Description of the Geology of
Allegheny County
Pennsylvania

 
Allegheny.- Area, 760 square miles; population in 1880, 355,869. In its center, Pittsburgh, the chief city of Western Pennsylvania, at the head of the Ohio river, occupies the space between the Allegheny and Monongahela rivers, having Allegheny City opposite to it on the triangular flood-plain of the Ohio west of the Allegheny, and South Pittsburgh stretching slong the southern river bank. The three rivers flow in narrow valleys sunk at least 400 feet beneath the level of the county; and along the upper part of the steep slopes crops out of the Pittsburgh bed, about six feet thick, nearly horizontal, and penetrated by colliery workings, the produce of which is lowered on inclined-planes to the railroads and river pools, for transportation in flat boats down the Ohio and Mississippi rivers. The bed extends beneath the southern townships into Washington county, its outcrop gradually, but not uniformly, falling lower and lower on the Monongahela slopes, until it passes beneath water level at Bridgeport, 56 miles above Pittsburgh. But in the other direction, northward, the bed creeps into the very hill tops, and the only relics of it that have been preserved from erosion, in the townships north of the Ohio river and west of the Allegheny river, are a few small spots in Ross and Indiana townships, three others on the highest summits along the Franklin township lines, and the last solitary outlier in Pine township. Between the Allegheny and Monongahela rivers, however, the higher lands east of Pittsburgh preserve large areas of the bed, more or less separated by valleys of erosion. A description of 200 collieries, with 76 sections of the coal bed, showing the gradual increase in size of its lower workable portion from about 6 feet at Pittsburgh, to 9 feet higher up the river, will be found in Mr. Wall's Report of Progress, K⁴. At Pittsburgh the bed lies 330 feet above low river water; 13 feet beneath it a 10-foot bed of limestone; several other thin limestones and coal beds too small to work crop out on the hill sides, but no workable bed exists down to the Freeport upper coal, which lying more than 200 feet beneath the city of Pittsburgh, makes its appearance at water level in the Allegheny river valley 2 miles above Springfield in East Deer township, and thence northward takes the place of the Pittsburgh coal bed in the mining operations of the valley. The Pittsburgh coal is of the finest cokeing quality, ranging in solid carbon from 59 to 64 per cent; volatile matters, from 30 to 34 per cent; from 3 to 6 per cent ash, about 1 per cent water, and 1 per cent sulphur. Its value was known to the French before Fort Duquesne became the English Pittsburgh, and its abundance has made this city the principal center of the iron manufacture of the United States. Whether or not the new fuel - natural gas from deep wells - shall replace the solid coal in the furnaces of Pittsburgh for any length of time, the pool made by the new dam at Davis Island (four miles below the city) must always be the starting-point for fleets of boats loaded with Monongahela Valley coal and Connellsville coke for the Western and South-western States. Coal and gas, with a little limestone, much excellent building stone, and salt water flowing from wells, are the only minerals of the county. No productive oil wells have been got, and the iron-ore deposits of the coal measures are insignificant. The soil is everywhere good, except along the more massive sandstone outcrops. (See Reports K, K², K⁴, and Q.)

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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