104 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



sylvania to northeastern Tennessee, over 500 miles long. Rack of the 

 Pulaski thrust front are several fensters, as illustrated in sections 12 and 

 13 of Fig. 8.17. See also Fig. 8.16. Sections 14, 16, and 17 of Figs. 8.18 and 

 8.19 also illustrate the thrusts of the central and eastern parts of the belt. 



The nature of the strong folds is illustrated in sections 15, 16, 18-21, 

 and 24 of Figs. 8.17 to 8.20. Most of the folds are asymmetrical and 

 steepest on the northwest flank. According to the orthodox view, this 

 marks active pressure from the southeast, as do almost all the thrusts. 



SILURIAN 



The folds of this region are some of the best known in North American 

 geology, and some are markedly long and regular. See the Tectonic Map 

 of the United States. Keith ( 1923 ) points out that the troughs of the folds 

 extend downward to almost a common level, whereas the anticlines 

 extend upward to variable elevations. Some of the anticlines are over- 

 turned and have broken into thrust faults. Most of the more eastern 

 thrust sheets have extensively flat or folded lower surfaces. 



The faults die out in southern Pennsylvania, and from there northwest- 



6ASS ISLAND 



L. KEVSER 7 



10 N T A R I 

 I 

 CAPE HORD OWEN SOUND 



NEW YORK 



! 

 GUELF HAMILTON LAKE ERIE 



CINCIN N ATI A N 



PENNSYLVANIA 

 I 

 ALLEGHENY FRONT 



HARRISBURG 





30 MILES . 



Fig. 8.7. Late Ordovician and Silurian stratigraphy of Pennsylvania, western New York, and western 

 Ontario. After Kay, 1942. 



