Description of Mercersburg-Chambeesburg District
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DESCRIPTION OF MERCERSBURG-CHAMBEESBURG DISTRICT. By George W. Stose. INTRODUCTION. anticlinal folds and exposed by erosion and the absence of Drainage. The drainage of the province flows in part east some of the resistant sandstone, the surface is more readily ward into the Atlantic Ocean, in part southward into the Gulf LOCATION AND AREA. worn down by streams and is lower and less varied than that of Mexico, and in part westward into Mississippi: River. All The|Mercersburg and Charnbersburg quadrangles are located of the mountain and plateau divisions on either side. This is of the western or Appalachian Plateau division, except a small in the south-central part of Pennsylvania, between parallels true likewise of the eastern part of the northern portion of the area in Pennsylvania and another in Alabama, is drained "by 39.° 45' and 40° and meridians 77° 30' and 78°, and contain Appalachian Valley, but in the western part sharp ridges and streams flowing westward into the Ohio and thence into the about 458 square miles. This area embraces the larger part narrow valleys of great length follow the narrow belts of Mississippi. The northern portion of the Appalachian Moun-' of Franklin County and a small part of Fulton County. upturned hard and soft rocks. tain division is drained eastward into the Atlantic, but south Its southern border is within 2 miles of the Maryland State Appalachian Mountains and Piedmont Plateau. The eastern of New River all except the eastern slope is drained westward' boundary. (See fig. 1.) division of the province embraces the Appalachian Mountains into the Ohio by tributaries of the Tennessee or southward and the Piedmont Plateau. The Appalachian Mountains are into the Gulf by tributaries of the Coosa. made up of many minor ridges, which under various local In general the streams of the Appalachian Valley flow^for names extend from southern New York to central Alabama. long distances in the lesser valleys along the outcrops of softer Chief among these are South Mountain in Pennsylvania, the rocks parallel to the mountain ranges bounding the Great Blue Ridge and Catoctin Mountain in Maryland and Virginia, Valley. These longitudinal streams empty into larger trans the Great Smoky Mountains in Tennessee and North Carolina, verse rivers, which cross one or the other of the mountain and the Cohutta Mountains in Georgia. The Piedmont barriers. In the northern portion of the province Delaware, Plateau is a vast upland which lies at the eastern foot of the Susquehanna, Potomac, James^and Roanoke rivers pass through Appalachian Mountains. It stretches southward from New the Appalachian Mountains in narrow gaps and flow eastward York to Alabama and merges on the east into the Coastal to the sea. In the central portion, in Kentucky and Virginia, Plain, which borders the Atlantic Ocean. The mountains and the longitudinal streams form New River, which flows west the plateau grade into each other with no sharp boundary. ward in a deep, narrow gorge through the Cumberland Plateau The same rocks and the same structures appear in each, and and empties into Ohio River. From New River southward to the form of the surface varies largely in accordance with the the boundary of Georgia the Great Valley is drained by tribu ability of the different streams to wear down the rocks. Most taries of Tennessee River, which at Chattanooga leaves the of the rocks of this division are more or less crystalline, beirig broad valley and, entering a gorge through the plateau, runs 39 39 either sediments which have been changed to slates, quartzites, westward to the Ohio. South of Chattanooga the streams flow schists, and gneiss by varying degrees of metamorphism, or directly to the Gulf of Mexico. igneous rocks, such as granite and diabase, which have solidi General geology. The rocks which appear at the surface of 77 C 76° 78 fied from a molten condition. the Appalachian province comprise igneous rocks, sedimentary FIGURE 1. Index map of the vicinity of the Mercersburg and Chambers- burg quadrangles. Appalachian Plateau. The western division of the Appala rocks, and crystalline rocks which, on account of their age and The dark shaded rectangle represents the Mercersburg and Chambersburg quadrangles. Other chian province embraces not only the Cumberland Plateau of extensive alteration, are of unknown origin. The geologic published folios covering parts of the area are as follows: Nos. 10, Harpers Ferry; 28, Pied mont ; 32, Franklin; 70, Washington; 82, Masontown-Uniontown; 94, Brownsville-Connells- Tennessee and Georgia and the Allegheny Plateau of Pennsyl history of the region is preserved in these formations, but at ville; 102, Indiana; 110, Latrobe; 115, Kittanning; 123, Elders Ridge; 125, Rural Valley; 183, Ebensburg; 187, Dover; 152, Patuxent; 160, Accident-Grantsville. vania and West Virginia but also the