ECOLOGICAL SIGNIFICANCE OF EAGLE CREEK FLORA 5890 
We may now turn to the plant record to see how the biological 
evidence corresponds with the geological. It will be recalled that 
there are two distinct ecological types represented in the flora, the 
one composed of fewer species but including the larger number of 
individual leaves, indicating an exposed habitat; the other com- 
posed of many species, indicating a protected habitat. Clearly the 
xerophytic association would have found its place on the ridges, the 
mesophytic association in the depressions such as are furnished by 
the bajada topography. Leaves from the oaks on the ridges might 
easily be blown or washed into the depressions below, there to be 
mixed and buried with the leaves of the mesophytic maples and 
elms growing along the streams. The bajada topography fits 
well with the ecological requirements of two habitats, one exposed 
and xerophytic, the other protected and mesophytic. 
However well these ecological requirements are fulfilled by the 
bajada topography, there should be some actual record of the latter 
if it existed during the Eagle Creek epoch. During the first season’s 
work no such evidence was discovered. ‘The requirements of the 
plants were unsatisfied until a second visit to the gorge, during 
which several situations were found where the topography developed 
during the epoch showed distinct relief. Along the Columbia 
River Highway cut west of the Tanner Creek bridge, two places 
were noted where a soil line slopes sharply into a depression. In 
one of these (see Fig. r) the soil line slopes down at an angle of 
22° and the depression below contains the upright stump of a large 
tree. Similar situations on Moffatt and Eagle creeks were observed. 
In the latter the material deposited over the leaf-bearing bed is 
clearly a valley fill, as shown by the horizontal bedding of the 
gravelly layers in the sandstone. Further, as previously noted, 
the upper surface of the formation has marked relief. Clearly 
the old surfaces within and at the top of the formation show that 
there were ridges and depressions such as the evidence of the plants 
demands and such as would have been present in a bajada deposit. 
A consideration of the climatic conditions indicated by the 
plants is next in order. The numerical predominance of the 
xerophytic form, Quercus pseudo-lyrata, and its occurrence in 
nearly all the deposits where large collections were made indicate 
