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Description of the Geology of
York County

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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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