68 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



strikingly in contrast with the hills than in districts farther north, yet tlie 

 regular crest lines of the dividing* ridges are a rather striking feature even 

 there. The plain extends westward across the Cinciiniati arch into Ken- 

 tucky, Ohio, and Indiana, to the Knobstone formation of southern Indiana 

 and northern Kentucky. It covers northern Kentucky, northwestern Ohio, 

 and the southern part of Michigan, and thence stretches westward far 

 beyond the Mississippi River, constituting the great interior plain of the 

 United States. 



In both the plain and the hilly country there are variations in topog- 

 raphy which need to be considered in some detail. The hard and resistant 

 rocks of the plain country stand in relief as escarpments or, in some cases, 

 as low dividing ridges, while those of the hilly country remain as elevated 

 divides between drainage lines or occasionally as island-like outliers. The 

 glacial deposits have greatly concealed the topography of the rock forma- 

 tions and disturbed the old systems of drainage. In the plain portion of 

 the region the old valleys have been so greatly filled that they can be traced 

 only in limited districts. In the hilly portion the drift seldom fills up the 

 lowlands to the height of the uplands or ridges, yet it obscures the old 

 drainage lines to a great degree. 



The present discussion is natiu'ally devoted chiefly to the eastern part 

 of the region, where the old topography is least concealed. Inasmuch as 

 the topographic features depend largely upon the rock formations, they are 

 to some extent discussed in belts that follow the outcrops of the formations. 

 The plain south of Lake Ontario in western New York is first discussed, 

 and then the region lying southward and westward, comprising the later 

 formations, is described. 



LOW PLAIN SOUTH OF LAKE ONTARIO. 



The low country south of Lake Ontario in western New York is 

 separable into three distinct plains, with two escarpments above the level 

 of the lake shore. These features are in part represented on PI. III. The 

 first or lowest plain extends from the lake southward to the Niagara 

 escarpment, a distance of 6 to 12 miles; the middle plain extends from the 

 Niagara to the Corniferous escarpment, a distance of 10 to 15 miles; while 

 the highest plain extends from the Corniferous escarpment southward to 

 the base of the hill)^ country, with a width averaging less than 10 miles. 

 These plains are underlain by shale formations, except for a short distance 



