UNI-"!";*.-! TOPOGRAPHY OF THE REGION. 629 



cliffs, to a depth of from 200 to 700 feet. The low mesas, surmounted 

 by several buckles or domes of basalt a few hundred feet high, rise 

 gradually toward the edge of the mountain. Near the mouth of the 

 Boise River the basalt mesas cease, and from here down to Weiser, 

 where the great Snake River canyon begins, several large tributaries 

 enter, such as the Payette and the Owyhee, and, at elevations of from 

 2,200 to 2,700 feet, level bottom lands and broad low terraces flank 

 the water courses. 



Between the mouth of the Boise and Weiser flat-topped hills of soft 

 sandstones rise on both sides of the Snake River to a height of 600 to 

 800 feet. Similar complexes of higli sandy mesas rise between the 

 lower courses of the Boise and the Payette and north of the Payette. 

 The mountains of older rocks surrounding the tectonic trough of the 

 Snake River Valley rise gradually, on the north side of the river, 

 beyond the sloping mesas of Tertiary rocks, their margin having a 

 northwesterly direction in this vicinity. The transition between 

 mountains and mesa is abrupt only at the Boise River, near Boise, 

 and the abruptness is here due to the extensive erosion of the Payette 

 sandstones by the river. 



The mountain region extending up to the Sawtooth Range, dividing 

 with a north-northwesterly trend the waters of the Boise and the south- 

 ern branches of the Payette from those of the Salmon, has an average 

 width of 5,5 miles and culminates in summits with an elevation of from 

 10,000 to 11,000 feet. This mountain complex, which is made up chiefly 

 of granitic rocks, does not form a well-defined range, but rather a 

 broad uplift dissected deeply and in the most intricate manner by the 

 forks of the Boise and the Payette. The summits of the narrow ridges 

 generally form gently sloping lines. If a surface were constructed 

 containing all these lines it would be of undulating, curved character, 

 sloping gently from elevations of 9,000 down to 4,000 feet. From the 

 southwestern edge a steeper slope carries the granitic rocks below the 

 surface of the Tertiary rocks of the Snake River Valley. The can- 

 yons of the Boise and the Payette have cut down in the uplift to a 

 maximum depth of 3,000 feet, and are joined by deep lateral canyons, 

 dividing the whole region into a maze of narrow aretes. The grade of 

 the main rivers is relatively low, from 10 feet up to 50 feet per mile, 

 and only well up toward the head waters are grades of 100 feet per 

 mile attained. The grades of the lateral canyons are also often rela- 

 tively slight in their lower course, but extremely steep cirques rise 

 near their head waters. The Idaho Basin quadrangle offers excel- 

 lent illustrations of these relations, which are the result partly of a 

 considerable antiquity of the drainage, partly of the crumbling char- 

 acter of the granite. At the main divide (Bear Valley quadrangle) 

 the broad valleys and gentler slopes of the Salmon River drain- 

 age contrast strongly with the deeply incised canyons of the Boise 

 and Payette. The latter streams are continually capturing territory 



