of older Quaternary age can be divided into cold-loving and warmth- 

 loving groups, whose distribution shows that even in the unglaciated 

 region of southern Germany an arctic climate i^revailed during the 

 glacial periods. In north Gennany the glacial faunas are found 

 chiefly in gravels, in south Germany in loess and sand loess. True 

 interglacial MoUusca occur on two horizons of different age, sepa- 

 rated by glacial deiwsits. The older one is characterized by P. d'llu- 

 v'mmi^ Bithynko tentaculata^ and Dreissensia polymorpJia^ the 

 younger by P. Duboisiana, Belgrandla^ and Planorhis alhus. 



In West Prussia the lower interglacial age of the marine CaTdhmi 

 and Cywma cla3's and Eem beds is confirmed by the occurrence in 

 them of Dreissensia polymorph-a and P. diluirianu-. In Posen P. dilio- 

 vlana has been found in interglacial deposits associated with Cm-bi- 

 cvla fiuminalis, and these two have also been found associated near 

 Odessa and as derived fossils in the middle bowlder clay of East 

 Prussia. 



A. Penck (8) was the first geologist to bring forward evidence in 

 support of a threefold glaciation of the district. Since 1880 a num- 

 ber of other more or less successful attempts at classitication have 

 been made, the net result of which is very much in favor of Gagel's 

 classification into three glacial and two interglacial horizons (9). 



GLACIAL, SUCCESSION IN THE RiVEU VALLEYS. 



Detailed consideration of the glacial succession in the valleys of 

 the north fioAving rivers south of Berlin is not necessary, as the 

 conditions are essentially similar to those of the Alps. 



The deposits of the Weser Valley, studied by O. Grupe (10) and 

 L. Siegert (11), niay, however, be considered here, as they differ con- 

 siderably, and the differences illustrate the scheme of classification. 

 According to Grupe, the valley of the Weser originated in middle 

 Pliocene, and was cut to a depth of at least 25 meters below the 

 present level of the river. The process of erosion left " Old Pliocene " 

 gravels at a height of 120 to 160 meters above valley level. In late 

 Pliocene times the valley was partially filled by clays and sands con- 

 taining Mastodon arvemensis and M. Borsonl. Belonging to the 

 Quaternary there are three gravel terraces — upper, middle, and 

 lower. The upper terrace interdigitates to the north with deposits 

 of the first glacial period on the Porta River, and is accordingly con- 

 temporaneous with this period. This was the maximum glaciation 

 in the district and, correspondingly, the upper terrace reaches the 

 great thickness of 60 to 70 meters at Hameln. The lower part of the 

 middle terrace includes at Nachtigall, peat with Corylus aveJlana, 



