318 A. KEITH OUTLINES OF APPALACHIAX STRUCTURE 



HEIGHT AXD DEPTH OF FOLDS 



The faults of the mountain border zone are situated on the northwest- 

 ern flank of the great anticline of the system, which manifests itself in 

 the Sutton Mountains of Canada and their analogue in the Green Moun- 

 tains of Vermont and Massachusetts, the Highlands of southern New 

 York and Xew Jersey, the Reading Hills and South Mountain of Penn- 

 sylvania and Maryland, the Blue Ridge and Catoctin Mountain of Vir- 

 ginia, and the mountain belt between the Blue Ridge and the Iron, Great 

 Smoky, and Frog Mountains of Xorth Carolina and Geargia, and the 

 extension of the belt into Alabama. This is the strongest known anticline 

 of the Appalachians and is compounded of numerous lesser and local 

 anticlines ; it is perhaps no greater, but it is better known than the belt 

 of uplift which lies still farther southeast. Equally continuous anticlines 

 parallel the Blue Ridge-Green Mountain fold on the northwest, but they 

 are lower and less intense and on them younger rocks form the surface. 



The major synclines are also parallel to the anticlines but display 

 much less variety. They are slightly deeper with reference to sealevel 

 and contain younger rocks at the southeast, but they held a similar posi- 

 tion at the beginning of the Appalachian revolution. The major syncline 

 in the closely folded belt is much sharper but nearly as deep as the prin- 

 cipal synclines farther northwest. The principal syncline southeast of 

 the Blue Ridge anticline is sharply folded but is also very deep. In New 

 England and Canada it carries Devonian beds down to sealevel and is 

 deeper than any other syncline lying northwest of it. The syncline in 

 Virginia, the Carolinas, and Georgia which has a similar position 

 carries the Cambrian sediment to and below sealevel. 



A series of cross-sections, therefore, brings out two facts, that the 

 S3^nclines differ only moderately in depth, while the anticlines are enor- 

 mously different. From this it is necessary to conclude that the anticline 

 is the constructively important feature of Appalachian structure, and 

 that the syncline is merely the part left behind with the least change. 

 This is precisely what should be expected from a theoretical consideration 

 of a mass relieving itself from strain. The motion which affords the 

 relief must necessarily be in the direction of least resistance, which is 

 mainly upward. The same conclusion must be drawn from all experi- 

 ments in the folding of artificial beds or in the laboratory deformation 

 of actual rocks. In all of them, where there is any shortening, the relief 

 is given by upward or outward motion; in other words, anticlines are 

 always raised but synclines are not deepened. 



When this conclusion is applied to the case of geosynclines it is seen 



