324 A. KEITH OUTLINES OF APPALACHIAN STRUCTURE 



section is taken as a whole. Xot much is known of the northern exten- 

 sion of the geosyncline. As far as higher beds are concerned, it appears 

 to pass through the heavy Devonian sediments of the Catskills and along 

 the Ordovician of the Hudson Valley. This syncliiiorium is not as deep 

 as that which lies east of the Green Mountains, however, which contains 

 Silurian and Devonian strata as well as Ordovician and Cambrian. In 

 Tennessee the thick Paleozoic sections of the east side of the Appalachian 

 Valley include a full section as high as the ]\Iississippian. The Penn- 

 sylvanian 1)eds only come to the west side of the valley, so that the posi- 

 tion of the Paleozoic geosyncline is probably in the closely folded valley. 

 In Alabama the thickest Mississipj^ian and Pennsylvanian sections are 

 ■extended well toward the east side of the close folds of the valley. Mid- 

 way of the system the thick sections of Pennsylvania and Virginia lie in 

 the middle of the closely folded belt. Thus, to sum up, in the system 

 as a whole the Paleozoic geosynclines and the closely folded belt of strata 

 correspond closely, although in the central Appalachians the geosyncline 

 appears to lie farther west than it does either at the north or south. 



"When the geosynclines are separated according to geologic systems, 

 important relations appear which the generalized Paleozoic section does 

 not brino- out. These mav best be described in terms of the maior Blue 

 Eidge-Green Mountain anticlinorium. The Algonkian geosyncline lies 

 almost ^vholly to the east of that uplift, on which are exposed older and 

 deeper rocks. In northern Vermont and Canada the geosyncline also 

 extended somewhat to the west of the Green Mountain axis. The Lower 

 Cambrian geosj^ncline had the same general position as that of Algonkian 

 time, but its western margin lay farther west, from 20 to 80 miles beyond 

 the Blue Ridge arcli. In Upper Cambrian time this plan was reversed, 

 and the geosyncline extended far to the west of the folded zone, its east- 

 ern l^order being near the Blue Eidge arch. This general arrangement 

 held for the later Paleozoic sediments, with the eastern border of the 

 seas onlv occasionallv extendino- east of the Blue Ridoe line. After the 

 Paleozoic, deposition began again east of the Blue Eidge and has there 

 remained. 



It is of dominant interest in these relations that the heavy deposits of 

 the Algonkian and Paleozoic geosynclines overlap each other and form 

 a maximum total section where now are situated the Blue Ridge arch and 

 the Appalachian Valley. Thus, in general, the highest piling up on folds 

 and faults coincides with the thickest masses of strata. This fact, to- 

 gether with the double reversal in position of land and sea, is of crucial 

 importance to the theory of isostasy. 



