270 



PHYSICAL GEOLOGY 



Conformity and Unconformity 



When strata are deposited one upon another in unbroken succes- 

 sion and without disturbance, they are said to be conformable (Fig. 

 I, p. 23). When, however, one set of beds is deposited on another which 

 has been above the sea and there eroded, the two beds are said to be 

 unconformable, and an unconformity, represented by the erosion sur- 

 face which separates the strata, is said to exist. Unconformities may 

 exist between stratified and igneous or metamorphic rocks (Fig. 322, 



Fig. 270. — A diagram showing an unconformity or erosion interval, AB. 



p. 326). Usually, an unconformity is marked by some change 

 in the relative dip of the beds, one set resting upon the upturned 

 edges of an older series (Fig. 270). Such an unconformity is spoken 

 of as an angular unconformity, because the strata below the uncon- 

 formity meet those above it at an angle. An unconformity, however, 

 sometimes occurs between strata which have n©t suffered any rela- 

 tive change in dip (Fig. 271); i.e., both the younger and the older 



Fig. 271. — Diagram showing unconformities. The stratum C is separated from 

 the strata B and D by unconformities. The unconformity between B and C is not 

 readily noticeable, because the strata of both are horizontal. An unconformity exists 

 also between the glacial drift A and the limestone B. (Near Milwaukee, Wisconsin.) 



strata may be horizontal. In such cases the unconformity car 

 usually be recognized by old erosion channels or by basal conglom- 

 erates (p. 240), but occasionally such a break is difficult to detect. 

 Importance of Unconformities. — Unconformities are of much 

 importance in the study of the geology of a region, because of the 

 geological history which they reveal. From the unconformity 



