PERIODS OF FOLDING 333 



central block was itself gently arched and faulted at many places, espe- 

 cially at its borders, where it was relatively lowered one or two miles by 

 marginal faults. The Triassic deformation shown in normal faults and 

 tilted blocks involved a slight extension of the earth's crust, thus indi- 

 cating that the great compression of the Appalachian movement was 

 entirely satisfied and its causes removed. The visible movements of this 

 deformation were almost wholly vertical, and in explaining them it is 

 most reasonable to postulate a simple vertical force. 



While it is not intended to discuss here the Triassic deformation, it is 

 important to state that Triassic normal faults have caused serious eiTors 

 in interpreting the structure where they have cut the older rocks at a 

 distance from the Triassic basins. This is particularly the case in west- 

 ern New England and eastern I^ew York and is one of the factors which 

 entered into the Taconic controversy. The normal faults produced rela- 

 tions just the opposite of the Appalachian thrust faults, and the result 

 of a failure to observe them can be readily understood. Little or no 

 attention has been paid by geologists to these faults in the Appalachians, 

 probably for the reason that few of them are found northwest of the 

 Blue Ridge, where studies of the Appalachians were focused. 



The Appalachian region as a whole was also deformed slightly by 

 Cretaceous, Tertiary, and later uplifts. These have taken the form of a 

 general bowing up of two great areas — one coinciding in general with 

 the most folded part of the Appalachians and another roughly parallel 

 area running through Texas, Oklahoma, Missouri, and the upper Missis- 

 sippi basin. Between these uplifted areas lies the Mississippian embay- 

 ment, in which the Tertiary and Cretaceous rocks are only partly raised 

 above sealevel. The changes of attitude caused by these folds are very 

 slight and only visible when large areas are considered. Exceptions to 

 this statement are seen in certain definite belts of faulting in Texas and 

 Arkansas, where there are normal faults of considerable throw. As a 

 rule, however, the Cretaceous and later displacements are very gentle 

 warps and are of the same character as the many warps which affected 

 the Appalachian region in Paleozoic time. There appears to be no ele- 

 ment of compression associated with them, a fact which puts them into a 

 class quite distinct from the Appalachian structures. 



One great fact stands out from the distribution of these movements' in 

 time and space, and that is that the Appalachian system, and especially 

 its most folded part, represents a weak belt of the earth's crust in strong 

 contrast with the Canadian shield and the plateau region of the Missis- 

 sippi Valley. It has been deformed again and again from Precambrian 



