CHAPTER XVI 



THE ORDOVICIAN (LOWER SILURIAN) PERIOD * 

 FORMATIONS AND PHYSICAL HISTORY 



The general conformity 2 between the Cambrian and Ordovician 

 systems shows that no great change took place in the relations of 

 land and water in North America at the close of the Cambrian period. 

 At the opening of the Ordovician, therefore, an epicontinental sea 

 stood over much of the continent. 



Sedimentation During the Ordovician Period 



While the principles of sedimentation during this period were 

 the same as during the Cambrian, the conditions, so far as our 

 continent is concerned, were somewhat different, chiefly because 

 the smaller areas of land yielded less sediment. During much of I 

 the period the deposition of land-derived detritus was confined tol 

 littoral tracts. Since the land areas were of various sizes, of various 

 sorts of rock, and presumably of various heights, conditions existed 

 for the deposition of all sorts of clastic sediments about their borders, 

 and for their deposition at very different rates. Sedimentation was 

 doubtless more rapid near the larger and higher lands than about 

 the smaller and lower ones, and more rapid on that side of any land 

 towards which the larger part of its drainage flowed. Where clastic 

 sediments failed, the shells and other secretions of marine animals 

 and plants were accumulating, making limestone. 



The known formations of the Ordovician period are in keeping 

 with these general principles. Adjacent to the broad, shallow 



1 Recently it has been proposed to recognize a system of rocks, the Ozarkian, 

 between the Cambrian and the Ordovician, the Ozarkian would include the lower 

 part of the Ordovician (Beekmantown formation and its equivalents), and the 

 upper formations of certain regions commonly referred to the Cambrian. Ulrich, 

 Hull. (k-ol. Soc. Am., Vol. XXII. 



2 There are local unconformities between these systems, as in some parts of 

 New York, and the evidence is increasing that they are more wide-spread than 

 formerly was supposed.- 



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