CONTRIBUTIONS TO THE GEOLOGY OF KENTUCKY
FIGURE 18.--Physiographic diagram of Kentucky.
The physiography of Kentucky can be generalized as a series of dissected plateaus and gently rolling plains separated by scarps. Most of the regions are characterized by erosional landforms on gently dipping Paleozoic elastic or carbonate rocks. Karst topography and residuum of variable thickness from the weathering of carbonate rocks typifies the Inner Bluegrass and Mississippian Plateau regions. Deeply incised dendritic stream patterns and colluvium-covered mountain slopes are common features of the elastic rock sequences of the Cumberland Plateau and Western Coal Field regions. Westernmost Kentucky (in the Mississippi Embayment of the Gulf Coastal Plain) is a low-lying, dissected plain underlain mostly by Mesozoic and Cenozoic sand and gravel. This region is known as the Jackson Purchase.
The border areas between terrains underlain by elastic rocks and carbonate rocks are marked by escarpments. Abrupt changes in slope occur at these border areas because the rocks on either side erode at different rates. Resistant Lower Pennsylvanian sandstones cap the Pottsville Escarpment between the Cumberland Plateau on the east and the Mississippian Plateau and the Bluegrass on the west. A similar but less profound escarpment marks the physiographic boundary between the Western Coal Field and the Mississippian Plateau. Muldraughs Hill, another prominent escarpment developed on Lower Mississippian elastics, locally separates the carbonate terrain of the Mississippian Plateau from the carbonate terrain of the Bluegrass region.
The escarpments are deeply incised by headward-eroding streams. As the drainage network develops, the escarpment marking the boundary between two regions retreats. Outlying erosional remnants mark former limits of the scarps. The outliers commonly erode to a characteristic form known in Kentucky as "knobs." Belts of knobs are common at the base of all the major scarps, but they are most extensive and best known around the outer margin of the Bluegrass at the base of Muldraughs Hill. The belt of knobs surrounding the Outer Bluegrass is commonly described as a distinctive physiographic region called simply The Knobs.
Locally, the hilly borderland between the Mississippian Plateau and the Jackson Purchase has been known as "the breaks." Much of this region has been flooded by damming of the Kentucky and Cumberland Rivers, forming the area known as the Land Between the Lakes.
Most runoff from Kentucky drains into the Ohio River, which forms the northern boundary of the Commonwealth; some passes directly into the Mississippi River, which forms the westernmost border. The great valleys of the Mississippi and Ohio and the lower reaches of their tributaries contain terraces underlain by thick deposits of alluvium and glaciofluvial sediments.
The Kentucky Geological Survey has published several general studies of the topography and scenery of Kentucky (Jillson, 1927; McFarlan, 1943, p. 218-247, 1958). These publications offer largely archaic interpretations of erosion cycles and time-dependent landforms, but they are well-illustrated geographic overviews. The continental setting of Kentucky's physiography is presented in the National Atlas of the United States (U.S. Geological Survey, 1970, p. 59-63).
Generally, the knife-edge crests of the mountains are as narrow and sinuous as the valley bottoms. Flatlands on either the ridgetops or the valley bottoms are commonly of small extent. Most of the terrain is in steep slopes. Whether the local topographic relief of this region spans as little as 200 ft or exceeds 2,000 ft, the landforms are similar. The mountain slopes underlain by shale and sandstone are mantled by complex accumulations of rock fragments and weathered debris (colluvium) that move downslope by debris avalanche, landslide, creep, and sheet wash. Deeply weathered soils are uncommon and occur on isolated, nearly level ridge crests and high-level terrace deposits. Cliffs of resistant sandstone cap many ridges and spurs. Scenic erosion remnants include pinnacles or "chimneys," shallow eaves known as "rock houses," and arches or natural bridges.
Pine Mountain and Cumberland Mountain.--The sinuous pattern of ridges and ravines is interrupted along the southeast border of Kentucky by the northeast-trending linear ridges of Pine Mountain and Cumberland Mountain, which are generally included in the western part of the Valley and Ridge physiographic province. The steep northwest face of Pine Mountain exposes the Pine Mountain overthrust fault. The rocks on Pine Mountain dip to the southeast and form jagged hogback ridges. To the southeast, these same beds dip to the northwest and underlie hogback ridges of Cumberland Mountain. Between these structurally controlled ridges are the Black Mountains and the Log Mountains, which include the highest summit in Kentucky, 4,150 ft.
Bluegrass.--The Bluegrass region of Kentucky (Davis, 1927) is a gently rolling lowland that is coextensive with the outcrop of Ordovician and Silurian carbonates and shales exposed on the crest and flanks of the Cincinnati arch.
The region, which takes its name from the lush growth of native bluegrass, is subdivided into the Inner Bluegrass, mostly on the Lexington Limestone, and the Outer Bluegrass, mostly on limestones, dolomites, and shales of Late Ordovician and Silurian age. A transition zone between these two subregions, underlain by the Kope and Clays Ferry Formations, is known as the Eden Shale belt.
The Inner Bluegrass is characterized by very low relief, abundant shallow sinkholes, thick, phosphatic
residual soils of exceptional fertility, and sparse outcrops. The Eden Shale belt (or Eden Hills) is a band of
rounded hills and ridges of moderate relief that forms a belt from I to 30 mi in width around the Inner
Bluegrass (Davis, 1927, p. 23). Soils are thin, but the bedrock, mostly shale, is poorly exposed. The Outer
Bluegrass typically has low to moderate relief and soils that range from thick, over limestones, to thin, over
shales; dolomites of the Silurian are commonly well exposed. Silurian shales, with abundant swelling clays,
typically result in small, low-angle landslides and underlie zones of unstable slopes. The soils developed on
some Silurian carbonate rocks are nearly as rich as those of the Inner Bluegrass.
