Cumberland. Area, 550 square miles; population in 1880, 45,977. The
Medina sandstone (IV) crest of the Blue or North Mountain carries the
northern county line from the Susquehanna River westward, 40 miles (in
a straight line) to the Franklin county corner. Two pot-hooks in the
course of the line are caused by two of the Perry county anticlinals
passing through the mountain and running on south-westward, past Bloserville,
through the slate country north of Newville. The stratification in the
mountain is everywhere very steep, (northward,) becomes vertical going
east, and is finally overturned as much as 20° beyond the vertical,
so as to dip 70° southward, in the gap of the Susquehanna River 4 miles
above Harrisburg. The Cumberland Valley is drained lengthwise by the
Connedogwinet Creek, the many horseshoe bends of which are most remarkable,
and can only be explained by the fact that the channel of the creek
is confined to the Slate belt (III), and that the southern points of
the bends touch or just enter the northern margin of the limestone belt
(II), while both the slates and the limestones are closely crumpled,
and squeezed over northward, so that nearly every exposure in both belts
shows a south dip, although the limestone formation creeps down
northward underneath the slate formation, and the slate formation gets
down beneath the Blue Mountain. The bends in the creek show a number
of close folds in the lower division of the Slate formation; a part
of it, which is very calcareous, forming a transition series of lime
slates in the place occupied by the Utica formation of New York.
Along the northern margin of the limestone belt the strata belong to
the Trenton formation, and are in some places very fossiliferous. Synclinal
prongs of slate project into the limestone at Kington, Plainfield, and
Newville, which aid in proving that the slate formation overlies the
limestone; and additional evidence is offered by an area of slate surrounded
by limestone stretching from Sheppardstown to the river two miles below
Harrisburg. Yellow Breeches Creek heads at the Big Pond on the Southampton
Township line, and flows eastward 33 miles (in an air-line) to the Susquehanna
2 miles below Harrisburg. As the Connedogwinet flows along the bottom
rocks of II, so the Yellow breeches flows along the bottom rocks of
II, at the foot of the South Mountains; and while it receives the Mountain
Creek and a score of smaller streams descending from the highland, it
is also fed by underground channels with the rainfall of the valley,
which issues to the surface in the Boiling Spring, Bigpond Spring, &c.,
powerful enough to turn mill-wheels. The whole limestone belt is honey-combed
with concealed caverns, in some of which brown hematite iron ore deposits
are no doubt now being formed, under the same conditions which governed
those deposits of the ore at the present surface which are now so extensively
mined east and south of Mt. Holly Springs, in front of Boiling Springs,
and along the foot of the mountain further west towards Franklin County.
Little red spots upon the map indicate 47 places north of the Yellow
Breeches Creek, and 35 places south of it (outside the mountain) where
more or less ore has been mined, all of which represent ancient sink
holes and caverns beneath a previous higher valley surface than the
present one. The South mountain area in Cumberland County, nowhere exceeding
2000` A.T., consists of quartzite, sandstone, and hard shale and slate
strata of doubtful age, which can be called Potsdam or Cambrian, * and
exhibits no useful minerals, although small pieces of red hematite iron
ore have been found lying on the surface, which indicate small beds,
or, perhaps, veins, which might here or there repay the expense of research.
Traces of a trap dyke have been detected crossing the mountain between
York County and Boiling Springs; but from the springs northward a well-defined
and nearly straight dyke runs along the two township lines between S.
Middleton and Monroe, and between Middlesex and Silver Spring, 11 miles
to the Blue Mountain, which it cuts through and passes across Perry
County. The ridge which it makes across the valley, about 50 feet high
and mostly left in the woods, can be seen for a great distance by travelers
on the railroad or turnpike. A small area of Mesozoic rocks is inclosed
by the bends of the creek at Lisburn, in the south-east corner of the
county. The limestone quarries opposite Harrisburg are very extensive
and expose a noble section of the strata.
* The pink coloration of this area on the little map,
and the name "Huronian" applied to it, are among the mistakes of this
hand atlas.
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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