ECOLOGICAL SIGNIFICANCE OF EAGLE CREEK FLORA 585 



favor the wide scattering of the leaves which fell upon it, there is 

 the added feature that the thick xerophytic leaves alone would be 

 strong enough to undergo transportation without being destroyed. 

 Surely we must assume that the perfect specimens of maple, 

 sycamore, and other broad thin leaves w^re not transported far 

 from the spot where they first fell. Further, the large number of 

 mesophytic species associated with the dominant maple and elm 

 leaves indicates that the deposits containing them were laid down 

 in the valley where they grew. 



The evidence of the plant fossils thus may be explained on the 

 basis of a twofold habitat such as would be furnished by an upland 

 region traversed by valleys. Whether or not this hypothesis is 

 supported by the geological evidence may be determined by a con- 

 sideration of the physical conditions which existed during the 

 Eagle Creek epoch. 



PHYSICAL CONDITIONS DURING THE EAGLE CREEK EPOCH 



The Eagle Creek formation is made up entirely of volcanic 

 materials. In the lower part, as exposed at Red Bluffs, beds of 

 tuff and ash are most conspicuous; above, the activity of streams 

 is evidenced by the predominance of conglomerate. Thus it is 

 clear that early in the epoch vulcanism was the dominant process 

 but that toward the close the streams were able to carry and assort 

 the volcanic material as fast as it was ejected, as well as erode the 

 lava flows bordering the craters. This sedimentary phase is the only 

 one represented on the Oregon side of the gorge and is the source 

 of practically all of the fossil plants. 



There are several conspicuous features, applying especially to 

 the conglomeratic phase, which require analysis. In the first 

 place there is a predominance of coarse material — rounded bowlders 

 ranging up to 15 feet in diameter and averaging from i to 3 feet. 

 The textural range is high in any outcrop, large bowlders mingled 

 with small, all bound together by a matrix which varies from coarse 

 gravel to fine mud, but is commonly sandy in texture. The abun- 

 dance of large bowlders and the almost entire lack of assortment 

 point to the deposition of these sediments by streams which had 

 high velocity. 



