296 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1917. 



this reaclvance. This was first described by Hartz and Milthers 

 near Allerod in 1901 (35). The following section was seen in a 

 brick kiln: 



Peat, 3 to 4 feet. 



Gray clay, free from stones, up to 6 feet. 



Bowlder sand. 



In the gray clay, 6 feet above the bowlder sand, lies 1 a bed of 

 Gyttja, 1 foot, thick, covered and underlain by clay. The flora of 

 the clay, both above and below the Gyttja, is purely arctic — Dry as 

 octopetala, Betula nana, Salix polaris, S. reticulata. The flora of 

 the Gyttja, on the other hand, includes Betula intermedia, B. ver- 

 rucosa, Juniper communis, etc., species not found in the clay and in- 

 dicating less arctic conditions and consequently a retreat of the 

 neighboring ice edge. 



A similar succession has been found at various other localities, and 

 Johansen (30) found the Allerod oscillation exhibited also in the 

 fresh-water Mollusca, though his latest researches tend to minimize 

 the subsequent fall of temperature (37). 



The older Yoldia clay belongs to the conclusion of the last inter- 

 glacial. Above this in North Jylland, and separated from it by a 

 bed of bowlder clay, is another marine clay with Yoldia arctica, as- 

 sociated with a slightly different fauna, the upper Yoldia clay, 5 

 to 20 meters thick. Its base is formed by unfossiliferous sand and 

 gravel, evidently a shore deposit, and it is similarly overlain by 

 other sand and gravel beds. This clay therefore indicates a sub- 

 mergence. Its fauna includes two high arctic speci&s, Tellina 

 ToreUi and T. Loveni, both of which occur living in north and east 

 Greenland, Spitzbergen, and the Kara Sea. 



The individuals of Yoldia arctica, however, are smaller, and the 

 fauna differs slightly from that of the older Yoldia clay, indicat- 

 ing slightly more favorable conditions; Nordmann estimates the July 

 air temperature as below 8° C. The bed evidently belongs to the 

 melting period of the ice, so that both stratigraphically and climati- 

 cally it falls on the same horizon as the older Dryas clay. It marks 

 a period of subsidence, during which the submergence reached its 

 maximum at Frederikshavn, where it amounted to about 50 meters, 

 decreasing gradually to the south and southwest; its upper limit, 

 however, is another shore deposit. 



The upper shore sand and gravel is unfossiliferous, but as it repre- 

 sents an elevation it must, correspond to the Allerod Gyttja, the air 

 temperature during the formation of which is considered by Nord- 

 mann to lie between 12° and 15° 0. More recent than this sand and 

 gravel is the Zirphaea sand of Jylland, and Vendsyssel, character- 

 ized by Zirphaea crispata and other boreal and boreo-arctic species, 



