292 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 8 



the end of the Pliocene Period and the present time, that is, geologic- 

 ally speaking, during the Pleistocene Period, there was a great exten- 

 sion of ice in all parts of the world. 



The map (fig. 1) shows the area which was under ice at some 

 time or other during the Pleistocene Period. The north polar ice cap 

 extended outward, reaching latitude 38° N. in North America, 50° N. 

 in Europe, and 60° N. in Asia. In the British Isles the ice sheet ex- 

 tended as far south as the Thames Valley but did not cross it. The 

 ice covering of the Antarctic was then much thicker than at present, 

 and Meinardus estimates that the flow of ice from the Antarctic 

 continent into the surrounding seas was at least three times as much as 

 at present. 



Not only were the polar ice caps more extensive and thicker, but 

 also the mountain glaciation was much developed and the glaciers 

 reached much farther down the valleys. The evidence shows that 

 this was so in all parts of the world. Not only were the mountains 

 which now carry ice more heavily glaciated, but also many mountains 

 which now carry no permanent ice had extensive glaciers upon them. 



Detailed study of the deposits left behind by the ice shows that there 

 was more than one advance and retreat of the ice during the Pleisto- 

 cene Period. Glacial and interglacial epochs have been detected in all 

 regions which were glaciated, both in regions covered by the extended 

 polar ice caps and on the glaciated mountains. The history of the 

 Pleistocene glaciation has been most completely studied by Penck 

 and Bruckner in the Alps. The result of their work is summarized in 

 figure 2, which is a reproduction of their classical diagram showing the 

 variation of the height of the snow line in the Alps during the Pleisto- 

 cene Period. From this diagram it will be seen that Penck and 

 Bruckner concluded that the snow line descended far down the moun- 

 tains on four occasions, the limits reached each time being practically 

 the same, namely, about 1,500 meters (4,000-5,000 feet) below the 

 present snow line. They gave the names: Gunz, Mindel, Kiss, and 

 Wurm to the four glacial epochs. It will be noticed that the four 

 glacial epochs occur in pairs — the Gunz with the Mindel and the Kiss 

 with the Wurm — with a rather longer interglacial epoch between 

 them. 



There is no certain evidence that the history of the ice worked out 

 in the Alps applies to other parts of the world ; but meteorological con- 

 siderations make it very improbable that there were different glacial 

 histories for each part of the world ; and as the evidence of glacial and 

 interglacial epochs has been found wherever the glacial history of a 

 country can be worked out in detail, one is justified in concluding that 

 these epochs occurred simultaneously as the result of a world-wide 

 cause. 



