Lancaster. - Area, 970 square miles; population in 1880,
139,447. Its extreme length from the Berks-Lebabon corner south to
the Maryland line is 43 miles. Its breadth from the Chester-Berks
corner west to Falmouth on the Susquehanna River is 47 miles. The
river bounds it for 41 miles (in an air line) and exposes sections
of its three geographical belts: the northern New Red (Trias) sandstone
belt, the central Lower Silurian (No. II) limestone plain, and the
southern Azoic (gneiss, mica-schists, roofing-slate, and serpentine)
country. The New Red formation, having been deposited on the ancient
uneven limestone surface, has been itself reduced by erosion to a
rolling hill country, (no where much more than 500` A.T.), very rough
along the Lebanon line, and traversed by several trap-dykes, one of
which extends from the river below Bainbridge eastward 25 miles, and
is largely developed approaching the Cornwall iron mines in Lebanon
county; another runs from the river at Falmouth 10 miles into Dauphin
county; but the trap is not confined to the New Red, since it crosses
the East Donegal arm of the limestone plain; and a third remarkable
dyke starts from the river at Peach Bottom ferry and runs (N.N.E.
26 miles) through the Azoic rocks, across the Chester county valley
limestone strip at May P.O. across the Salisbury arm of the limestone
plain (past Springville), and into the Potsdam rocks (No. I) of Mt.
Airy. Other smaller exposures of trap occur, notably one at the Wharton
nickel mine, near the Gap west of Christiana station P.R.R. One third
of the county is a limestone plain of unsurpassed fertility, under
the highest cultivation, drained centrally by the two forks of Conestoga
creek; its southern edge by the Pequea; and its north-western arm
by the Chiquesalunga. An extensive bay of limestone land around Rome,
Ephrata and Lincoln penetrates northward into the heart of the New
Red belt, and show how shallow the Mesozoic deposits must have been,
although their universally north dip would seem to make them thick.
In this bay are the old Warwick iron works. Back of Columbia are the
famous Chestnut Hill brown hematite iron ore mines near the contact
of the limestone and slate. Near Landisville, some lead and zinc ore
has been obtained from the limestone, but no extensive deposits like
those of the Saucon valley in Northampton county have been found,
nor is their possible existence indicated by the structure, although
the limestone strata are everywhere steeply upturned, and the whole
limestone region seems to be traversed by sharp compressed anticlinal
and synclinal folds, some of which are probably overturned towards
the north. Two of these little rolls bring the Potsdam sandstone to
the surface at Manheim 4 m. N. of Lancaster. Another is seen in the
cliffs of Chicques Rock facing the river above Columbia. Here the
Potsdam is thrown up by a fault against the limestone lying north
of it, and for a distance of 8 miles eastward. The edge of the limestone
plain is made irregular by projecting spurs of Potsdam, (overlying
gneiss) from Chester County; and Pequea valley and Conestoga valley
are two projections of low limestone land into the Azoic country of
the Welsh mountain region of that county. The edge of the limestone
is masked in many places by Potsdam quartzite fragments, the mother
rock of which does not appear. The contact of the limestone with the
New Red is usually masked by fragments of mica-schist; especially
is this true around the Ephrata limestone basin. It suggests that
the New Red rests upon these schists, and that the limestone was high
dry land, when the New Red estuary received its deposits. These "York
schists" are the iron-bearing rocks of the county. The limestone formation
does not consist wholly of pure limestone beds, but of alternations
of these with argillaceous slates, and some quartzose beds, and it
is impossible to assign a certain thickness to the mass. The southern
part of the county may be said to be a country of chlorite-schists
(exposed in the northern end of Turkey hill to the mouth of the Conestoga)
apparently 7,000 feet thick, and these overlying still older gneisses
brought to the surface along the river at the mouth of Tocquan creek
in a broad and gentle anticlinal, which must be considered as passing
under Martic, Providence, Eden, Bart, and Sadsbury townships into
Chester county. South of the Tocquan anticlinal the overlying series
descend again. South of Fishing Creek in Drumore Township and to Peter’s
creek is a chlorite-schist belt containing the roofing slates of Peach
Bottom so extensively quarried. (See pictures and descriptions in
Report C3.) South-east of this are two belts of serpentine,
separated by a belt of schists; the southern running along the Maryland
line, and holding the famous Wood chrome-iron mines (described
in Report C3, p.195) which at one time produced all the
chrome in the world, and in busy times as high as 500 tons per month.
The serpentine is here unstratified, 1,000 yards wide, striking N.
78º E., with
sandy chlorite slates north of it and hornblendic gneiss and syenite
south of it. The brucile crystals from this mine, once exceptionally
abundant and beautiful, are now rarely found.
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.
Return to Geologic
Maps of Pennsylvania Counties
Copyright © 2000, Pennsylvania State University
Libraries. All rights reserved.
Please send comments to: ems@psulias.psu.edu
Last modified: 5/3/01
|
