Chester. - Area, 760 square miles; population in 1880,
83,481. A perfectly straight valley, two miles wide on the Montgomery
county line at the Schulykill River, and less than one mile wide near
the Lancaster county line, separates the northern from the southern
townships. The Siluro-Cambrian limestones of No. II, which occupy
the "Chester county" or "Downington Valley," dip generally 300
to 500 southward, although small anticlinal rolls run diagonally
across their general strike, and the white-marble strata, confined
to its southern edge, stand quite vertical. The North Valley Hill
is made by the Potsdam sandstone, No. I, rising northward from
beneath the lowest limestones, and spreading in sheets and patches
over a considerable gneiss region, embracing Honeybrook, E. and W.
Nantmeal, W. Vincent, E. and W. Pikeland, Charlestown, Upper Uwchlan,
E. and W. Brandywine, and parts of W. Caln and Sadsbury townships;
and it is plain that the fundamental gneiss area now exposed was formerly
entirely covered by both the Potsdam quartzite and the overlying limestone.
The South Valley Hill, on the contrary, is the edge of a low
table-land, (500' to 600' A.T.) composed (1) of a belt of magnesian-mica
slate; also, vertical or dipping at the highest angles southward,
apparently in contact and conformity with and over the
marble beds of the south edge of the valley, but possibly overturned
and beneath the marble, in which latter case the valley is
a synclinal trough, and the slates south of it are equivalent to the
quartzite north of it; or else a fault runs along the south edge of
the valley. The belt of South Valley Hill slate is only 2 miles wide
at the Schulykill end; widens westward to three miles at West Chester;
4 1/2 at the West Branch Brandywine; and then
spreads over E. and W. Fallowfield, Highland, Londonderry, Upper and
Lower Oxford, and E. and W. Nottingham townships into Lancaster county;
(2) a belt of older and newer gneisses and mica-schists occupying
all the townships to the south and east. Isolated areas of limestone,
however, occur in this belt near West Chester, Doe Run, Kennett's
Square, Avondale, Landenburg, &c.; and Potsdam quartzite seems
to be preserved around London Grove and at points on the Delaware
State line. A long range of serpentine separates the two belts in
E. Goshen and Williston townships, and another still more extensive
serpentine belt ranges along the Maryland line into Lancaster County,
and carries deposits of chrome-iron sand. A trap dyke enters from
Delaware county at the south edge of the slate belt, and extensive
outspreads of trap bowlders occur along the Berks county boundary
in the north; other local exhibitions of trap being numerous in various
parts of the county. Between the Schulykill River and French creek
the country is wholly of Mesozoic and Gneissic rocks. The large magnetic
iron mines of Warwick, connected with both trap and New Red rocks,
but really belonging to the underlying Azoic floor, are still worked.
Small quantities of brown hematite ore have also been obtained from
the valley limestone. The white marble quarries are numerous, but
none of them large. (See Report C4.) The extensive Kaolin
Mines in New Garden Township are pictured with those of Delaware
County in Report C5.
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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