240 J. I., lilch — /,',<-, at Strut in Trenching in the 



seeking. These trenches arc still working their way up the 

 tributary valleys by headwater recession and sapping. This is 

 well illustrated by fig. '2. which shows the trench working back 

 into the still undisseeted Hat of a small tributary. 



The alluvia] flats are composed of line sandy loam and 

 gravel. The latter occurs in the form of irregular lenses inter- 

 spersed through the loam. Its component pebbles range in 

 size up to •">. 6, or 8 inches in diameter, but seldom larger, and 

 even these sizes are uncommon. Two or three inches in 

 diameter would represent a fair average for the coarser gravel. 

 From this there is every gradation down to fine sand. The 

 stratification is indistinct, but may usually be made out. In 

 characteristic exposure the material stands up in vertical bluffs. 

 From its general make-up and composition there can be no 

 doubt that it is a stream deposit of the type formed by aggrad- 

 ing temporary streams. 



Fairly uniform conditions of deposition are shown by the 

 nature of the sediment. That heavy floods were not common 

 during the time of deposition is indicated by the lack of heavy 

 bowlders in the deposit. The valley filling is not an ordinary 

 flood plain such as may be formed along permanent streams, 

 for, in the first place, the slope of the valley bottom, 120 feet 

 per mile, is too great for the formation of a flood plain so wide 

 and flat as the one we are considering ; and in the second 

 place, the surface of the flat is not a graded slope along the 

 stream course. Alluvial fans from the valley sides form an 

 integral part of the valley filling. Their prominence, and their 

 effect in breaking the normal valley profile, indicate that a 

 large proportion of the filling came from the sides down the 

 short steep tributaries rather than down the main valley. 



On the whole, then, the character of the valley filling indi- 

 cates accumulation under conditions such that there was no 

 permanent stream in the main valley, and such that accumula- 

 tions from the sides were as important as those from the main 

 stream. These conditions would be realized under an arid or 

 semi-arid climate during the time of filling. 



In the bottoms of the trenches and sometimes spread out 

 over the surface of the flats above, there is coarse stream 

 gravel with bowlders ranging up to two feet in diameter, and 

 much of it evidently of very recent deposition. This coarse 

 material presents a marked and significant contrast to the fine 

 textured alluvial valley filling revealed in the side walls of the 

 trenches. The significance lies in the fact that its transporta- 

 tion must have required a volume of water much greater than 

 that of the finer alluvial material on which it rests. Its pres- 

 ence overlying the finer material and the fact that it is asso- 

 ciated with flood debris in such a manner as to prove its 



