THE ORDOVICIAN PERIOD 511 



is perhaps equally worthy of note that in the latter part of the 

 period, mud (now shale) was deposited over an almost equally 

 extensive area. This may mean either that the lands were so 

 elevated "as to allow the streams to carry more sediment to the sea, 

 or that conditions favored the transportation of mud farther from 

 shore than formerly, or both. Associated with the Upper Ordo- 



Fig. 377. Trenton Falls, Trenton, N. Y. The locality whence the Trenton 

 formation derived its name. (Darton, U. S. Geol. Surv.) 



vician shales, there are considerable bodies of limestone in some 

 places, and of sandstone in others. All the Ordovician formations 

 of the interior and the east bear within themselves evidence of 

 shallow water origin. 



Western sections. In the Great Plains, the Ordovician system 

 appears at the surface but rarely, though it probably underlies the 

 younger formations. West of the Great Plains, the system is 

 present generally, and the sections are somewhat simpler than in 

 the interior or the east. As farther east, limestone is a conspicuous 

 part of the system here. 



