Ridge and Valley Province—The Ridge and Valley province of the Appalachian region, some 50 to 75 miles wide, lies between the Blue Ridge Mountains and the Appalachian plateaus, to the east and west, respectively. There is an exception in the extreme south, where the Blue Ridge is absent, and the Ridge and Valley province lies adjacent to the Piedmont foothills.
In the Ridge and Valley province, conglomerates, shales, and sandstones predominate, although seams of softer limestone occur among the rock formations. These strata have been subjected to lateral pressures that built them into long, narrow, rather even-crested ridges which, in turn, are separated by narrow valleys. Sandstone is exposed at the ridges, while the valley rocks are limestone and shale. The ridges are more pronounced in the northern section; to the south they may be so indistinguishable as to appear as a great valley when viewed from afar.
Some physiographers refer to the low gradients and fertile soils found in a nearly connected chain from Pennsylvania to Alabama as the Great Valley or the Appalachian Valley. These are the rich lands that long ago attracted migrants and settlers for agricultural enterprises.
Most soils of the valleys, especially the Great and Appalachian valleys, being derived from limestone, are therefore alkaline in reaction. They are fertile. Intense weathering, however, may encourage acidification of the soil, even if of limestone origin. Where shale is the weathered mantle, soils are less fertile, less well-drained, and shallower. Still shallower are the soils along the ridges. Often high in stone fragments, the relative proportion of sand (in contrast to the silt and clay fractions) in these soils depends on the parent material from which they are derived.
American chestnut was likely more abundant in the Ridge and Valley Province than in any other southern zone prior to its demise as an important tree of commercial value due to the chestnut blight fungus. The rich valley soils are now mostly in agriculture. Only culled-over remnants of the valley forest remain, while vast tracts of bottomlands and their adjacent valley soils have been inundated by power- and flood-control projects.
Appalachian Plateaus—Within the Southern Appalachian Mountains are the Appalachian Plateaus, a geological uplift without the lateral pressures necessary to build strongly folded forms. Instead, the rocks are nearly horizontal. Elevations almost everywhere extend above those of the Ridge and Valley province. The region is bounded on all sides by out-facing escarpments of dissected mountain fronts. The eastern boundary, from West Virginia to Alabama, is a prominent escarpment some 500 to 1000 feet in height, while the western boundary, from the Ohio River to Alabama, is a minor tableau cut by streams into promontories and buttes.
The part of the Appalachian Plateaus extending from the Kentucky River northward bears the local name Allegheny Plateau. The easternmost escarpment is called the Allegheny Front or, where strongly dissected, the Allegheny Mountains. The southern sector is often cataloged as the Cum-
berland Plateau, the Cumberland Front having formed to the east, and the Cumberland Mountains occurring where the cutting of terrain by rivers has been most severe.
Interior Low Plateaus—The Interior Low Plateaus of Kentucky and Tennessee also lie within the Southern Appalachian Mountain chain, west of the Appalachian Plateaus. Most of the Highland Rim, one of these plateaus, is underlain by cherty limestone, which weathers into stony soil. Although chert fragments help retard erosion, soil productivity is low. The best soils of the Interior Low Plateaus are in the Nashville and Blue Grass basins where nutrient-laden parent material has weathered into deep, slightly acid, fine-textured soils.
Formed along the summit and flanks of the Cincinnati Arch, the low plateaus form a broad upwarp approximately parallel to the Appalachian Mountains. Geologists say the summits of the arch, originally domed in two places, were eaten away by erosion to produce topographic depressions like the Bluegrass Plain around Lexington, Kentucky, and the Nashville Basin.
Surrounding the Nashville Basin is an extensive plateau at elevations of 700 to 1300 feet and, until cultivated, supported prairie grasses. Around the limestone outcropping of the gently rolling Lexington Plain is a belt of steep hills formed on shales. This area, too, is now largely farmed.
The calcareous origin of the soil of the Interior Low Plateaus often excludes pine forests while encouraging the growth of calcium-loving eastern redcedar. Nearly 80% of the present forests of the region are classed as oak-hickory types, a figure obscuring the great variety of species found there. The original forests of these plateaus have been described as transitional between the mixed mesophytic of the Appalachian Plateaus and the oak-hickory types of the prairie transition zone to the west. In this situation, numerous major vegetative components formed a mosaic in response to local environmental factors. Cavernous limestone underdrainage, called karst, for example, favors grasses over trees, giving rise to open savannas like the so-called barrens of Kentucky, the Blue Grass Region, and the Nashville Basin. From the Nashville Basin westward, lower slopes afford sites on which more-valuable hardwood species develop into well-formed stems, while upper slopes and ridges produce stands of xeric species like post and blackjack oaks, as they did in the virgin forest.
The level and fertile land of the plateaus encouraged farmers to clear the forests for agriculture. Where not cleared of timber, valuable black walnut and eastern redcedar have been severely overcut, and these trees then replaced by scrub hardwoods. Pine forests predominate in the Highland Rim south of the Nashville Basin, in the western coal fields of the Ohio River, on abandoned farm lands, and on both reclaimed and unreclaimed banks of surface-mined soil.
The climate of the Interior Low Plateaus is not markedly different from that of the Appalachian Plateaus. Rainfall amounts to 40 to 55 inches annually, and the growing season lasts almost 200 days.
Beech Gaps and Balds—Beech gaps, small islands of gnarled, broken, deciduous trees in dense stands with sharply marked boundaries, occur above 5000 feet in the Southern Appalachians.15 The persistence of American beech in these south-facing slopes and in slight dips in ridges that lie east-west has been attributed to the species' ability to withstand strong winds that funnel through gaps in the mountains, even when calm weather prevails elsewhere. Surrounding the beech gaps, forests of spruce and fir trees deposit deep layers of acidic foliage relatively low in nutrients. In contrast, the thin, less-acidic leaf mold under the beech trees, rich in plant nutrients, stimulates decomposition of more recently deposited foliage by micro- and macro-organisms. This decomposing activity leaves humus layers thinner than under adjacent spruce and fir forests. The soil pH, too, under the beech trees usually measures a unit above that of the conifers in these high mountain zones.
Balds high in the Appalachian Mountains occur as treeless, dome-shaped summits on well-drained sites with deep soils at elevations above 4000 feet. Why the balds remain treeless can only be conjectured: Neither fire, browsing wildlife, freezing temperatures, wind, shallowness of soil, influence of ancient burial grounds, nor postglacial climatic fluctuations seem to be the cause of the phenomenon.
On abandoned farms within the Southern Appalachian Mountains are small barren areas called galled spots, where old-field plant succession has failed to develop. Inadequate phosphorus could be the cause, preventing the invasion of nitrogen-fixing legumes often needed for normal plant succession.
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