

FACTS ABOUT MOUNTAIN REGIONS. 789 



Carpathians, and in the folded strata of other regions, the Sierra Ne- 

 vada, which is peculiar in being a single mountain mass, the western 

 side has a slope of 100 to 250 feet to the mile, and the eastern a 

 slope, for a large part, of 1,000 feet to the mile. 



Where flexures have an inclined axis and axial plane, a twist or 

 torsion of the strata must often have taken place. A marked example 

 of torsion is represented in the cut below (by Gardner), representing 

 upturned beds in the Elk Mountains, Western Colorado, from Hayden's 

 Survey of 1873. Through a twist in the upturning, the Cretaceous, 



Fiff. 1143. 



Upturned strata of the west slope of the Elk Mountains, Colorado. The light-shaded stratum, Tri- 

 assico- Jurassic ; that to the right of it, Carboniferous ; that to the left, Cretaceous. 



which is the overlying rock in the back part of the scene (to the left), 

 is really the underlying in the front part and the Carboniferous the 

 upper part, the pressure having so pushed forward the mass that the 

 order of superposition is the reverse of the order of age ; and the Car- 

 boniferous beds of the front ridge incline 45° beyond the vertical. 



Again, in regions where the folding was feeble in degree, there has 

 sometimes been an upward bend along a line of much length, with no 

 corresponding downward slope. Such folds are described by Powell 

 as occurring in the Colorado basin, and are illustrated in the figure 

 from his Report given on page 792. 



Extensive elevated plateaus are a feature of the gently folded mar- 

 gins of mountain ranges. They sometimes pass by a gradual passage 

 into the more folded region of the mountains ; but often they stand 

 apart in consequence of a great fault or series of faults. The region 

 of Alleghany Mountain, together with its extension northward and 

 eastward to ]Sew York and the Catskill Mountain mass, and south- 

 ward through the Cumberland Table Mountain of Tennessee to 

 Alabama, lies in the western section of gentle slopes of the Appala- 

 chians. The following figure, from Saflford, represents the abrupt 

 succession from the folds of Eastern Tennessee to the nearly horizon- 

 tal and boldly browed " Cumberland Table-land," just west. The 

 folded region, which is that of the more upturned portion of the Ap- 

 palachian range (lying west of the Archaean range on the eastern bor- 



