griswold.1 AREAL GEOLOGY AND STRATIGRAPHY. 17 
north-south ridge which divides the waters flowing directly into the 
Ohio River from those flowing west and south and finally entering the 
Ohio at Marietta. The streams have cut their channels into the easily 
eroded formations to a depth of nearly 300 feet, leaving a confused 
mass of hills and valleys, with very little or no level land. The hill 
slopes are seldom too steep for farming, and the country is in a high 
state of cultivation. The many beds of limestone disintegrate readily, 
forming arable land of excellent quality. 
Areal geology and stratigraphy. — The geologic section exposed by 
the stream erosion includes about 150 feet of the Upper Productive 
Coal Measures (Monongahela) and 300 to 400 feet of the Lower Bar- 
ren Measures (Conemaugh). The Pittsburg coal (No. 8), the bottom 
of which is the dividing line between these two formations, outcrops 
near the bottom of the deepest valleys in the southeast corner of the 
quadrangle. Here is obtained a section above the Pittsburg coal 
nearly 300 feet in thickness. Toward the northwest the Pittsburg 
coal rises until, at a point a few miles south of the northern border of 
the quadrangle, it is found only upon the summits of the highest 
ridges. Here is obtained a section below the Pittsburg bed fully 300 
feet in thickness. In this vertical distance of 600 or TOO feet are found 
a number of strata that can be easily recognized whenever seen in 
outcrop, and may be used as guide horizons in determining the geo- 
logic structure. 
The upper surface of the workable coal in the Pittsburg bed is taken 
as the datum plane from which the vertical distance, above or below, 
is calculated to other horizons, in all cases the top of the stratum men- 
tioned being taken. 
The Pittsburg bed has a thickness of about 4i feet of workable coal, 
and above this is a thin bed of fire clay, followed by 1£ feet of impure 
coal, or coal blossom. 
In ascending order from the Pittsburg bed the following easily rec- 
ognizable strata are found: (1) a hard, dark-blue limestone, lying but 
a few feet above the Pittsburg coal, and called 4i Upper 8 limestone; " 
(2) a thin bed of coal, rarely over 6 inches, occurring in the shale and 
known as the " Pittsburg rider vein;" (3) a coal bed with a thickness 
of from 8 to 1(> inches, known as " Meigs Creek coal;" (4) an 8-inch 
bed of cream-white limestone, intersected by thin seams of crystallized 
calcite, here referred to as " Meigs Creek white limestone." 
Below the Pittsburg coal are found (1) a bed of limestone very simi- 
lar to that above, here called "Lower Pittsburg lime;" (2) a bed of 
coal less than 1 foot in thickness, and called " No. 7 b;" (3) the Ames 
limestone, from 2 to 4 feet in thickness, of a greenish color, and car- 
rying a great number of crinoidal stems; (4) the Ames coal, often 
found directly beneath the Ames limestone; (5) a bed of coal from 1 
to 3 feet in thickness, called " No. 7 a." 
Bull. 198—02 2 
