ECOLOGICAL SIGNIFICANCE OF EAGLE CREEK FLORA 579 



vegetation cover. Conditions become progressively more arid 

 toward the east, with only a sparse occurrence of pines and oaks 

 at The Dalles. 



GEOLOGIC RELATIONS OF THE EAGLE CREEK FORMATION 



A recent study of the geology of the Columbia River Gorge by 

 Drs. I. A. Williams and J H. Bretz under the auspices of the Oregon 

 Bureau of Mines has given us a rather complete knowledge of the 

 general geology of this little-known region.^ As far back as 1873 

 LeConte recorded the presence of fossil leaf impressions in the 

 volcanic conglomerate at the base of the basalt series." In 1895 

 Diller secured a small collection of leaves near the mouth of Moffatt 

 Creek which has been described by Knowlton,^ and four years 

 later Gilbert made a larger collection from a talus block near 

 Cascade Locks, a collection which has never been described. The 

 collections which are the basis of this paper are, however, the first 

 which are sufficiently complete to give any conclusive evidence 

 regarding the age of the Eagle Creek formation. 



There are but few cases of such an illuminating record of the 

 history of a mountain range as has been furnished by the Columbia 

 River in its path across the Cascades. Following is the generalized 

 section exposed by the Columbia: gravels and river terraces of 

 recent origin; Herman Creek lava — andesitic basalt; Satsop 

 formation — stream gravels and volcanic ash; Columbia River 

 lava — successive flows of basalt; Eagle Creek formation — volcanic 

 conglomerate, ash, and tuff. 



The Eagle Creek formation. — The Eagle Creek formation is 

 exposed along the bottom of the gorge from Warrendale to Viento 

 on the Oregon side with a corresponding distribution on the north 

 side of the river. It is the oldest formation recognized in the region, 

 and is brought to the surface in the axis of the great north-south 

 anticline which is the backbone of this portion of the range. The 

 thickness of the exposed part of the formation varies from 2,700 



' Ira A. Williams, Bull, of Ike Ore. Bureau of Mines and Geol., Vol. II, No. 3, 1916; 

 J H. Bretz, unpublished manuscript. 



' J. LeConte, Am. Jour. Scl., 3d Series, VII, 167-80. 



3 F. H. Knowlton, Twentieth Ann. Rept. U.S. Geol. Surv., pp. 37-64. 



