THE BUILDING OF THE COLORADO ROCKIES 231 



sideration because they are truly a part of the plains and do not 

 properly belong to the folded belt. The truly folded section is 

 thus reduced to 132 miles in length (Figs. 5-1 1). 



THE SHORTENING OF THE CRUST 



Folding of strata under horizontal compressive stress is neces- 

 sarily accompanied by a horizontal shortening of the deformed 

 portion of the earth shell. The compressed mass is shortened in at 

 least one of its horizontal dimensions, while it relieves itself by 

 bulging upward in the direction of least resistance. The first 

 objective in our problem is to determine the amount of horizontal 

 crustal shortening which the Rocky Mountain region underwent 

 while writhing in the throes of diastrophic revolution. The original 

 length of the Lyons-Grand River section before folding may be 

 determined with some approximation by measuring the total 

 length of a selected layer through all the sinuous bends and wrinkles 

 which are recorded on the plotted cross-section. A small corrective 

 factor for the compacting of materials under the compression is 

 to be introduced. The present horizontal length to which the 

 section has been compressed is simply the map distance between the 

 two ends of the section measured along the slight curve which was 

 chosen so that the section shall cross the folds at right angles as 

 explained on page 227. The difference between the length of the 

 strata followed through all the folds and faults as reconstructed 

 and the present map distance of the same section is the approxi- 

 mate shortening which the earth shell has suffered in being crumpled 

 into mountains. 



For measuring the length of the wrinkled strata all members of 

 a thick series of sediments are not likely to be of equal value. In 

 folds of the type technically known as similar, the individual beds 

 have all been folded with the same curvature.^ But in parallel 

 folds, where the bedding surfaces are mutually parallel, the curva- 

 ture of no two beds is exactly the same, and this difference in cur- 

 vature implies the dying out of the folds in one direction or the other 

 frorfi a given bed. Hence an element of error is likely to be intro- 

 duced in the hypothetical projection of younger beds over eroded 



^ C. K. Leith, Structural Geology (1913), pp. 106-7. 



