54 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS 



formations, are not known certainly to have outcrops, and will therefore be 

 passed by without further description. 



Hudson River group. — The Hudsou River group, as described in the Indiana 

 and Ohio reports, consists of alternating beds of limestone and shale. Its 

 outcrop covers about 4,000 square miles in Ohio, 1,800 square miles in 

 southeastern Indiana, and also a large area in the adjacent part of Kentucky. 

 Its outcrops in the three States embrace the central part of the Cincinnati arch. 



The crown of the arch, capped as it is by the soft and friable rocks of 

 this group, has suffered greater reduction than the slopes. Much of it is 

 slightly lower than the portion of the slopes capped by the resistant Lock- 

 port (Niagara) limestone. 



Medina group. — Tlie Mediua group outcrops extensively along the south 

 border of Lake Ontario in western New York. It has only a limited out- 

 crop in Indiana and Ohio, on the borders of the Hudson River outcrop. 

 In Indiana and western Ohio the Medina is almost entirely a shale forma- 

 tion, but in eastern Ohio and farther east sandstones are interbedded with 

 the shale. The heaviest beds in New York are shales, but there are also 

 resistant sandstones of considerable thickness. This formation has usually 

 a reddish color, both as shale and as sandstone. The part taken by the 

 Medina sandstone in forming the waterfalls in western New York is discussed 

 farther on. 



Clinton group. — The Cliuton limestone outcrops in western New York 

 immediately south of the outcrop of the Medina. In Ohio and Indiana it 

 is restricted, like the Medina, to a narrow strip on the margin of the outcrop 

 of the Hudson River group. The most characteristic stratum is a firm . 

 limestone, but the drill has shown that in parts of Ohio it is a sandstone, 

 Avhile in eastern Ohio it becomes shaly. In New York it is a limestone 

 with ferruginous bands. In New York and to some extent in Indiana and 

 Ohio it has produced waterfalls in connection with the underlying shaly 

 beds of the Medina group. 



Niagara group. — Thc outcrop of the Niagara group in western New York 

 is a narrow strip leading eastward from the Niagara River to the vicinity of 

 Utica. It forms a conspicuous part of the Niagara Falls and the gorge 

 below the falls, as well as other falls and gorges in western New York. The 

 name Niagara was given because of the outcrops on the Niagara River. 

 The group is divided into two formations. The niain member is the Lockport 



