588 Op) RAITT VWs CEEAINEY, 
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 (one 
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 
débris 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 stream 
deposition here, and the presence of conditions suitable for the 
development of plant life. 
Some idea of the topographic zalatiogs 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 
débris. 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. : 
tA. C. Trowbridge, op. cit. 
