ECOLOGICAL SIGNIFICANCE OF EAGLE CREEK FLORA 589 



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. i) 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 



