54 



A NATURALIST IN THE GREAT LAKES REGION 



the lower end of Flathead mound where, however, it is nearly 

 covered by the crumbling Niagara limestone above it. 



The map will show the nearest point at which the Galena- 

 Platteville is exposed. In several counties along the crest of 

 the arch the streams have cut down sufficiently to expose the 

 St. Peter's sandstone that underlies the Galena-Platteville and 

 in a very few spots to expose the deeper Lower Magnesian lime- 

 stone (Prairie du Chien). Such cutting to the St. Peter's occurs 

 in eastern Carroll County, in Ogle County south of Oregon on 

 the Rock River, and along many streams in southwestern 

 Wisconsin. The cutting runs deeper, even to the Lower Mag- 



FIG. 37. Diagrammatic section across the arch of rocks in Illinois. The 

 lower section cuts through Joliet (J), Marseilles (M), Ottawa (O), and Prince- 

 ton (P). The upper one is farther north, (i) Lower Magnesian formation; 

 (2) St. Peter's sandstone; (3) Galena-Platteville formation; (4) Richmond shale; 

 (5) Niagara limestone; (6) Devonian; (7) Pennsylvanian. 



nesian on the Illinois River in the neighborhood of La Salle 

 and Utica. Between these it is exposed along the railroad 

 tracks. It is also seen near the mouths of several creeks flow- 

 ing into the Vermilion River. 



The Illinois is here flowing through country of which the sur- 

 face rock belongs to the Pennsylvanian coal measures, so the river 

 has cut through these, through the Galena-Platteville limestone 

 and St. Peter's sandstone, to the Lower Magnesian limestone. 

 The rocks of the Pennsylvanian formation are most easily reached 

 from Chicago at Braiderwood or Coal City, on the Chicago & 

 Alton Railroad and Atchison, Topeka & Santa Fe Railway, 

 respectively, where they may be examined in the rock dumps 



