Description of the Fairfield and Gettysburg Quadrangles
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DESCRIPTION OF THE FAIRFIELD AND GETTYSBURG QUADRANGLES By George W. Stose and F. Bascom 1 INTRODUCTION The Appalachian Highlands include three well-marked Ridge comprises many minor ridges, which, under local names, longitudinal subdivisions, each characterized by a general extend from southeastern New York to northern Georgia. LOCATION AND AREA similarity of sedimentary deposits, geologic structure, and Chief among these are the Highlands of southeastern New The Fairfield and Gettysburg quadrangles are in the south- topography. The western subdivision includes the Appalach York and New Jersey, South Mountain in Pennsylvania, the central part of Pennsylvania, between parallels 39° 45' and ian Plateaus; the middle subdivision is the Appalachian Blue Ridge and Catoctin Mountain in Maryland and Virginia, 40° and meridians 77° and 77° 30', and together contain about Valley and Pvidges province; the eastern subdivision includes the Great Smoky Mountains in Tennessee and North Carolina, 458 square miles. They embrace the larger part of Adams the Blue Ridge and Piedmont provinces. Topographically the and the Blue Ridge in Georgia. County, the east side of Franklin County, and the south corner Blue Ridge and Piedmont provinces are distinct, but geologi The Piedmont province is a wide belt of upland southeast of of Cumberland County. The southern border of the area is cally they are so closely allied that they may be better treated the Blue Ridge province. It descends gently southeastward within 2 miles of the Maryland State boundary. (See fig. 1.) as a unit. These three subdivisions are well defined from and grades into the Coastal Plain, which borders the Atlantic Alabama to southern New York, and the following description Ocean. The Blue Ridge and Piedmont provinces grade into applies chiefly to that part of the Appalachian Highlands. each other in places, with no sharp boundary. The form of (See fig. 2.) Farther north and east are the St. Lawrence the surface differs largejy in accordance with the proximity of Valley, Adirondack, and New England provinces. large trunk streams and their activity in wearing down the rocks. The rocks of this subdivision are largely crystalline, APPALACHIAN VALLEY AND RIDGES being either sediments that have been metamorphosed to The middle subdivision has generally been called the Appa quartzite, slate, schist, and gneiss, or igneous rocks that have lachian Valley province, but in most places, especially in solidified from a molten condition, such as granite, diorite, Pennsylvania, ridges form so large a part of it that the gabbro, rhyolite, and basalt, also more or less altered by name Appalachian Valley and Ridges is more appropriate, metamorphism. and that name was proposed by the writer and has been APPALACHIAN PLATEAUS adopted by the United States Geological Survey. It is the most uniform and best defined of the three, being sharply The western subdivision of the Appalachian Highlands delimited on the southeast by the Blue Ridge province and comprises the Cumberland and Allegheny Plateaus and several on the northwest by the Appalachian Plateaus. In its south lower plateaus in Tennessee, Kentucky, and Ohio. On the ern portion it coincides with the belt of closely folded and west it is bordered mainly by the Interior Low Plateaus and faulted rocks that form the Coosa Valley of Alabama and Central Lowland provinces of the Interior Plains. Its eastern Georgia and the valley of east Tennessee and Virginia; this border is sharply defined in most places by the Allegheny Front portion ranges in width from 40 to 125 miles. Throughout and the Cumberland escarpment. its central and northern portions only the east side of the The rocks of this subdivision are almost entirely sedimen FIGURE 1. Index map of south-central Pennsylvania and portions of adjacent States province is marked by great valleys; these range in width tary and are but gently folded. The form of the surface, which The location of the Fairfleld and Gettysburg quadrangles is shown by darker ruling (No. 225). from 8 to 13 miles and comprise the Shenandoah Valley of is in part dependent on the character and attitude of the rocks, Published folios describing other quadrangles are indicated by lighter ruling and are listed on the back cover of this folio Virginia, the Cumberland Valley of Maryland and southern is that of plateaus in various stages of dissection. In the Pennsylvania, the Lebanon Valley of eastern Pennsylvania, southern half of the province large portions of the plateau APPALACHIAN HIGHLANDS and the Kittatinny Valley of New Jersey. The west side of country are very flat, but more commonly the plateaus are These quadrangles form part of the major physiographic this part of the province throughout most of its extent is a