422 



HISTORICAL GEOLOGY 



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The classic section of the Ordovician in the United States is in 

 New York, where it was first extensively studied. 

 Pulaski stage (shale) 

 Frankfort stage (shale) 

 Utica stage (shale) 

 Ordovician I Trenton stage (limestone) 

 system ] Black River stage including Lowville limestone 

 Chazy stage (limestone) 

 Beekmantown stage (limestone) 

 Tribes Hill stage (limestone) 

 In this region limestone deposits prevailed during the Lower 

 (Canadian) and Middle (Mohawkian) Ordovician, but shales in the 

 Upper (Cincinnatian) Ordovician. 



Close of the Ordovician. — The close of the Ordo- 

 vician was marked by horizontal and vertical move- 

 J^5 ments of considerable importance in eastern North 



c -£ America (Taconic deformation) and Great Britain. 

 .2 c3 . . ... 



'•% V During the Cambrian and Ordovician in North America 



~rt ^ sediments had been accumulating in a subsiding trough 



j** «S lying between the Adirondack land mass and a land 



ja ^r mass in New England, and which stretched from the 



js ~ St. Lawrence River to the City of New York and to 



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the south (Fig. 374, p. 404). These sediments, after 

 having been accumulated to a thickness of more than 

 a mile, were subjected to great lateral pressure which 

 folded them and brought them above sea level to form 

 a mountain range of which the present Taconic Moun- 

 tains of western New England are perhaps rather 

 insignificant remnants. The folding was so intense 

 that limestones were recrystallized to marbles, of which 

 the most famous are those of Vermont and Massa- 

 chusetts; the sandstones were changed to quartzites 

 and schists, and the muds and shales to slates and 

 schists. These disturbances afFected the region east of the Taconics, 

 but were comparatively local, as is shown by the slightly folded 



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Fig. 393. — An east-west section through the Appalachian Mountains near Mercers- 

 burg, Pennsylvania, showing the relation of the Ordovician, Osr, Oc, Om, Oj, and the 



Silurian, Sc and St. (U. S. Geol. Surv.) 



