Hebshey.] 



The River Terraces of the Orleans Basin. 



465 



evidence of fossil representatives of Pseudotsuga and Sequoia 

 from other regions of the West, which shows a definite recession 

 from previously occupied areas." 



The most startling result of this study is the discovery of 

 practically conclusive evidence of a comparatively short period 

 of excessive rain-fall. This was first suggested by the torrent 

 fans on the 120-foot terrace. It must be remembered that these 

 fans occur only at the months of such small ravines as did not 

 produce similar fans on any higher or lower terrace. Corre- 

 sponding fans in the mountain gulches away from the river are 

 so coarse and so peculiarly ridged along the central line leading 

 up the small gulches as to imply rapid formation as the result 

 of cloud-bursts. Now, cloud-bursts are characteristic of arid 

 climates, and for a time it was a question in my mind whether 

 these torrent fans indicate excessively humid conditions or semi- 

 arid conditions. I could not find any satisfactory solution for 

 the problem until it occurred to me that the landslides furnish 

 the key. 



It is a remarkable fact, established by abundant observation, 

 that the great majority of the landslides bear an intimate rela- 

 tion to the 120-foot terrace. A few landslides occurred earlier, 

 and some later, even down to the historical period, but the larger 

 landslides, and by far the greater number, occurred during a 

 short period which was identical with the torrent fan period. 

 In this connection there are three hypotheses worthy of consid- 

 eration : 



1. That in cutting from the 475-foot to the 120-foot terrace 

 such high, steep slopes were produced as especially favored land- 

 sliding. The objections to this are that many large slides along 

 the serpentine belts moved several miles, and from positions not 

 at all affected by erosion below the 475-foot level, and that these 

 landslides did not occur in considerable numbers until after the 

 upper channels under the 120-foot terrace were completed. They 

 clearly date from a particular portion of the time occupied in 

 forming the 120-foot terrace. 



2. Many of the settlers attribute these landslides to earth- 

 quakes. The region is one of the most stable on the continent, 

 and, so far as I am aware, no severe earthquakes have been expe- 



