APPALACHIAN FOLDS OF CENTRAL PENNSYLVANIA 235 



Southeast of the Susquehanna bridge above Harrisburg the data 

 obtained were inadequate and unsatisfactory. The strata to which 

 we must look for guidance here are Hudson River shales and slates, 

 and they are mashed and disturbed to such an extent that often there 

 is danger of mistaking secondary structures for bedding. Wherever 

 observed these beds all dip southeastward at angles varying from 

 5o°-8o° in such a way as to indicate overturned folds. But these 

 soft shales furnish the poorest sort of criterion for determining the 

 true character of the folding, because a weak formation of this sort 

 may be much wrinkled, crushed, and appressed, while the stronger 

 strata above may have been merely bowed upward and may not have 

 suffered crumpling to the same extent. In projecting the younger 

 formations over these folded shales, the wrinkles were made to die 

 out slowly. The shale layers were allowed considerable thickening 

 and thinning, but in the stronger layers above little of this distortion 

 was taken into account. It may well be that considerably more 

 thickening of the layers on the crests and troughs, and thinning on the 

 limbs of the folds should be allowed in the upper formations. Since 

 the observed data for this slate and shale belt are so meager and the 

 restoration of the younger formations which once covered this region 

 is so precarious because of this limited knowledge, and because the 

 whole is so much a matter of personal opinion, it has not seemed 

 advisable to state any measurements made upon the reconstructed 

 curves. Some of the layers were measured, however, and upon this 

 basis a rough guess that the original length of these strata was about 

 twice their present horizontal length is ventured. They can scarcely 

 have suffered much less shortening than this, though they may have 

 suffered much more. Claypole believed that these overturned folds 

 resulted from such intense crumpling that into each horizontal mile 

 of present distance there have been squeezed what were originally six 

 miles of flat-lying strata^, but this figure seems to me somewhat 

 excessive. 



To put these figures together, we have, on the basis of the assump- 

 tions made, and subject to other limitations to be mentioned shortly, 

 71.8 miles reduced to 61.6 miles west of the Susquehanna bridge, 

 and 9 . 5 miles jammed into the present distance of 4 . 75 miles between 

 Harrisburg and the Susquehanna bridge, making a total west of 

 Harrisburg of 81 miles compressed into 66 miles. 



