STRUCTURAL FEATURES 



273 



unconformity. Most unconformities are developed by the erosion, 

 tlu- deformation, or both, of the older and lover set of beds, before 

 the deposition of the younger and upper set. The interval of time 



IML'. J47. I'm onformity in Bighorn Basin, Wyoming. The lower (Laramie) 

 beds dip notaMy to tin- left, and tlu- upper horizontal (Wasatch) .strata rest upon 



their cut-off edges. (Fisher, U. S. Geol. Surv.) 



T- 



between the deposition of the unconformable sets of beds may be 

 very long when the unconformity is great, or short when the 

 unconformity may be slight. Unconformities are of great signifi- 

 cance in the interpretation of geological history. Unconformities 



Fig. 248. One phase of unconformity. The beds at the right were tilted to 

 their present position before the deposition of the beds at the left. 



exist between stratified rock and igneous rock, and between strati- 

 fied rock and metamorphic rock, as well as between different series 

 of bedded rocks. 



Structural Ventures Arising from Disturbance 

 Inclination and folding of strata. The original attitude of beds, 

 whether formed by water or by lava-flows, commonly departs but 

 little from horizontality. Locally, however, both kinds of deposits 

 are made on considerable slopes. 



