hershev.i The River Terraces of t lie Orleans Basin. 



4G1 



slightly tilted upstream. To determine the amount of this tilt- 

 ing would require a more accurate survey. However, a channel 

 level which at the Pearch Mine Las an altitude of about 572 feet 

 seems to have a corresponding level at Graham Flat of 583 feet. 

 The distance involved is about a mile and a half, and, making 

 allowance for an original downstream grade, this would indicate 

 an upstream tilting of about ten feet per mile. It is a matter 

 of observation that the interval between the forty-five-foot and 

 seventy-foot terraces rapidly increases downstream. From this 

 I infer that the river abandoned the flood-plain of the seventy- 

 foot terrace because of a tilting of the basin in an upstream 

 direction. Subsequently there was a reverse tilting toward the 

 southwest, but not to as great an amount as the former north- 

 east tilting, so that the upper terraces still retain part of the 

 earlier slant. 



The Upper Group of terraces were apparently formed under 

 comparatively low-grade conditions. This is indicated by the 

 fineness of the gravel, great thickness of the flood-plain deposits 

 and great width of the valleys excavated. At the time that the 

 surface of the 850-foot terrace was the valley floor, the flat 

 central portion was probably from a mile to a mile and a half 

 wide, and this was bordered on each side by a gently sloping 

 tract about one-half a mile wide. The gradual rise of the valley 

 floor to the steeper mountain slopes was not due to local alluvial 

 fans, as in the case of the 120-foot terrace, but to a gradual rise 

 of the bed-rock floor. This gave the upper valley a broad 

 U-shape, indicative of considerable age. Subsequently cut por- 

 tions of the present valley are more distinctly canon shaped. 



The terraces indicate an uplift, or a series of uplifts, of the 

 Orleans Basin. The bed-rock floor of the highest terrace is 

 1,200 feet above sea-level. If this has not been uplifted since 

 the river abandoned it, the stream, granting it a grade equal 

 to the present, must have extended over seventy-five miles 

 beyond the pesent coast-line. However, the evidence is that 

 during the formation of these terraces the river has been 

 extended. At least the lower of the marine terraces along the 

 coast must be the correlatives of these river terraces, as they 

 all date from the latter portion of the Quaternary Era. With 



