INTRAFORMATIONAL CORRUGATED ROCKS 593 



DIFFERENTIAL MOVEMENT ACCOMPANYING NORMAL FAULTING 



Up-drag of strata commonly takes place as a result of friction 

 during movement on the downthrow sides of normal faults. Under 

 proper conditions drag folds of the nature of intraformational corru- 

 gations develop as a result of differential movement of the bending 

 strata. A good example has been observed by the writer in the bed 

 of the Connecticut River just below the dam at Holyoke. A normal 

 fault is clearly traceable in the bed rock across the river a few rods 

 below the dam. From the fault south for 150 yards the strata 

 (sandstone and shale) show dips of 25 to 40 degrees to the south due 

 to up-drag, and they strike parallel to the fault. About 150 yards 

 south of the fault there is a notably disturbed zone of thin-bedded 

 dark shale with strike parallel to that of the fault. It shows clearly 

 along the strike for 200 yards. The disturbed zone, varying in 

 thickness between 4 and 10 feet, is overlain by fairly well-bedded 

 red sandstone, and underlain by thin-bedded dark shale much like 

 that of the disturbed zone. Next below there is sandstone. The 

 folded zone does not terminate abruptly at either summit or base, 

 but the top is much the more regular, coming close to the over- 

 lying sandstone. Even the sandstone, for a foot or so above the 

 shale contact, is locally somewhat bent. The whole body of the 

 rock in the bed of the river south of the fault plainly shows the effects 

 of differential movements which took place during the process of 

 normal faulting. One belt of weak, thin-bedded shale overlain 

 by relatively rigid sandstone became moderately corrugated, the 

 corrugations being of the nature of drag folds produced by differ- 

 ential movements. That the corrugations must have developed 

 after the deposition of the overlying sandstone layers is proved by 

 the fact that the lower part of the sandstone is in many cases moder- 

 ately bent just like the immediately underlying shale. It seems 

 impossible to escape the conclusion that this corrugated zone is of 

 tectonic origin, that is, the result of differential movement accom- 

 panying normal faulting. 



DIFFERENTIAL MOVEMENT ACCOMPANYING REGIONAL FOLDING 



The principle of differential movement of strata is well illus- 

 trated in many regions of notably folded rocks. Differential move- 



