ANTICLINAL MOUNTAIN RIDGES 



175 



feet of Miocene sediments, the thickest section of the EUensburg 

 formation measuring 1,600 feet. Subsequent erosion, however, 

 removed the EUensburg, and even some sheets of the Yakima 

 basalt over considerable areas, so that the later deformation was 

 of the nature of flexing at essentially the present surface. This 

 uplift began probably in the Pliocene and continued long enough 

 so that even now erosion has not essentially modified the ridges 

 of deformation. The exception to this statement is found in the 

 water gaps of the Yakima and Naches rivers, in which cases 



Pig. I 



streams coming from the more humid region to the northwest 

 have possessed sufficient corrosive power to withstand the defor- 

 mation forces and maintain their right of way across the uplifted 

 ridges. The mechanics of such surface deformation would alone 

 constitute an interesting subject of investigation. It is evident 

 to any observer that a series of basaltic flows in which the col- 

 umnar jointing is so common a feature would act quite differently 

 under deforming forces than more rigid formations. The type of 

 deformation found here is that termed by Van Hise ^ "joint folds." 

 One distinction must be kept in mind, namely, that the joints in 

 the Yakima basalt are not the product of the deformation forces, 

 but owe their origin to contraction in the cooling lava. During 

 the later deformation these thousands of joints may have been 

 opened sufficiently to allow the production of the broad flexures 

 of the basalt sheets. Adjustment along the planes between suc- 

 'Van Hise, Jour. Geol., Vol. V, p. 191. 



