THE CANTWELL FORMATION 125 



here shows steep, though variable, dips, both to the north and to 

 the south; and transverse to the trend of the sequence displays 

 considerable variation in the degree of dynamic metamorphism, 

 the rocks ranging from fairly massive to schistose. These obser- 

 vatiolis indicate that the rocks have been subjected to considerable 

 compression, and it is beheved that the gentle folds on the west 

 pass eastward into more intense and comphcated folds, not iso- 

 clinal, but with axial planes variously inclined on an east-west 

 axis. 



Although no faults were directly observed in the portions of the 

 Cantwell examined, faulting was undoubtedly a resultant of the 

 compressive forces to which the terrane has been subjected. It 

 would appear from the increasing pressure effects to the east that 

 the formation as a whole had suffered a differential degree of com- 

 pression. Yet the possibility must not be overlooked that in the 

 massive areas extensive faulting may have relieved the stresses 

 that in the absence of these places of yielding would have produced 

 close folding and schistosity there also. In the westward part of 

 the formation (west of the Nenana), where the rocks are massive 

 and folding gently. Brooks^ observed a number of faults consequent 

 upon the folding.^ 



Contact relations. — -Along the northern border, according to 

 Brooks, the Cantwell rests unconformably upon Paleozoic rocks. 

 Along the eastern portion of the southern border, the relation to the 

 bordering Paleozoic sediments is obscured by granite intrusions 

 and glacial gravels, but the contact is beheved to be characterized 

 by an unconformity, although there is some evidence that the 

 plane of separation is one of fault movement. The coming to 

 place of the extensive granite bathoHths seems to have had little 

 structural or mineralogic effect upon the invaded sediments. 



Thickness. — ^The thickness of the Cantwell formation was 

 obtained with greatest accuracy in the section east of the mouth of 



^ Op. cit., pp. 79-80. 



^ Brooks (op. cit., p. 79) suggests that the Cantwell is not so sharply folded as 

 the adjacent formation, "because the massive beds were able to resist the movement, 

 some of which has been taken up by lines of shearing that have followed the shale beds 

 and the lines of parting between the shales and the massive beds." 



