By F. Bascom, W. B. Clark, N. H. Dartoii, H. B. Kummel, R. D. Salisbury, B. L. Miller, and G. N. Knapp. INTRODUCTION. tains and plateau. Topographically there is similar prominences rise 100 to 200 feet above the subsequently uplifted with more or less folding. usually a more or less abrupt passage from a upland. These facts are indicated by their constitution, LOCATION AND AREA. trenched upland to level-topped mountains and In Maryland the western edge of the upland their structure, and their mutual relations. They The area herein referred to as the Philadelphia ridges. reaches an altitude of 700 feet, and slopes to an include some of the oldest materials known to have district lies between 39° 45' and 40° 15' north On the east the plateau is separated from the eastern margin in the vicinity of Baltimore, 300 thus accumulated and some of the most recent. latitude and 75° and 75° 30' west longitude. It seaward margin of the continental shelf by a belt feet above tide level. Rocky Ridge and State The first sediments that were deposited in this covers one-quarter of a square degree, which is of coastal province 250 miles in width. The Ridge are monadnocks on this upland. Atlantic belt were of arkosic and argillaceous equivalent in this latitude to 915.25 square miles, greater part of this is under water in this latitude; In Virginia, North Carolina, South Carolina, character, and their deposition took place in pre- or about 34.50 miles from north to south and 26.53 the land portion is called the Coastal Plain. This and Georgia the plateau slopes southeastward and Paleozoic time. The subsequent compression and miles from east to west. dips gently eastward under the sea and increases southward. Its summits range from 500 to 1500 folding to which the arkose and argillites were The district consists of four quadrangles the greatly in width toward the south. The boundary feet in height along its western border, and its subjected have developed from them a hard, Germantown, Norristown, Philadelphia, and Ches­ between plateau and plain is usually defined by a eastern margin has an altitude of 300 to 500 feet. crystalline, finely banded gneiss, composed largely ter each of which measures fifteen minutes of well-marked change in the topographic features Southwest Mountains, Brushy Mountains, South of quartz and feldspar, and a mica gneiss with latitude and fifteen minutes of longitude. It lies and geologic formations. Topographically the Mountain, and many others are residual masses on interbedded mica schist facies. These gneisses in three States Pennsylvania, New Jersey, and change consists generally of an abrupt transition this plateau. have been differently named in different portions Delaware and embraces in whole or in part ten from a diversified upland to a relatively undiver- The plane surface which, sloping gradually from of the plateau. In the Pennsylvania Piedmont counties, namely: Bucks, Montgomery, Philadel­ sified lowland. Geologically there is a transition western border to eastern margin, forms the the finely banded quartz-feldspar gneiss is inti­ phia, Delaware, and Chester counties in Pennsyl­ from consolidated and for the most part crystalline plateau, does not owe its even contour to the mately associated with the more massive gneiss and vania ; Burlington, Camden, Gloucester, and Salem rocks to unconsolidated clays, sands, and gravels underlying rock formations, for they possess highly forms with it the composite Baltimore gneiss, while counties in New Jersey; and Newcastle County in of more recent age. These Coastal Plain forma­ complex structures. The larger streams, which the mica gneiss is known as the Wissahickon mica Delaware. Within these limits there is a population tions always overlap the eastern border of the have cut into the plateau, converting it into a gneiss. of more than a million and a half. plateau, and in some districts are found far inland diversified upland, maintain courses which are The pre-Cambrian movements which metamor­ and somewhat obscure the passage from plateau to quite independent of the structure and character of phosed these formations were accompanied by the OUTLINE OF GEOGRAPHY AND GEOLOGY OF ATLANTIC plain. the rock floor. The tributary or subsequent intrusion of bosses, irregular masses, sills, and BORDER REGION. The margin of the plateau is always indicated, streams, on the other hand, show adjustment to the dikes of molten material which further altered the The Atlantic border region, of which the Phila­ however, by a change in the character of the constitution of the rock floor; and by means of squeezed sediments and which consolidated as delphia district is a part, is divisible into two streams which pass from plateau to plain. An them the heterogeneity of rock