232 ROLLIN T. CHAM BERLIN 



anticlines, or of older beds in the troughs of deep synclines. This 

 error increases with the vertical distance from the observed stratum. 

 It is clear therefore that cross-section measurements designed to 

 give the crustal shortening should be made on stratigraphic horizons 

 as near as practicable to those on which the field observations were 

 actually made. The error of mechanical projection is then less 

 serious. 



Competent and incompetent beds also behave differently in 

 folding. Various secondary adjustments between the different 

 beds are, in many cases, accomplished by minor contortions and 

 subordinate movements within the readily yielding incompetent 

 layers, which, by accommodating themselves in buffer-like fashion 

 to the major movements, facilitate the folding. The strong, 

 competent layers, less subject to local irregularities, are likely 

 to reveal truer estimates of the shortening of a region as a whole. 



The ideal horizon for measurement should therefore have the 

 following qualities: It should be stratigraphically near the beds on 

 which the observations were actually made; it should be the top 

 or bottom of a rather thin formation so as to limit variability; 

 the formation should be resistant and hence predisposed to outcrop 

 sharply in the field; it should be readily distinguished from other 

 formations ; and it should be so located in the column as to come to 

 the surface at many places in the section. Such a formation in the 

 Colorado Rockies is the Dakota sandstone. It is one of the most 

 persistent and conspicuous of the formations, and it occupies the 

 desired mean position in the stratigraphic column. The base of the 

 Dakota sandstone was therefore adopted as the line to be measured 

 in these studies. 



On the cross-section sheets the base of the Dakota was followed 

 through all the recorded folds and crenulations with a thin, flexible 

 copper wire, which was bent so as to conform to every change of 

 curvature. The wire was then straightened and compared with the 

 scale for measurement. The difference between the length of the 

 base of the Dakota, as thus measured along all the sinuosities of 

 the section, and the present horizontal length of the section gives 

 the amount of crustal shortening. Measuring each of the fourteen 

 sheets separately gave the following results: 



