Franklin. - Area, 760 square miles; population in 1880,
49,855. This important triangular area with its base line upon the
State of Maryland covers an enlargement of the Great (Cumberland)
valley between the South mountains (Blue Ridge) on the east, and the
North (Blue) mountain on the west. Shirley’s and Furnace branches
of Meade’s run, a branch of Connedogwinet Creek, separate it from
Cumberland County; but almost the entire rainfall of the county drains
southward by the Conococheague creek into the Potomac. The western
part of the county includes Path valley lying behind the North Mountain,
and also a cove in the south-west corner of Montgomery Township debouching
into Maryland. The main limestone belt of No. II, at the western
foot of the South mountains, is 6 ½ miles wide at Shippensburg, 8
½ at Chambersburg, 13 at Greencastle, and has a line of important
brown hematite deposits along the foot of the mountains: the furnace
banks on the Cumberland line, the Pond bank group opposite Chambersburg,
the Mt. Alto banks a little further south, and others. Smaller isolated
deposits of ore have been found scattered over the surface of the
belt. The great Slate belt of III, lying on the limestone,
is 6 ½ miles wide at Shippensburg, 5 at Chambersburg, and 3 ½ at the
Maryland line. Back of the great Slate belt, No. II comes up again
in a Second limestone belt 3 miles wide at Maryland
line, running north 28 miles and tapering to a point before reaching
Roxbury P.O.; but the sharp anticlinal fold which brings up the limestone
runs on in the slate and makes the great hook in the North mountain
in Cumberland county. Back of this is a Second slate belt trough
one or two miles wide, in which lies Casey’s mountain (IV) at the
Maryland line, and the long isolated mountain ridge of Parnell’s knob
opposite Chambersburg. Back of this lies the anticlinal Third limestone
belt, of Mercersburg, 1 mile wide and 11 miles long, pointed at
each end. Back of this runs the synclinal Third slate belt, holding
at its south end Two-top mountain (IV) and at its north end Jordan
mountain (IV), which is a long loop of the North mountain, inclosing
a long narrow elevated trough of red shale (V) drained northward by
the main head stream of Connedogwinet creek. Back of this slate belt
is the anticlinal Fourth limestone belt, 1 ½ miles wide and
11 miles long from its north point at Loudon, to its south point where
it passes under the slate in the "Corner," a little cove between Two-top
and Cove mountain. In Path valley a Sixth limestone belt, 1
½ miles wide and 11 miles long, is faulted along its western edge
at the foot of Tuscarora Mountain (IV); and here are the extensive
Richmond brown hematite ore banks. The northern head of Path valley
is closed by slate coves and sandstone spurs on the Perry county line.
Cove gap, west of Mercersburg, leads into a red shale valley of V,
which widens southward as the Cove, and contains an elliptical outcrop
of the Lower Helderberg limestone (VI), the Oriskany sandstone (VII),
and a small trough of Marcellus and Hamilton rocks (VIII), the only
place in the county where the Devonian rocks are preserved. The fossil
iron ore bed in V has been opened here. The parallel anticlinal and
synclinal belts of limestone and slate in Franklin county are of the
highest importance in proving the identity of formations II and III
with the Lower (Cambro-) Silurian series in New York State and elsewhere,
and with the limestone and slate belts of the Juniata river country.
Trenton fossils abound along the west edge of the main limestone belt
at Chambersberg and elsewhere; but the limestone formation (II), which
is nearly 7000 feet thick in Blair county, seem to be only 3000 feet
thick in Franklin county; and the Slate formation (III), which is
6000 feet thick in Lehigh and Northampton counties, may not be more
than 1500 or 2000 feet thick in Franklin county. Little lumps of a
coal-like substance occur in the slate along Cove mountain, and have some connection with the multitudes of graptolites
which then inhabited the sea. A great fault seems to run along the
foot of the South mountain; for the limestone strata commonly dips
east against and as if under the mountain rocks. Although basins and
rolls traverse the limestone belt in front of the mountain, and prongs
of slate project diagonally into the limestone belts, showing troughs,
while prongs of limestone penetrate the slate belts, showing arches,
the whole floor of the valley is closely crimpled into innumerable
folds, many of which must be overturned and compressed, as shown by
the general prevalence of east dips. The South mountain mass is divided
in two by a curious traverse fault of great size, which runs east
and west along the line of the Chambersburg and Gettysburg turnpike;
the whole country north of the fault, both mountain and valley, being
shifted about three miles westward. North of the fault the Franklin
county mountain mass consists of several thousand feet of sandstones
and slates, dipping 30°
and 40° east,
which may be called Potsdam (I) or Cambrian. South of the fault are
red schists and massive and conglomeratic sandstones, folded, and
at Mt. Alto standing nearly vertical. In the south-east corner of
the county appear what may be Huronian strata. (See map in Atlas D5.)
From: A geological hand atlas of the sixty-seven
counties of Pennsylvania :embodying the results of the field work
of the survey, from 1874 to 1884. By J. P. Lesley. (Report of
progress (Geological Survey of Pennsylvania), v. X ) Harrisburg, PA
: Board of commissioners for the second geological survey, 1885.
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