588 RALPH ]]'. CHANEY 



bajada deposit on the flanks of a mountain range lying to the north, 

 a range whose volcanoes threw out the great volume of pyroclastic 

 material, and which later were perhaps the sources of the basalt 

 flows. The variations in thickness from north to south are in 

 accord with this geographic relation, for the formation appears to 

 thin toward the south. 



We may outline the physical history of the Eagle Creek forma- 

 tion as follows: To the north of the gorge, an east-west range of 

 mountains contained volcanoes which were active throughout the 

 epoch, though probably less active toward its close. Large amounts 

 of ash and tuff were thrown out, covering the flanks of this range 

 to a depth of more than 2,000 feet. During intervals of volcanic 

 inactivity streams assorted this material, producing the beds of 

 volcanic conglomerate. Toward the close of the epoch streams 

 assumed dominance and transported large amounts of volcanic 

 debris out from the axis of the range, depositing it as far away as 

 the present south side of the gorge but in progressively lesser 

 amounts. Thin layers of ash in the sections on the south side 

 indicate that a small amount of pyroclastic material was carried 

 there directly from the vents. The dominance of clastic sediments 

 indicates, however, the relatively greater importance of strearti 

 deposition here, and the presence of conditions suitable for the 

 development of plant life. 



Some idea of the topographic relations of such a bajada deposit 

 on the flanks of a high range may be gained from the description of 

 similar deposits of the Sierras.^ Here the alluvial plain is traversed 

 by numerous streams flowing out at right angles from the range, 

 streams which on losing their gradient drop their loads of coarse 

 debris. This piles up until the deposit is considerably higher than 

 the areas on either side, whereupon the streams are shifted laterally 

 into the lower areas and deposition is continued there. For the 

 purposes of this discussion it is sufficient to note that the surface 

 of the bajada in the Sierras is characterized by numerous rather 

 steep-sided ridges which result from stream deposition, and that 

 most of the vegetation is found in the valley-shaped lowlands 

 between. 



'A. C. Trowbridge, op. cit. 



