QUATERNAEY DEPOSITS OF BRITISH ISLES — BROOKS. 295 



Above these glacial beds comes the marine interglacial " Skaerum- 

 hede series," divided into three zones as follows: 



3. Yoldia {Portlandia) arctica zone \Yith purely arctic fauna and beds of 



gravel and sand ^Yith fragments of boreal Mollusca; 40 meters. 

 2. Abra nitida, zone with a boreo-arctic fauna ; 9 meters. 

 1. Turritella terebra zone with a purely boreal fauna ; 74 meters. 



The TurriteUa terehr^a zone consists of a mild, dark-gray clay marl, 

 with black beds at the base and a rich marine fanna of pronounced 

 boreal type. The most "temperate" species are found only in the 

 middle of the zone; above and below are somewhat colder forms. 

 The fauna indicates that the lower part of the zone was accnmiilated 

 at a depth of 40 to 60 meters, the upper part at a depth of 60 to 80 

 meters. 



Upward this zone passes gradually into the Ahra nitida zone; 

 black, very mild, stone-free clays with a pronounced boreo-arctic 

 fauna accumulated at a depth of 20 to 40 meters. It is overlain with 

 a sharp boundary, by the Yoldia arctica zone, a hard, gray, marine 

 clay with isolated scratched bowlders and nests and layers of sand 

 and gravels, with rolled shell fragments. The primary Mollusca of 

 this zone, mostly broken, are high artic ; the rolled fragments in the 

 sand and gravel beds are secondary, and exclusively boreal, and must 

 have been brought, like the nests of sand and gravel in which they 

 lie, by ice from the deposits of the TurriteUa terebra zone. The 

 lower part was formed in 20 meters, the upper part in 10 meters 

 depth. In the upper 30 meters of the Yoldia clay are masses of 

 moss and occasional seeds and leaves of higher plants, almost all in- 

 dicating an arctic climate. {SaVm polaris, S. herhacea, Betula nana^ 

 etc.) 



The scratched erratics in the Yoldia clay come partly from the 

 Christiania region (Rhomb porphyry) and partly from the Skager- 

 rack; none are from the east Baltic, so that the ice must have had 

 a purely northern origin. A comparison of the fauna of the Yoldia 

 arctica zone with that of the older Yoldia. clay of Vendsyssel shows 

 that the two are identical and consequently of the same age. 



The last glacial period in Denmark is represented by the bed of 

 fluvio-glacial sand, gravel, and clay, o7 meters thick, at Skaerum- 

 hede, and the overlying sandj^ and stony moraine. Remains of this 

 glaciation are found over a large part of Denmark, either at the 

 surface or beneath later Quaternary deposits. So far, however, no 

 interstadial deposits have been found in it comparable to those in 

 the Baltic Hohenriicken of East and West Prussia, so that the Baltic 

 oscillation as a readvance of the ice edge over Denmark apparently 

 did not occur. 



The postglacial deposits, however, show a well-marked climatic 

 oscillation — the Allerod oscillation — which possibly corresponds to 



