184 CEMENT MATERIALS AND INDUSTRY. [bull. 243. 
width comes into the county from the north, and, extending southward along the 
western slope of Shriver Ridge, passes through the western part of Cumberland and 
across the Potomac into West Virginia. The Potomac in its very perceptible cast- 
ward bend nearly 3 miles above Cumberland, and again in the more prominent 
eastward bend about 6 miles above Cumberland, 'has carved out two small portions 
of this belt from the West Virginia area. These patches are mostly concealed, but 
their contact with the Salina is fairly well shown. Northward the Helderberg- 
Salina contact is largely concealed, but the limestone quarries which oc* ur in the 
lower part of the Helderberg along the western base of Shriver Ridge afford a con- 
venient means of judging the approximate western outcrop of the Helderberg. 
Shriver Ridge marks the eastern limit, as the contract lies on its western slope a 
short distance below the top. 
West of Wills Mountain there is a band of Helderberg corresponding in position 
to the eastern belt, but by reason of the perpendicular attitude of the strata this belt 
is considerably narrower than the one on the eastern side. Following closely the 
general direction of Wills Mountain, it crosses the Potomac River at Potomac station. 
Along the belt north of the National Road the Helderberg-Salina contact is usually 
not well shown, but the Helderberg-Oriskany contact is prominent, the latter being; 
represented by the steep ridges to the north and south of Corriganville. South of 
the National Road neither contact is well shown, although slight topographic features 
usually indicate their positions with reasonable accuracy. 
Another Helderberg area of considerable extent is exposed south of Rawlings. 
This forms the body of the steep isolated ridge known as Fort Hill, which extends 
southward along the Potomac for a distance of about 4 miles. 
In addition to the above-mentioned areas, two very slight exposures may be seed 
along the West Virginia Central Railroad, on the north and south sides of Monster 
Rock, near Keyser, W. Va. They are of little importance, except in so far as theyv 
are of value in helping to work out the structure in that part of the county. 
In addition to the areas above described as occurring in the westeri 
and central parts of Allegany County, four narrow belts of the Helder- - 
berg limestone outcrop in the western portion of Washington County, 
their location being shown on the map (PI. XV, p. 314). The beM 
exposures of these limestones in the county, so far as location is con- 
cerned, are those near Hancock, on Potomac River. 
DESCRIPTION AND STRATIGRAPHIC POSITION. 
The stratigraphy of the Lewistown limestone is thus described by 
Mr. O'Harra in the report previously cited: 
Lithologically, the Helderberg is preeminently a limestone formation. Argillace- • 
ous materials occur as impurities in some of the beds, but these are not important, 
and sandstones are almost wholly lacking. Thin bands of chert, which are white or | 
yellowish-white in color, occur sparingly throughout the upper part of the formation. 
Most of the limestone in the upper part is heavily bedded, and much of it is highly 
fossiliferous. The lower part of the Helderberg is a dark-blue thin-bedded limestone, 
which in breaking gives a decided ring. This corresponds to the Tentaculite lime- 
stone of New York, which in Maryland is over 400 feet thick. In the field the con- 
tact between the Salina and the Tentaculite limestone is very marked because of the 
different weathering qualities of the two rocks. The Salina rock weathers into soil 
very completely, while the Tentaculite limestone leaves innumerable small, thin, 
dark-blue slabs upon the surface. 
The thickness of the formation is nearly 800 feet. The two partial sections given 
