3 68 ORDOVICIAN PERIOD 



arm of the ocean which covered the larger part of the Mississippi 

 basin (Fig. 310) there appear to have been no sources of abundant 

 sediments during most of the period. Along the western base of 

 Appalachia, clastic materials were being deposited. Alternating 

 beds of coarse and fine sediment indicate either (i) that the adjoin- 

 ing land was higher at some times than at others, or (2) that the 

 climatic conditions or (3) the vegetal covering changed, or (4) that 

 waves and currents varied in their effectiveness. 



Conditions for the formation of limestone prevailed widely in 

 the epicontinental sea. Plants and animals secreting calcium car- 

 bonate may have been no more abundant far from land than near 

 it, but away from shore their shells, etc., were more abundant 

 relative to the sediments derived from the land. 



The development of the Ordovician system meant the destruc- 

 tion of an equivalent body of older rock. The material which 

 entered into the new system came from all preceding formations 

 so situated as to be exposed to erosion. Even the limestones of the 

 system had their ultimate source in older formations, for the mineral 

 matter extracted from the sea to make the shells had been dissolved 

 from older formations during their decay, and brought to the sea in 

 solution, largely by the same streams which carried the clastic sedi- 

 ments. 



Sections of the Ordovician. The Ordovician system of North 

 America was first studied carefully in New York, and the section of 

 that State is, in some measure, the standard to which others are 

 referred. In New York the system is divided as follows: 



( Richmond beds l (in Ohio and Indiana) 

 Upper Ordovician \ T . , , 

 . . ,. N < Lorraine beds 

 (or Lincmnatian) / T ,,. , , 

 V Utica shales 



C Trenton limestone 



Ordovician 



Middle Ordovician 



, . , i Black River limestone 

 (or Mohawkian) / T . .. 



v. Lowville limestone 



\JJi"< '%JVfef '- s . - r 



Lower Ordovician \ Chazy limestone 



(or Canadian) ( Beekmantown limestone (Calciferous) 



The classification of New York is not applicable in detail in other 

 parts of the continent. In Wisconsin, Iowa, and Minnesota, for 

 example, the formations commonly recognized, numbered in the 



1 Question has been raised as to the propriety of including the Richmond beds 

 in the Ordovician. Hartnagle, N. Y. State Mus. Bull. 107, 1907. In Illinois, 

 beds of Richmond age are unconformable on the older Ordovician. Weller, Jour, 

 of GeoL, Vol. XV, p. 519; and Savage, Am. Jour. Geol., Vol. 125, p. 431, 1908. 



