80 



Referred to sea level, the southern or downstream end of the upper 

 terrace is about 20 feet higher than the northern, or upstream end. That 

 is, in a distance of a little more than two miles the terrace level rises. 

 20 feet higher above sea level, while the surface of the water in the stream 

 has a fall of about 37 feet in the same distance, making a total differen- 

 tial level between the surface of the terrace and the surface of the water 

 of about 57 feet. 



The terraces of the lower level are from 25 to 35 feet lower than those 

 of the upper level and are found only in the lower portion of the canyon- 

 like part of the valley ; their width is about the same as that of the upper 

 terraces, averaging perhaps 25 or .30 feet, with a maximum of from 50 to 

 00 feet. The height of surface of this lower .series above the stream also 

 increases in the downstream direction, but at a much smaller rate than in 

 the case of the upper terraces; in a total distance of about two-thirds of 

 a mile the difference in elevation is about (! feet. The fall of the stream 

 l8, in the same distance, a little more than (5 feet, which leaves the surface 

 of the lower series of terraces with a very slight gradient in the down- 

 stream direction, when referred to sea level as datum. 



Considering, then, the upper terrace level, the lower terrace level, and 

 the present gradient of the stream in their relations to each other, the 

 lower terrace level can be represented by a nearly horizontal line drawn 

 a little lower than midway between two other straight lines which diverge 

 in the downstream direction ; the upper representing the surface of the 

 upper terrace le\el. and the lower the present gradient of the stream. 



CONCLUSIONS. 

 The causes which operated to make the conditions resulting in the 

 terraces above described may. no doubt, be inchnhd for the most part in 

 the following: 



1. Variiitifins in the amount of sediment carried by the streams. 



2. Variations in the amount of water carried by the streams. 



3. Variations in gradient due to — 



(a) Diastropbism ; 



(b) Dams of ice and glacial debris more or less complete, 

 resulting in ponds, river-lakes, etc. 



While it is not po.ssible to say jiositively just what share each of these 

 causes may have had in the formation of e.'ich one of these terraces, the 

 following ]inrti;il ex|ilMiiations seem to be justified: 



