HISTORY OF THE WILLOWS AND POPLARS 25 



Species that are found fossil in the Pleistocene deposits. The 

 oldest known were the contemporaries of the dinosaurs of the 

 closing days of the Lower Cretaceous. One small leafed form 

 is found at this early day in the Potomac River valley and the 

 other, which was for a long time the oldest known dicotyledon, 

 comes from the Kome beds of western Greenland and was 

 named Populus primaeva by Heer, its describer. 



These first poplars are rare forms but their geographical 

 separation gives us a hint that their abundance was greater in 

 those early days than the records show, and this is also indicated 

 by the abundance and wide distribution of poplars during the 

 Upper Cretaceous, from which about thirty species have been 

 described. They are much less abundant than the willows in 

 the Upper Cretaceous of Europe but, unlike the willows, they 

 are common in Greenland, and they are exceedingly ubiquitous 

 in North America at this time, especially in the West where 

 they appear to have been very common along the borders of the 

 Upper Cretaceous sea that submerged so much of the then low 

 western country. In addition to the American, European and 

 Arctic records a petrified piece of a poplar root has been described 

 from the Upper Cretaceous of Japan, indicating that Asia had 

 its species then as nov\^. 



During the succeeding Eocene period there were upwards of 

 50 species, or twice as many as are living at the present time. 

 The rising land of what is now the Rocky Mountain countrj^ 

 shut off the moisture laden winds from the Pacific and the 

 lessening rainfall made of this vast region a quite different 

 country from what it had been during the Upper Cretaceous. 

 In the continental deposits of the Eocene, that is deposits laid 

 down on the bosom of the land rather than in the sea — the 

 deposits of wind blown materials and volcanic dust, laid down 

 in lakes, streams, floodplains, etc — deposits referred to the Fort 

 Union formation, leaves of poplars are the most abundant fos- 

 sils. 



Poplars appear to have covered at this time all of the plains 

 and mountain country of the West in great variety, extending 

 northward from the western provinces of the United States and 



