THE ORDOVICIAN PERIOD 



423 



rocks of this period in New York, New Jersey, and Canada, only 

 short distances from the scene of maximum deformation. The 

 region north of the St. Lawrence seems to have been little affected, 

 since the sedimentation continued from the Ordovician to the 

 Silurian with slight interruption, almost the entire record being 

 preserved in the strata of Anticosti Island in the Gulf of St. 

 Lawrence. The date at which the Taconic deformation occurred 

 is known, because the Silurian rocks rest upon the eroded and 

 upturned edges of the Ordovician (Fig. 394), showing that after the 

 deformation the strata were elevated and eroded for many years, and 

 were again covered by the sea and Silurian deposits laid down on 



Fig. 394. — An east-west section in eastern New York, showing the Silurian rest- 

 ing unconformably upon the upturned edges of the Ordovician. (After W. J. Miller.) 

 We have here the proof that this portion of the continent was raised above the sea 

 during the Ordovician, and that after the land was eroded it sank, and upon this old 

 land surface Silurian sediments were deposited. 



them. These disturbances, which culminated in the Taconic deforma- 

 tion and in the draining of the interior seas, were of long duration. 



Cincinnati Anticline. — The first evidence of a deformative move- 

 ment in the Middle States is found in the formation of the Cincinnati 

 and other anticlines, which appeared as low folds in the Middle Or- 

 dovician (Trenton). The Cincinnati arch, though later submerged, 

 was again elevated and greatly enlarged at the close of the period. 

 This fold extends over an oval area in Ohio, Indiana, and Kentucky, 

 with the longer axis in a north and south direction. 



The withdrawal of the epicontinental seas at this time may have 

 been due to the sinking of the ocean bottoms (oceanic segments, 

 p. 366) or to the raising of the land. 



Volcanism. — There is little evidence of igneous activity in North 

 America during the period, although in England and Wales great 

 masses of lava and volcanic ash form thick strata. Indeed the 

 Ordovician volcanism of Great Britain was one of the most extensive 

 in Europe since Pre-Cambrian times. 



Ordovician of Other Continents. — Ordovician rocks occur in Great 

 Britain, where a thickness of 24,000 feet has been measured. A 



