148 HISTORICAL GEOLOGY 



Physical Histoky 



Earlier Mississippian. — This time was marked by a still 

 further expansion or transgression of the epicontinental sea which 

 had already become pretty extensive in the late Devonian. By 

 the close of the Osage epoch of the Mississippian, most of the Mis- 

 sissippi Basin and Appalachian region, as well as much of the 

 Rocky Mountain region, had become submerged, though with 

 considerable islands like the Cincinnati anticline area, Ozark 

 Mountain area, and others in the Rocky Mountain district. 

 During this time coarse, clastic sediment (Pocono sandstone) 

 accumulated along the western shore of Appalachia; red beds, with 

 interbedded salt and gypsum, deposited in Michigan in lagoons 

 bordering the Canadian land; and chiefly limestone, with some 

 clastic sediment, were laid down in the great interior sea. 



Later Mississippian. — During the St. Louis epoch the Mis- 

 sissippian sea reached its maximum extent, when even the islands 

 of the Osage epoch were submerged, and all of North America was 

 covered by the sea except the northeastern part of the continent, 

 Appalachia, and probably some lands on the Pacific Coast. Map 

 Fig. 87 shows the geography of the continent during that time. 

 A comparison with the map (Fig. 45) of the mid-Ordovician shows 

 the Ordovician sea to have been more interrupted with islands 

 from Appalachia westward across the continent than the Missis- 

 sippian (St. Louis) sea, but, because of the extensive arm of the 

 Ordovician sea over the Hudson Bay region, more of North America 

 was then submerged than during the Mississippian. 



In the east vast quantities of clastic sediments continued to 

 deposit as muds (now Mauch Chunk shales) above the Pocono 

 sands along the western shore of Appalachia. Locally conditions 

 were right for coal formation as proved by some coal beds in the 

 Mauch Chunk. The interior sea, however, had clearer waters 

 than at any time since the Onondaga epoch of the Devonian, and 

 limestone deposition greatly prevailed. This clear sea extended 

 westward even across the site of the Rocky Mountains. More 

 red beds with associated salt and gypsum continued to form in 

 southern Michigan lagoons. Also red beds and gypsum were 

 formed in Nova Scotia. 



Later in the period (Chester epoch) there was considerable 

 withdrawing and shoaling of the sea, as indicated by sandstones 



