86 HISTORICAL GEOLOGY 



Late Ordovician. — Soon after Trenton time in the Wichita 

 Mountain district of Oklahoma there was a rather local crustal 

 disturbance, when the strata were upturned and granite intrusions 

 are thought to have taken place. 



Since the later Ordovician sediments are mostly clastic (shales 

 and sandstones), it is evident that an important change in the 

 physical geography conditions took place during the latter portion 

 of the period. Considerable areas of sea bottom were gradually 

 converted into dry land, and other portions were shoaled. Either 

 an uplift of the land, or a withdrawal of epicontinental sea waters 

 by sinking of the ocean bottoms (Atlantic and Pacific), or both 

 may have produced the change. The newly exposed lands, and 

 the relatively higher old lands, suffered pretty rapid erosion, and 

 clastic sediments accumulated comparatively fast. As we shall 

 presently see, considerable crustal disturbances (orogenic), ac- 

 panied by uplifts, reached their climax toward the close of the 

 period in eastern North America, and there is much evidence to 

 show that such actual uplifts began well before the close of the 

 period. 



Close of the Ordovician (Taconic Revolution). — In marked 

 contrast with the quiet close of the Cambrian, the Ordovician 

 ended with important physical or crustal disturbances and moun- 

 tain-making. Much of the interior (epicontinental) sea appears to 

 have been drained as a result of relative changes in level between 

 land and sea, gradually increasing during late Ordovician time. 

 In this interior region the land was mostly but slightly elevated 

 to remain dry only till the early part of the next period. 



We have learned that sedimentation was practically uninter- 

 rupted during the Cambrian and Ordovician periods, and that 

 some thousands of feet of strata had accumulated in the seas which 

 covered eastern New York, all of the regions of the present Berk- 

 shire Hills, Green and White Mountains, as well as southward at 

 least to Virginia and over the region occupied by the present Pied- 

 mont Plateau. Toward the close of the Ordovician period a great 

 compressive force was brought to bear in the earth's crust upon 

 the mass of sediments which reached from New England to 

 Virginia, and possibly farther southward. As a result of this com- 

 pression the strata were tilted, highly folded, and elevated far 

 above sea level into a magnificent mountain range which has 

 been called the Taconic Range, and this great physical (orogenic) 