dissected by many valleys and ravines into numerous small, division known as the Appalachian Highlands, which lies succession of narrow longitudinal valleys separated by long flat-topped hills. In portions of West Virginia and Pennsyl between the Atlantic Plain on the east and the Interior Plains parallel ridges. Its general altitude, however, is markedly vania the plateau surface is so largely dissected as to leave on the west and extends from central Alabama to Canada. lower than that of the mountainous provinces on both sides. irregular rounded knobs and ridges that bear little resemblance This region had throughout its extent a similar physiographic The rocks of the Appalachian Valley and Ridges are almost to the original flat surface. The western part of the province . wholly sedimentary and has been extensively eroded, and portions of its surface are of consist of limestone, shale, much lower altitude but are still comparatively level or rolling. and sandstone. The strata were deposited in nearly ALTITUDE horizontal layers, but they The Appalachian Highlands attain their greatest height in are now inclined at vari North Carolina, where Mount Mitchell, in the Blue Ridge ous angles, and their out province, reaches an altitude of 6,711 feet. From this cul crops form narrow belts minating point the mountains descend to less than 1,000 feet of the different kinds of at the south end of the province in Alabama. The highest rock. The surface relief mountain in Virginia is Mount Rogers, in the Blue Ridge, depends upon the differ which is 5,719 feet above sea level. The valley ridges in ences in hardness and Virginia reach over 4,700 feet, but in Maryland and Pennsyl solubility of the outcrop vania they do not rise above 2,200 feet. The plateau province, ping rocks. In the south west of the Appalachian Valley, has at its southern limit an ern part of the province, altitude of 500 feet, ascends to 2,000 feet in Tennessee, reaches owing to the absence of 4,000 feet in eastern Kentucky, and culminates at 4,800 feet some of the resistant sand in West Virginia, whence it descends again to 3,300 feet in stones that occur farther Maryland and 2,200 feet in north-central Pennsylvania. north and to the corre The height of the floor of the Appalachian Valley is deter spondingly larger amounts mined largely by the drainage basins of the trunk streams that of calcareous and shaly cut through the mountain barriers on each side at irregular rocks which are brought intervals, and it has therefore numerous culminating points on up in the anticlinal folds the watersheds between these streams. Thus it rises from less and exposed by erosion, than 500 feet in Alabama to 2,700 feet on the divide between the surface has been more Tennessee and New Rivers in Virginia, whence it descends to readily worn down and is 2,200 feet in the New River valley. It rises and falls likewise lower and generally less over the divides and into the valleys of James and Potomac varied than that of the Rivers in Virginia, declining to 500 feet in the Potomac Basin. FIGURE 2. Relief map of part of southern Pennsylvania showing mountain ridges, valleys, and drainage systems adjacent mountain and In Pennsylvania the floor of the valley does not rise above The Fairfleld and Gettysburg quadrangles include the eastern part of South Mountain and the area to the east as far as Hanover and extend from a line a little north of the State boundary northward nearly to York Springs plateau provinces. This 1,000 feet. Throughout the length of the province the stream is true likewise of the channels are cut 50 to 250 feet below this valley floor, and the and geologic history, which is recorded in its rocks and in its northeastern part of the Appalachian Valley and Ridges prov ridges rise 500 to 2,000 feet above it. topographic features. Only a part of this history can be ince, but in its northwestern part high, sharp-crested parallel interpreted from so small an area as the Fairfield and Gettys ridges and intervening valleys follow the narrow belts of up DRAINAGE burg quadrangles, and it is therefore desirable to consider the turned hard and soft rocks respectively. The drainage of the Appalachian Highlands south of New area in its relation to the entire division. York flows in part eastward to the Atlantic Ocean, in part BLUE RIDGE AND PIEDMONT PROVINCES southward to the Gulf of Mexico, and in part westward to 1 The descriptions are chiefly by G. W. Stuse; the description of the pre Cambrian rocks is by F. Bascom and G. W. Stose, and the petrographic The eastern subdivision of the Appalachian Highlands Mississippi River. The greater part of the Appalachian portion by F. Bascom. embraces the Blue Ridge and Piedmont provinces. The Blue Plateaus is drained by streams flowing westward to Ohio River. The northern portion of the Blue Ridge province is drained next cycle of sedimentation.