NATURE OF THE GEOLOGIC RECORD 



133 



FIG. 7.1. Folding followed by overthrust, one way in which 

 the time sequence of strata is disturbed. (By permission 

 from Historical Geology, by Hussey, p. 306. Copyright, 

 1947. McGraw-Hill Book Company, Inc.) 



although the latter may not be evident if study is confined to a small area. 



Over much of the earth — in fact all localities lacking high mountains or 



deep canyons — the number of strata exposed on the surface in any one 



locality is strictly limited. In more level regions one or a few strata will be 



