38 OIL AND GAS; OHIO, WEST VIRGINIA, PENNSYLVANIA. 
is exposed, by far the larger part of the surface being occupied by 
the Monongahela, which, however, toward the south passes below 
water level and is succeeded by the Washington formation. 
KEY HORIZONS AND INTERVALS. 
Within the formations exposed are a number of easily identifi¬ 
able beds that make excellent guides by which to follow the geology. 
Of these the Pittsburg coal is by far the most prominent. • Its base 
has therefore been selected as the key horizon upon which to show the 
surface structure. This coal shows in outcrop in all of the northern 
half of the quadrangle, except a small area in the northwest corner. 
Throughout most of this section the outcrop appears at enough 
places to furnish sufficient data for complete structural work. South 
of the Pittsburg, Cincinnati, Chicago and St. Louis Railway the dip 
of the formations carries the coal under cover a short distance from 
the railroad. It appears again on Chartiers Run and Westland Run 
in the southeast corner and on Cross Creek in the southwest corner. 
Below the Pittsburg coal the principal marking stratum is the 
Ames limestone, which occurs near the middle of the Conemaugh 
formation. This limestone outcrops in the valley of Raccoon Creek 
from the north edge of the quadrangle to the mouth of Brush Run, 
also in Harmon Creek and Aunt Clara Fork of Kings Creek, but 
was not noted on Kings Creek. The conditions are not favorable 
for measuring the interval between the Ames limestone and the 
Pittsburg coal within the Burgettstown quadrangle. This distance 
as determined in the Steubenville quadrangle to the west varies from 
219 to 224 feet, and in the Beaver quadrangle to the north it is 
about 230 feet. These intervals were accepted along the borders of 
the quadrangles. 
The Monongahela formation has a number of members that may 
be used as key rocks. The Sewickley and Uniontown coals at the 
bottom and top of the Benwood limestone are available for this 
purpose. They were little used, however, owing to'the fact that 
both of these coal beds are small and their outcrops inconspicuous. 
Although small, the coals are usually present and they were identi¬ 
fied in a number of places. The Sewickley coal is about 102 feet 
above the base of the Pittsburg, and the Uniontown coal is from 
206 to 222 feet above the same horizon. 
The two beds in the lower section of the Benwood limestone called 
the Dinsmore and Bulger limestones, which are described in the 
chapter on stratigraphy in Part II of this paper (pp. 69-70), are 
the best marking strata in the middle of the Monongahela forma¬ 
tion. these two beds have been employed more" extensively in 
