398 GEOLOGY 



of unconformity. An unconformity is developed by the erosion, 

 the deformation, or both, of the older and lower set of beds before 



Fig. 305. An unconformity between the Devonian (the thick-bedded rock) 

 below, and the Coal Measures (thin-bedded and broken) above. The 

 surface of the lower rock was eroded before the deposition of the upper 

 part upon it. (Udden.) 



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

 between the deposition of the two unconformable sets of beds may 

 be very long when the unconformity is said to be great, or short 



Fig. 306. An unconformity in Bighorn Basin, Wyoming. The lower(Laramie) 

 beds dip notably to the left, and the upper horizontal (Wasatch) strata 

 rest upon the cut-off edges of the dipping beds. (Fisher, U. S. Geol. Surv.) 



