

FORMATIONS AND PHYSICAL HISTORY 



\ \ 



: 



al>-c-nce is in harmony with the quiet which characterized the 

 period. 



General Conditions and Relations of the Ordovician System 



Position of beds. As originally deposited, the Ordivician beds 

 probably dipped away from the lands of the period. Over great 

 areas in the interior, this original and simple plan of stratigraphy 

 has been but little modified (Fig. 329). In other regions, deforma- 

 tion of the strata has completely changed their original positions. 

 Thus in the Appalachian Mountains (Fig. 330) and in some parts of 

 Arkansas (Fig. 331), Oklahoma, x ^ 

 and various mountains of the \ \ \ 

 west, the strata are folded arid 

 some places faulted. 

 Metamorphism. The sediments 

 ave undergone more or less al- 

 teration since their deposition. In 

 me places the changes have 

 en slight, and in others great, 

 he larger part of the Ordovician 

 sands have been changed to sand- 

 tone, the larger part of the muds 

 shale, and most of the lime- 

 stone is still essentially non-meta- 

 morphic. But where dynamic 



tion has been great, and where the original position of the strata 

 as been changed greatly, the changes in the rock have been 

 greater. 1 Thus in the Taconic Mountains (southeastern New York 

 and southwestern New England), the limestone has been changed 

 to marble, the sandstone and quartzite to quartz schist, and the 

 shale to slate and schist. 



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

 nd the Ordovician system is no exception. In the Appalachian 

 Mountains it is thousands of feet thick, while in the interior it is 

 only hundreds. In Wisconsin and Iowa, the aggregate thickness 

 is rarely more than 800 or 900 feet. 



Outcrops. In the interior, where the system is relatively thin, 



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

 (Mass.) folios, U. S. Geol. Surv. Compare with folios of the Appalachian Moun- 

 tains, the interior, and the western part of the United States. 



: 



IIH 



r 



ha 



Fig. 331. Section showing the 

 position and relations of the Ordovi- 

 cian beds in the mountains of Arkan- 

 sas. Length of section, about \y$ 

 miles. (Penrose, Ark. Geol. Surv.) 



