(FORMATIONS AND PHYSICAL HISTORY 373 



t appears at the surface in relatively wide belts or areas (Fig. 332), 

 while in the eastern mountains, where it is thick, it appears at the 

 surface in a succession of narrow and parallel belts (p. 372). The 

 outcrops are largely adjacent to older rock. 



Close of the Ordovician Period 



The close of the period was marked by geographic changes of 

 more importance than those at its beginning. The greatest change 

 was the withdrawal of the epicontinental waters from a large part 

 of North America, converting extensive stretches of shallow-sea 

 bottom into land. The cause of this change may have been the 

 sinking of the ocean bottoms and the drawing off of the epiconti- 

 nental waters. The altitude of the new land must have been slight 

 or its exposure brief, for it suffered little erosion before much of it 

 was again submerged and covered by sediments of later age. It is 

 indeed the widespread absence of the lower part of the Silurian 

 system, apparent or real (p. 388), rather than a pronounced strati- 

 graphic unconformity between it and the Ordovician, which indi- 

 cates the extensive emergence of land in the interior at the close of 

 the Ordovician period. , 



Folding movements were limited. The most considerable were 

 in the Taconic Mountains, where both the Cambrian and Ordovi T 

 cian systems were thick. The date of the folding is known, because 

 Silurian formations overlie the Upper Ordovician unconformably 

 in this region. It is not to be inferred that all the mountain-making 

 movements which have affected western New England occurred 

 at this time. There had been folding earlier, in pre-Cambrian 

 times, and there were movements later as will be noted. The 

 principal deformation of the strata in the Appalachians and in 

 Arkansas came much later. 



Between folding and the more gentle movements already noted 

 there are all gradations. The "Cincinnati arch" is an example. 

 This arch is a very low anticline with a general north-south course, 

 extending through Cincinnati. The beginning of this arch may 

 have been as early as mid-Ordovician. Another similar arch ! may 

 have come into existence at about the same time in Arkansas and 

 Oklahoma. 



The crustal movements referred to above have been mentioned 



i 



: 



I 1 Branncr, Am. Jour. Sci., Vol. IV, 1897, p. 357. This very suggestive article 

 lias l>i a rings on many questions besides the Ouachita Uplift. 



