Northampton. - Area, 380 square miles; population in 1880, 70,312. Its
geology is that of Lehigh county, already described, extended eastward
from the Lehigh to the Delaware, 28 miles along the Kittatinny (Blue)
mountain, and 14 miles along the Lehigh from Allentown to Easton. The
slate belt (III), nine miles wide, has a nearly straight edge, 23 miles
long, and a pretty uniform height of 200` above the flat limestone belt
(II), which is 8 miles wide, and crossed by the Monocacy and Bushkill
creeks, which drain the slate belt southward through the little gaps
at Bath and Stockertown, where it is plain to see that the limestone
goes under the slate; a short prong of slate at Nazareth, and two small
outliers of slate at Howertown and Weaversville show the same fact;
as does also the limestone area inside of the slate belt east of Kreidersville,
and three other smaller ones between it and Bath. But the best show
is made by the anticlinal limestone valley of Cobus creek, 3 ½ miles
south of the Delaware water gap; in which interval the whole of No.
III rises to the surface with a thickness of 5240` (1540` upper massive
series; 3700` lower thin beds. See Report D3, Vol. I, p.85.)
At the Lehigh III is faulted and folded, but seems to measure about
6000`. The limestone formation No. II cannot be measured with any accuracy
on account of its extremely crumpled condition, as shown in the large
quarries worked for the blast furnaces between Newport and Allentown.
(See pictures in Reports D2 of quarries at Coplay and Catasauqua.)
Several well-defined large anticlinals and synclinals traverse the belt
lengthwise. (See dips marked by arrows, on the six-sheet topographical
map in Atlas to D3.) The general plain of limestone land
(400` A.T. rising in swells to 450`) is broken by uplifts of Potsdam
sandstone No. I and old gneiss in two places (apparently on one anticlinal
range) viz: Chestnut Hill, 4 miles long, gapped by the Bushkill; and
Quaker Hill, 3 miles long, gapped by the Monocacy. The belt may therefor
be considered as two grand basins, in one of which the lower Lehigh
flows; the other is sub-divided into two sub-basins by a line of local
anticlinals, which govern the underground drainage; for the surface
is almost destitute of streams, and numerous sinkholes testify to concealed
caverns ramifying beneath it; one of these of an extraordinarily instructive
character, 4 miles north of Easton, and another one mile east of Catasauqua
are described and reasoned from in Report D3, Vol. I, pp.
16,17. The farmers distinguish the beds as "rock limestone" and "slate
limestone," the former, massive, making a stronger farm lime. Thin pure
dolomite beds occur giving a sonorous sound when struck. Hydraulic limestone
beds are quarried on the west bank of the Lehigh above Coplay for making
Saylor’s Portland Cement (see analyses and tests in D2, pp.
59+); the purer limestone beds are carefully selected by the iron-works.
(See analyses in D2, pp. 17+.)
The comparative absence of ore banks in this county, north of the
Lehigh River is remarkable; only 27 being described in D2
(pages 197-); but along the foot of the South mountain there are 51
(pp. 191+) and 5 more in the limestone valleys inclosed in the mountains,
viz: the lower reach of Saucon valley, and the Raubsville, Uhlersville
and Riegelsville (Durham) valleys which are merely the western ends
of the long limestone valleys of New Jersey, inclosed between the
ranges of the Highlands. The summit of the mountain south of Bethlehem
is only 960`A.T.; the three summits east of the Saucon valley 1000`,
820`, and 940`; the Hexenkopf south of Smith’s Island 1030`; Morgan
hill south of Easton 820`; the Raubsville ridge 770`; and Brougher
hill at the river 650`, rising westward in a series of knobs along
the Bucks county line to 940` A.T. The knob at the east end of Chestnut
hill is only 690` A.T., the Delaware River beneath it being 170`,
making a picturesque gap. The hill is interesting as being the westward
disappearance underground of the old gneiss rocks of the northernmost
ranges of the New Jersey highlands; for the development of serpentine
beds among the lower limestone strata upon its flank; and for being
a fine collecting ground for rare species of mineral crystals. The
principal mineral wealth of the county lies in its range of roofing-slate
quarries in front of the Blue mountain, from Bangor and Penargyl through
Jacobsburg, Milgrove, Douglassville, Cherry Hill, Chapman’s, Danielsville,
and Berlinsville, to Walnut Port and Slatington in Lehigh county.
More than one hundred quarries and exposures are located on the colored
geological county map, and described or noted in Report D3
Vol. I, with pictures and sections, showing the contortions of the
beds, and the relation of the slaty cleavage to the planes of stratification;
with tables of the quantity of slates made; sizes and mode of working.
The South mountain gneiss is supposed to be of Laurentian age; it
is very similar throughout; mostly a granulite; feldspathic; with
little or no mica; but much magnetite, generally disseminated, and
also in lenticular beds, mined near Vera Cruz, and on Mine Hill just
east of Durham P.O. Some corundum is found just north of Shimersville.
The Potsdam sandstone or quartzite beds, only 25` thick at Allentown,
lies unconformably on the gneiss. Great rounded (weathered?) bowlders
of gneiss cover the southern slope of Lehigh Mountain, and are common
elsewhere; some of these are 20` high, evidently fragments from cliffs
now no longer existing. (See one in Plate 3 page 240 of D3.)
The Terminal Moraine crosses the Delaware at Belvidere, and
curves by Mt. Pleasant, Factoryville, and Bangor to Fox Gap, a slight
notch in the crest of the Blue Mountain. All east of this is covered
with ice drift. A fine kame, or under ice, gravel, and sand
ridge runs from Portland up Jacobus creek valley. (See descriptions
and sketches of it in Report Z.)
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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