York. - Area, 920 square miles; population in 1880, 87,841. This large
and wealthy county, stretching for more than 50 miles along the west
bank of the Susquehanna River, and 42 miles along the Maryland State
line, has a remarkably varied geology, as yet very imperfectly understood.
It is naturally divisible into three nearly equal areas: The northern
townships of Mesozoic (New red) sandstone and shale (with an unusual
quantity of trap ridges* and bowlders, and a good deal of magnetic iron
ore), traversed centrally from S.W. to N.E. by Conewago Creek (which
drains most of Adams County) and by Little Conewago Creek along the
southern margin. A middle belt of townships along Codorus Creek,
and its west branch, flowing from the south-west (the south branch heads
at the Maryland line) consists of (1) the southern appearance at the
present surface of the Great Valley limestone (No. II) which undoubtedly
floors the northern area beneath the Mesozoic rocks; (2) the underlying
Potsdam sandstone of Chiques cliffs, and the Pigeon hills; (3) belts
of hydro-mica schists connected in some as yet undetermined manner with
Nos. I and II; and (4) a more southern belt of chlorite schists (of
equally uncertain age) about 4 miles wide, running through Lower Windsor,
Windsor, York, Springfield, Codorus, Manheim, and W. Manheim townships
to the extreme S.W. corner of the county. The southern townships,
with a general surface of gneisses and schists, evidently belonging
to the Philadelphia Belt system, whatever that may be, are drained transversely
by the extensive water-tree of Muddy Creek (precisely as the same region
in Delaware County is drained transversely by Darby, Cobb’s,
Ridley, and Chester creeks) which turns at a right angle where it meets
the roofing slates of Peach Bottom township, and flows along their northern
edge into the river, as if the drainage had been established by a fault,
or by the basset edge of a synclinal; which last conjectured cause would
have operated if the Peach Bottom slates with its fossil plants were
of Hudson river age )No. III). The Tocquan anticlinal
uplift of ancient (Laurentian?) gneiss mentioned in the account of Lancaster
County (page 1xiv above) undoubtedly runs on through York County, and
must be the starting-point for any thorough investigation into the age
and structure of this Azoic strata of the county. Carefully detailed
sections along the Susquehanna River are published with Report C3
on Lancaster County; maps and sections further west, with Reports C
and C2 on York and Adams. There are 126 iron mines described
in C, with chemical analyses of the ores, and a map showing the locations
of those which lie at and west of Dillsburg, Rossville, Wellsville,
and Franklintown in front of the South Mountain range (the Blue Ridge
of Virginia); which comes to its northern end in the north-west corner
of York County; and against its slope the upper strata of the Mesozoic
are deposited, as in Bucks County. Heavy deposits of clay lie at the
foot of the mountain. An extensive manufactory of bricks (3,000 per
day) is carried on at Barnetts, one mile from Hanover (on the H. and
G. R.R.) from a mixture of clay (produced by decomposition of the mica
slates) with red Mesozoic sand. The Great Valley limestone No. II at
the north edge of the Mesozoic region is seen in the most interesting
exposures along Yellow Breeches Creek; and there is a certainty that
this formation is identical with the blue limestone of the Cadorus
valley; as it is with the blue limestone of the Chester Valley in Lancaster,
Chester, and Montgomery counties. But there is a belt of white limestone
in York County, as there is a belt of marble in Chester County,
which seems to be older than the blue limestone; the limestone beds
(so extensively quarried along the river) set in 3300 feet north of
the Wrightsville end of the long bridge at Columbia, and 350` from the
southern most visible outcrop of 50º south dipping slates (a
formation in all apparently 5000` thick,) with nothing to show the nature
of this 350` interval. The overlying (?) limestone beds dip also 50º
southward, and are extensively quarried for 1600 feet along the river
bank, the dip increasing to 60º and finally (at Kerr’s & Cook’s
quarry) to 76º S. 4º E. (In Wrightsville 70º , S. 18º E.) Total thickness
of limestone exposed 2800`. In Kerr’s and Detweiler’s quarries
the stone is crystalline, and in Detweiler’ the layers roll much; just
south of it white (sometimes pink) limestone pebbles occur
in a blue limestone matrix; and this is taken as a (doubtful)
indication that the white limestone of York County is an older formation.
From the bridge southward 2300 feet of probably overturned (83º to 86º
south dipping) nearly vertical limestone beds are apparently a repetition
of those north of bridge, coming up again on the south side of a deep
compressed synclinal trough, the central axis of which is at the bridge;
the total thickness of beds preserved in this trough at river is nearly
3000 feet. A fault seems to cut off the south side of the trough at
Creitz’s Creek with an upthrow of the same sandy pyritous slates as
those north of the trough, all dipping S. 10º E. at various very steep
angles; but for half the horizontal distance of 2500` along the river
bank the slates dip 60º , 80º , 65º , 70º , 50º , 75º , and for the
other (southern half) 90º , 85º , 87º , 85º , to within 300` of Wilton’s
run; consequently it looks as if there was a closely compressed trough
in which 1250` of slates were preserved. Against the last vertical slate
exposure has south of it the first exposure of the Wilton run belt of
limestone beds dipping S. 5º E. 54º
, then 60º , 1200
feet horizontal distance (1100` of limestone) to Walton’s ore bank,
at the north edge of the next belt of crystalline chlorite and dromica
schists (7000` wide); apparently a huge anticlinal arch crimpled along
its crest. North of Cline’s run 1400 feet, the limestone beds begin
again and continue down the river for 12,060 feet of exposed beds always
dipping to the south, but at lower and higher angles alternately, suggesting
several collapsed and overthrown anticlinals and synclinals. This third
limestone belt is limited southward by a ravine and blank of 1400 in
the exposures, and then recommence the precipitous river hills of compact
crystalline mica-schists dipping back diagonally toward the limestone
(N. 30º W.)
at angles increasing from 49º
to 52º and immediately
to 68º and 77º
. This dip was considered by Prof. Rogers to be cleavage; but there
seems no good reason for not considering it stratification. The remarkable
straightness of the southern edge of the Codorus valley limestone belt
at its contact with the slate, from Littlestown N. 61º
E. for 29 miles to a point nearly a mile S.E. of the Widow Fritz’s ore
bank (where it curves a little and runs on 8 miles about N. 75º
E. to the lock below the Columbia dam) taken with a non-conformity of
dip in the two formations everywhere along the line, proves the existence
of at least one great fault crossing York County; but no outcrop is
discernible along the line. A similar long straight southern edge to
the Chester County limestone at its contact with the South Valley Hill
slates suggests a similar fault in that region. For two instructive
instances of unconformable contact see Report C, p. 135. A curious feature
of some of the York limestone beds consists in the powdering of the
surfaces of the laminae of sedimentation with minute scales or flakes
of mica-schist, making hand specimens look like pieces of genuine chlorite
or hydromica slate, (see C, p. 133.) At many of the quarries beds of
ore are interpolated conformably between the beds of limestone. On the
other hand, at Figley & Brillhart’s ore banks, layers of limestone
are seen in the mass of ore. The slates are sometimes charged with disseminated
crystals of magnetic iron ore, from microscopic size up to 1/8 inch,
with scales of specular ore, and with pyrites; but usually the whole
mass has moldered into brown hematite iron ore clays with a varying
percentage of magnetic and specular ore. (Dr. Frazer discusses the possible
origin of the limestone ores in C, pp. 136+.)
* For a chemical discussion of the different kinds of trap see Report
C, pp. 115 to 129, with two microscopic sections.
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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