lowlands of eastern Ten no one place is the record complete. Only by combining the nessee, Kentucky, and Ohio. Its northwestern boundary is data from the various parts of the province can the general APPALACHIAN PROVINCE. indefinite but may be arbitrarily regarded as a line coinciding sequence of geologic events be determined. These quadrangles form part of the Appalachian geographic with the eastern boundary of the Mississippi embayment as far The oldest rocks of the region consist chiefly of gneisses and and geologic province, which extends from the Atlantic Coastal up as Cairo and thence crossing the States of Illinois, Indiana, schists and occur in the Piedmont Plateau. The original char Plain to the Mississippi lowlands and from central Alabama to and Ohio. Its eastern border along the Appalachian Valley acter of these rocks is entirely obliterated by the extensive Canada. This region had throughout its extent a similar is sharply defined in most places by the Allegheny Front and alteration which has taken place during the long ages since history, which is recorded in its rocks and its topographic the Cumberland escarpment. their formation, and it is in large measure impossible to d.eter- features. Only a part of this history can be interpreted from The rocks of this division are almost entirely of sedimentary mine whether they were a part of the cooled surface of an so small an area as the Mercersburg and Chambersburg quad origin and are but gently folded. The surface, which is original molten mass or sediments laid down in the earliest seas. rangles, and it is therefore desirable to consider the area in its dependent on the character and attitude of the rocks, is that The great pressure and heat to which the rocks were subjected relation to the entire province. of a plateau in various stages of dissection. In the southern while deeply buried in the earth caused the recrystallization of The Appalachian province is composed of three well-marked . half of the province the plateau is in places extensive and very the mineral particles in new forms and associations and pro divisions, each characterized throughout by similar sedimentary , flat, but it is more commonly intersected by numerous valleys duced in the rocks a gneissoid or schistose structure. During deposits, geologic structure, and topography. The western and ravines, leaving small flat-topped areas. In West Vir an early period, while these schists and gneisses were still deeply division is named the Appalachian Plateau; the middle the ginia and portions of Pennsylvania the plateau is $o largely buried, they were penetrated from below by great masses of Appalachian Valley; the eastern the Appalachian Mountains dissected as to leave irregularly rounded knobs and ridges molten material, which now occur upon the surface as granites, and Piedmont Plateau. These divisions extend the entire that bear little resemblance to the original surface. The west diorites, and other crystalline igneous rocks. This basal com length of the province from northeast to southwest, but the ern part of the plateau has been completely removed by erosion plex is generally regarded as belonging to the Archean period. following description applies more directly to the portion south and the surface is now comparatively low and level or rolling. Upon these once deep-seated rocks, which were brought t6 of the State of New York. Altitude. The Appalachian province attains its greatest the surface by uplift and long-continued erosion, lavas were Appalachian Valley. The central division is the Appalachian elevation in North Carolina, where the Appalachian Mountains poured in pre-Cambrian time. Whether these ancient lavas Valley. It is the most uniform and best denned of the three, have a maximum altitude of 6700 feet. From this culminat represent a late portion of the Archean period or are of Algon- being sharply delimited on the southeast by the Appalachian, ing point the eastern range descends to less than 1000 feet at kian age is .not certain. They are separated from the overlying Mountains and on the northwest by the Appalachian Plateau. its south end in Alabama and, toward the north, to 3500 Cambrian strata by an unconformity, and fragments of the In its southern portion it coincides with the belt of closely feet in Virginia and 2000 feet in Maryland and Pennsyl lavas form basal conglomerates in the Cambrian. These pre- folded and faulted rocks that form the Coosa Valley of Alabama vania. The plateau division on the west of the Appalachian Cambrian rocks are confined for the most part to the Piedmont and Georgia and the Great Valley of eastern Tennessee and Valley has at its southern limit an altitude of 500 feet, ascends Plateau. In general, the oldest rocks lie on the east side of the Virginia; this portion varies in width from 40 to 125 miles. to 2000 feet in Tennessee, and culminates in eastern Kentucky province and successively younger strata appear toward the west.