CHAPTER XVII 

 THE ORDOVICIAN (LOWER SILURIAN) PERIOD 



FORMATIONS AND PHYSICAL HISTORY 



The general conformity l between the Cambrian and Ordovician 

 systems shows that no considerable physical 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, 

 as at the close of the Cambrian, an epicontinental sea stood over 

 much of the continent. 



Sedimentation During the Ordovician Period 

 The conditions of sedimentation during the Ordovician period 

 were somewhat different from those of the Cambrian. All the 

 common processes of weathering were operative on such lands as 

 still existed, wasting the rocks and preparing sediment for removal 

 to the sea; but the small area of land within North America yielded 

 but little sediment, and during much of the period the deposition 

 of land-derived sediment was confined to littoral tracts. Farther 

 from land the shells, skeletons, and other secretions of marine 

 animals and plants were accumulating, making limestone. Since 

 the land areas of the period were of various sizes, of various sorts 

 of rock, and presumably of various heights, it is probable that con- 

 ditions 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 

 land masses than about the smaller and lower ones, and more 

 rapid on that side of any land towards which the larger part of its 

 drainage was directed. 



The sedimentary formations of the Ordovician period are in 



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

 of New York, and they may be more wide-spread than has been supposed. 



507 