character and the granite, gabbro, pyroxenite, and peridotite. distinct geologic provinces the Appalachian and abrupt decrease in stream velocity characterizes complexity of rock structure are finding expression. With the opening of Paleozoic time there were the Coastal. The Philadelphia district comprises this passage, and so frequently do falls or rapids The general trend of the highlands, which is deposited upon these gneisses successively arena­ portions of both provinces, each of which is char­ mark the margin of the plateau that this boundary northeast-southwest, in harmony with the strike of ceous, arenaceous-argillaceous, calcareous, and argil­ acterized by special topographic features which has been called the "fall line." This term is the underlying rocks, does not accord with the laceous sediments. This deposition, taking place record its geologic and physiographic history. imperfectly descriptive of the boundary, which is main drainage lines of the plateau but with the in Cambrian and Ordovician time, extended over The Appalachian Province is composed of three actually a zone of appreciable width. East of this courses of the secondary streams. The main a considerable portion of the Piedmont region, but well-defined geographic districts, which extend zone navigable streams leading to tidal estuaries streams of the plateau, rising either in the Appa­ not throughout the region. The intense folding, throughout its length. The Allegheny and Cum­ afford good shipping facilities; west of it the lachian Mountains or on the inland border of the faulting, and accompanying metamorphism to berland plateaus form the most western of these streams cease to be navigable and occupy narrow Piedmont, pursue courses consequent upon the which they were subjected by the close of Paleozoic divisions; the Greater Appalachian Valley, com­ turbulent channels. In the southern extension of slope of the upland i. e., to the east, southeast, time converted the arenaceous materials into quartz- prising in Pennsylvania a group of valleys and a the plateau the "fall line" gradually rises until in and south, and either empty into estuaries heading ite, the arenaceous-argillaceous sediments into inter- succession of narrow ridges, constitutes the central the Carolinas and Georgia, although falls and at the eastern margin of the Plateau or, crossing bedded quartzites and mica schists, the calcareous district, while the eastern division of the province rapids still mark its location and furnish power the Coastal Plain, empty directly into the Atlantic material into marble and calc schists, and the embraces the Appalachian Mountains, which for factories, it lies considerably above the tidal or into the Gulf of Mexico. The Delaware, Sus- argillaceous deposits into slates or micaceous schists. border the Greater Valley, and the group of plateaus limit. quehanna, and 'Potomac are such master streams These formations, which are widespread though which, stretching away to the east, constitute a The position of this "fall line," at the head of which have cut into the plateau rugged valleys not continuous throughout the Piedmont belt, are vast upland known as the Piedmont Plateau. navigation and at the source of water power, has transverse to the strike of the rock formation. respectively designated in the Pennsylvania Pied­ Of the three divisions of the Appalachian been a dominant factor in determining the location South of an east-west line drawn just north of mont the Chickies quartzite, the Shenandoah lime­ Province, only a part of the eastern, or Piedmont of the large cities of the Atlantic States. A line New Brunswick, N. J., the interstream areas are stone, and the Octoraro schist. Plateau, is embraced within the district described passing through New York, Trenton, Philadelphia, covered by a mantle of residual soil and are These crystallized sediments and igneous intru­ in this folio. Wilmington, Baltimore, Washington, Fredericks- characterized by absence of rock ledges. North of sions constitute the foundation of the Pennsylvania burg, Richmond, Petersburg, Raleigh, Columbia, this line the rock mantle is glacial drift, the depth Piedmont Plateau, but are uncovered only in PIEDMONT PLATEAU. Augusta, and Macon will therefore mark in a of which varies from zero to 200 feet. Rock detached belts trending northeast and southwest. By F. BASCOM. general way the passage from plateau to plain. ledges become an increasingly prominent feature They have been folded in synclinoria and anti- GEOGRAPHY. While the plateau exhibits diversity of scenery, toward the north, and contours are controlled, not clinoria made up of compressed anticlines and The Piedmont Plateau lies at the southeastern there are certain general features common to the by streamwork