THE ORDOVICIAN PERIOD 515 



been greatly changed, the changes in the rock have been greater. 1 

 Thus in the Taconic Mountains (southeastern New York and south- 

 western New England), the limestone is mainly in the condition of 

 marble, the sandstone and quartzite have been largely changed to 

 quartz schist, and the shales to slate and schist. Metamorphic 

 rocks of Ordovician age are known also in some parts of the Pied- 

 mont plateau. 



Thickness. The rocks of all systems vary greatly in thickness, 

 and the Ordovician system is no exception. In the Appalachian 

 Mountains it is to be measured by thousands of feet, while in the 

 interior it is to be measured by hundreds instead. In Wisconsin 

 and Iowa, where sedimentation seems to have been interrupted 

 but little from the beginning of the period to its end, the aggregate 

 thickness is rarely 700 feet. 



Width and position of outcrops. In the interior, where the 

 system is relatively thin, it sometimes appears at the surface in 

 relatively wide belts or areas (Fig. 382) , while in the eastern moun- 

 tains, where it is thick, it appears at the surface in a succession of 

 narrow and parallel belts (p. 488). 



Close of the Ordovician Period 



The close of the period was marked by geographic changes of 

 more importance than those at its beginning. The greatest 

 change was the withdrawal of the epicontinental waters from a 

 large part of North America, converting extensive stretches of 

 shallow-sea bottom into land. The cause of this change may have 

 been the sinking of the ocean bottoms and the drawing off of the 

 epicontinental waters. The altitude of this new land must have 

 been slight or its exposure brief, for it suffered little erosion before 

 much of it was again submerged and covered by sediments of later 

 age. It is indeed the wide-spread absence of the lower part of the 

 Silurian system (p. 536), rather than a pronounced stratigraphic 

 unconformity between it and the Ordovician, which indicates the 



1 See, for example, the New York City, Holyoke (Mass.-Conn.), and 

 Hawley (Mass.) folios, U. S. Geol. Surv. Compare with folios of (1) the Appa- 

 lachian Mountains, (2) the interior, and (3) the western part of the United 

 States. 



