APPALACHIAN FOLDS OF CENTRAL PENNSYLVANIA 231 



the bottoms of the synclinal troughs, and assuming that the remaining 

 twenty-nine miles of strata possessed an average dip of 40°, he arrived 

 at the conclusion that these forty-nine miles of strata, if flattened out, 

 w^ould measure fifty-eight miles. It is not easy to determine the 

 number of folds occurring in the Cumberland Valley, but on the basis 

 of the thickness of strata included in these plications, Claypole 

 reasoned that there could scarcely be less than eight overthrown 

 anticlinal arches in the sixteen miles considered. Adopting Profes- 

 sor Rogers' lowest angle of dip for the southeast legs (45°) and 60°, 

 his lowest estimate for the northwest limbs, he calculated that ninety- 

 five miles of strata had been compressed into these sixteen miles. 

 Considering both sections together, this would mean that a tract of 

 the earth's surface measuring originally one hundred and fifty-three 

 miles had been compressed into sixty-five miles. As Claypole 

 frankly stated, this estimate took account only of the eleven principal 

 folds and ignored the minor flexures. 



The present estimate of crustal shortening is made from a measure- 

 ment of the strata in the plotted cross-section formed as above stated 

 and made to include as much as possible of the minor contortions of 

 the beds. The data for these sections were obtained at the railroad 

 horizon. From the dip-angles, the locations of the formation-contacts, 

 and the thicknesses assigned to the formations, the whole series of beds 

 up to the top of the Pottsville conglomerate were projected over the 

 entire restored section. But as the youngest of these arched strata 

 had to be projected many thousands of feet above the railroad-level 

 to do this, it is to be noted that the farther up in the stratigraphic 

 series the restoration of structure is carried, the more uncertain does 

 it become. The separate strata, being of different material and offer- 

 ing varying resistance to the thrust, cannot always be supposed to 

 wrinkle alike. Minor flexures and local bits of crumpling may 

 fade out in passing up or down, and new ones appear. Hence for the 

 purpose of measuring the length of strata over this section it seemed 

 advisable to choose the stratigraphic horizon which remained nearest 

 to the railway-level. It is to be recognized, of course, that the maxi- 

 mum amount of crustal shortening was probably suffered by the 

 surface beds and that the folds slowly die out downward, but it is 

 believed that whatever lessening of the flexures there may be in the 



