OUTLINE OF ROCK FORMATIONS. 63 



colored, blue or gray shales, but carries occasional thin beds of sandstone. 

 In southern Ohio, notably in Ross, Pike, and Scioto counties, the sandstone 

 beds become more prominent. The sandstone is extensively quarried at 

 Buena Vista, on the Ohio River, a few miles below Portsmouth. There are 

 also developments of sandstone in northeastern Ohio, notably in Trambull 

 County, where numerous quarries have been opened. Its outcrop usiially 

 covers a width of several miles. 



From the Ohio River northward to Chillicothe the Cuyahoga shale 

 outcrops on both sides of the Scioto River, but north of that city it lies 

 east of the river and forms the east border of the Scioto Basin. It outcrops 

 in a narrow strip south of Lake Erie from Lorain County eastward to 

 Geaug-a County, but there swing-s southward around the head of Grand 

 River Basin. 



In northwestern Pennsylvania this formation, under the name of 

 Pocono sandstone and Crawford shales, covers the uplands in a belt 10 to 

 15 miles wide in Crawford and southeastern Erie counties and extends 

 southeastward along the lowlands into Mercer and Venango counties. It 

 forms much of the surface in the northern half of Warren County and 

 extends southward along valleys and low parts of the upland beyond the 

 limits of that county. It outcrops only on a few ridges in southwestern 

 New York. 



This formation apparently constitutes the lower part of the Knobstone 

 group of Indiana, so well developed on the west border of the Cincinnati 

 arch. It there forms the slopes of the prominent escarpment west of the 

 trough occupied by the Devonian shale, but it lies mainly outside the limits 

 of the region under discussion. 



Logan conglomerate (part of Pocono sandstone of Pennsylvania). The Log"an COnfiflomerate 



is well developed from Wayne County, Ohio, southward into Kentucky, 

 but seems to be feebly developed or wanting in the vicinity of the Cuj^ahoga 

 Valley. It is characterized at two horizons by a cong'lomerate which 

 carries small pebbles. The conglomerate phase is not so conspicuous in 

 southern as in central Ohio, and in the latter district occupies only a small 

 part of the formation, there being fine-grained sandstones and even shales 

 embedded with the conglomerate. The average thickness of the Logan 

 conglomerate group is about 200 feet, but the maximum is much above the 

 averag'e, reaching probably 400 feet. The resistance of this conglomerate 



