Description of the Geology of
Berks County
Pennsylvania

 
Berks.- Area, 900 square miles; population in 1880, 122,597. The Schuylkill river crosses this county from north to south; and two large streams, Maiden creek draining the northern, and Tulpehocken creek draining the western townships, in the Great Valley, fall into it above and opposite to the large city of Reading, one of the principal centers of iron manufacture in the State. The eastern townships cover a low mountain region (1000'-1200' above tide) of gneiss, covered with patches and belts of Potsdam quartzite and slate No. I, one or two hundred feet thick, and inclosing the Oley limestone valley of II. Mulbaugh hill, on the west county line, is another summit of the floor-gneiss of the State, carrying Potsdam on its northern slope. Between the two gneiss areas a gate ten miles wide, mostly closed with Potsdam, lets the limestone of the great valley out southward; but it immediately sinks beneath the great belt of the Mesozoic new red country; rising again however to the surface at the southern corner of the county. All along the south slope of the mountains east of Reading the limestone is seen descending beneath the Mesozoic. And undoubtedly forms its floor under the southern townships. Extensive outbursts of trap form high hills along the northern edge of the Mesozoic, and a peculiar conglomerate, called Potomac marble, finely exposed in the cuttings of the railroad at the bend of the river below Reading, has been shot into the new red estuary by torrents from the north at the close of the Mesozoic age. The thickness of the whole new red brown sandstone and shale deposit on the Delaware seems to be at least 30,000 feet, but in Berks county it cannot be 10,000 feet, even if the dip be not false-bedding. The Great Valley, between the gneiss mountains and the Kittatinny or Blue mountain, along the crest of which (1500' A. T. ) runs the north county line, is divided into two belts: one, of the limestones of II (2000'? thick) at the foot of the gneiss mountain; the other, of the slates of III (5000'? thick) extending to the Blue mountain and far up its slope. The limestone belt is famous for its levelness and fertility, and for its numerous local and isolated deposits of brown-hematite ore. The slate belt is elevated about 200' above the limestone belt; is sculptured into innumerable vales; contains some rofiong slate and flag-stone beds of good quality, especially in Albany and Greenwich townships; contains no other minerals of value; and although the soil is good, is still in an inferior state of cultivation. These two great formations are excessively crimpled by side pressure, and the whole valley is a labyrinth (underground) of sharp, small anticlinal and synclinal folds, all pressed over to the north and collapsed, so that the dips exposed along the Schuylkill river from Reading up to the Port Clinton gap (17 miles in straight line) are all steep to the south; whereas the limestone as a whole passes down northward under the slate, and the slate as a formation dips northward under the Medina sandstone of the Blue mountain. Another consequence of the plication is, that strips of slate run lengthwise in the limestone belt, marking sharp downward folds or narrow basins; and strips of limestone run lengthwise in the slate belt, marking the sharp tops of upward folds, or compressed arches eroded off at the present surface. In the northern corner of the county, ten such folds traverse the slate, and show themselves in as many spurs of the Blue Mountain on the Little Schuylkill side in Schuylkill county. The great fold in front of the gap broke, and its northern limb was thrust vertically upward several thousand feet in the air, as is shown by the attitude of the Medina and Clinton strata in the gap. Besides the brown-hematite deposits of the valley limestone (the principal of which is the great old Moselem bank on its northern edge) there are important magnetic-iron mines at Boyertown and Siesholtzville, and the Jones mine in the southern corner of the county. (See Report D3, Vol. 2.)

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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