the United States of America

Appalachian Ridge-and-Valley Province.

West of the Piedmont, Blue Ridge and New England provinces is a narrow belt of folded sedimentary rocks stretching from the St. Lawrence valley to the Gulf coastal plain. The length is i,000 m. and the width rarely 75 m., more often half that amount. Between New England and the Adirondacks it is very narrow. In its present state this belt consists of alternating ridges and valleys, the former being parallel, even-topped and rarely 3,00o ft. high. For the most part they are nearly straight and in line with the major belt but a zigzag pattern appears in Pennsylvania. The valleys occupy more than half the space. Throughout its length the Paleozoic sandstones, shales and limestones are folded and often faulted by great lateral compression which made mountains of great height. The ridges now seen are not those made by the folding. Those lay far above the level of the present mountain tops. Even the synclinal trough bottoms were above the present crests. All were eroded away to a peneplain covering this and ad jacent provinces. At the time of its making it could not have been, at highest, more than a very few hundred feet above the sea. The tilted strata came to the surface of this plain in parallel strips. Later came a general rise of the whole Appalachian region and the limestones and shales were worn down to a new and lower level (often a peneplain) while the sandstones, being stronger, along with the hard rocks of the Blue Ridge province and the sandstone-capped plateau on the west, retained more nearly their former level. The tops of the even-crested ridges indicate the old peneplain somewhat uniformly worn down.

The greatest breadth of this Ridge-and-Valley province is in Pennsylvania where it is almost 90 miles. From here to southern Virginia the eastern third or fourth is almost without ridges.

Its several parts from the Lebanon valley in Pennsylvania to the Shenandoah valley in Virginia are well known for their agricultural wealth. In this central portion the master streams cross the belt transversely, their longitudinal tributaries being developed on the softer outcrops. In the northern section of the province, Hudson-Champlain valley, and again in the southern section, Valley of East Tennessee and neighbouring States, the main streams are longitudinal. In the latter case this probably came about by the headward elongation of Tennessee river capturing the former streams which flowed from the Blue Ridge to the Ohio.

Appalachian Plateaux.

West of the Ridge-and-Valley prov ince is a plateau of almost equal length with a minimum width of 35 miles at the south but more than 200 miles wide farther north. On the side toward the Ridge-and-Valley province, it is at least as high as the ridges already described, 2,000 to 3,00o ft., but it declines north-westward and is barely r,000 ft. high where its edge approaches Lake Erie. Its rocks are almost horizontal, generally outcropping on the south-east in an escarp ment overlooking the adjoining valley of the folded province. Ex cept at the north end the rocks at the surface are Carboniferous and Permian. Thick beds of strong sandstone are common and generally cap the hills. Soils, therefore, are generally not good.

They are least fertile at the south end where the sandstones are most dominant, and best at the north end where Devonian rocks form the plateaux and glaciation has left its heterogeneous drift.

The larger part of the province is almost or quite completely dissected by stream valleys. These may approximate i ,000 ft. in depth on the higher south-east side, but the relief diminishes to ward the north and west. Nevertheless the hilltops are about at the same altitude and the horizon is nearly level. The Cumber land plateau at the southern end, mainly in Tennessee, is excep tional in preserving large patches of undissected upland on beds of strong sandstone. In parts of the eastern margin the beds are mildly folded though not enough to develop the topography of the next province to the east. The elevation is less uniform than elsewhere in this province and the horizon less level. As these are also places of greater height these districts are known as mountains, the Allegheny mountains in Pennsylvania, and the Cumberland mountains in Kentucky and Virginia, but these names are applied somewhat broadly, and without exact bound aries, to the deeply dissected eastern margin. The Catskill moun tains in south-eastern New York are like the adjacent plateau but covered by an additional thick plate of strong conglomerate now deeply and maturely dissected. The plateau in New York, and to some extent in Pennsylvania and Ohio, was glaciated, leaving the hills and valleys less angular; leaving also many lakes and displaced streams, some of which in carving new valleys have made picturesque falls and gorges. The main plateau ends at the north in an escarpment not far south of the 43rd parallel and 600 to i,000 ft. high. West of Rome, N.Y., this escarpment overlooks the lacustrine plain along Lake Ontario, a part of the central lowland. East of Schenectady the descent from the plateau is to the valley of Hudson river (Ridge-and-Valley province). Be tween these two cities, the descent is only to a lower bench of the plateau province. In this bench Mohawk river has cut its valley connecting the two lowlands named. As all strata here dip slightly southward, the valley of the Mohawk is a strike valley. It is the greatest avenue of travel and commerce between the Atlantic coast and the central lowland.

The glaciated northern portion of the Appalachian plateau is favoured agriculturally and is one of the leading dairy districts of the United States. The remainder of the province is almost co-extensive with the great Appalachian coal fields, 75,000 sq.m. in extent. As this area was never folded the coal remains bituminous. In the province to the east the folds were so high that the carbo niferous beds, being uppermost, were almost wholly eroded away. Only in a few small spots were the synclines so deep as to leave the coal below the base level of erosion. Here the coal was meta morphosed to anthracite. The total original area of the anthracite fields was 484 square miles. In the central part of the plateau province are also the great Appalachian oil and gas fields.