Rivers crossing the Bluegrass flow through meanders entrenched 200 to 300 ft below the plains and low hills. The river bottoms are narrow, sinuous, and confined by limestone cliffs and steep, wooded slopes covered with colluvium. These confined terraces and flood plains are narrow and discontinuous. River and stream valleys widen at their confluence with the Ohio Valley. Tributary terraces are graded to those of the Ohio. Along the Ohio Valley, steep ravines and bluffs descend from the Bluegrass plains to the river terraces.
The plains surrounding the entrenched meanders of the major rivers include high terraces of weathered river alluvium which predate rapid downcutting to the Ohio River's current base level (see description of high-level fluvial deposits in section on the Cretaceous and Tertiary Systems).
Although most Bluegrass topography is subtle, structural control of the drainage pattern is evident. The Licking River is locally entrenched along cuestas of the east flank of the Cincinnati arch. The Kentucky River follows the Kentucky River and Lexington fault systems and cuestas on the west side of the arch. The Salt River and its tributaries flow westward down the dip slope of the arch, and local tributaries are bounded by cuestas.
The Knobs.-Erosional remnants or outliers of the backwasting Pottsville Escarpment on the east and Muldraughs Hill on the south and west of the Bluegrass form The Knobs region (Burroughs, 1926). The region lies in a narrow belt 10 to 15 mi wide and marks the outer limit of the Bluegrass region. Elsewhere, other knobs occur along the outer margins of the Pottsville Escarpment between the Cumberland and Mississippian Plateaus and in front of the Dripping Springs Escarpment. Although solitary knobs rise above surrounding plains or valley bottoms, most occupy narrow interfluves between broad alluvial flood plains of the rivers and creeks dissecting a nearby escarpment. Trails of knobs extend like fingers from the continuous cliffs of the escarpment into the adjacent plains. As the distance from a highland rim increases, knobs become lower, more rounded, and farther apart.
Individual knobs are characterized by symmetrical concave-upward slopes which rise gently out of the bottomlands or surrounding plains. The slopes steepen upward into cliffs on knobs with resistant caprocks. Knobs that have lost their protective caps have rounded crests. Well-developed knobs may be nearly circular or elliptical in plan view.
Knob terrains generally develop where resistant caprocks overlie easily eroded shale and siltstone. As drainage systems work their way headward into a highland rim, streams cut once-continuous spurs and ridges of the caprock, creating incipient knobs. Mass wasting sculpts the hills into their characteristic symmetrical shapes. Thick colluvium on the lower slopes merges with alluvium on the valley bottoms. On Carbonate plateaus, thick colluvium around the bases of limestone knobs may be graded into sinkholes.
Mississippian Plateau.--The Mississippian (or Pennyroyal) Plateau of south-central and western Kentucky is an upland region underlain by Mississippian rocks, mostly limestones (Sauer, 1927). The plateau borders all of the other physiographic regions in Kentucky except the Bluegrass, from which it is separated by the narrow belt of The Knobs. On the east, the Cumberland River flows into the Mississippian Plateau from the Cumberland Plateau. The river valley includes broad, fertile bottomlands, cliffs, gorges, and knobs distributed around wide meander bends. On the north, the Mississippian Plateau forms the summit of Muldraughs Hill, the escarpment that bounds the belt of The Knobs and Bluegrass regions; it forms an arcuate outcrop belt of Mississippian rocks on the flanks of the Cincinnati arch. Part of the northern boundary of the Mississippian Plateau is formed by the Ohio River valley where steep bluffs descend to low river terraces. On the west, the boundary is a rim of low ridges and escarpments (Pottsville Escarpment of fig. 18) along outcrops of basal Pennsylvanian strata which mark the limits of the Western Coal Field region. The boundary is transacted by rivers originating on the Mississippian Plateau and flowing through the Western Coal Field to the Ohio River. On the westernmost boundary, the Mississippian Plateau forms highlands capped by basal Cretaceous and Tertiary gravels above the Cumberland and Tennessee River valleys, which border the Jackson Purchase region of westernmost Kentucky. Short, steep slopes are dissected by a dense network of tributaries to the two rivers. The Mississippian Plateau topography extends southward into central Tennessee.
The upland plain of the Mississippian Plateau, or Pennyroyal Plain area (Sauer, 1927, p. 36-45), is overprinted by complex patterns of sinkholes. Surface-runoff drainage systems are poorly developed because of the extensive network of subsurface channels eroded in the soluble limestone. The karst topography is characterized by knobs and glades (elongate depressions). Creeks disappear into sinkholes and reappear at lower elevations as springs in the glades. Extensive solution weathering of the limestones and dolomites has produced a mantle of insoluble residuum, including fragments of chert, which is locally tens of feet thick where transported and concentrated in sinkholes. Prior to modern agriculture, much of this terrain was known as the "Barrens," a pioneer term for grassland prairies.
The western part of the Mississippian Plateau is crossed by high-angle normal faults which strike nearly east-west (Rough Creek and Pennyrile fault systems) and a major intersecting fault zone trending to the southwest and disappearing beneath the Jackson Purchase (see fig. 16). The horsts and grabens of the faulted area in the western part of the Mississippian Plateau commonly exhibit inverted topography. The downdropped blocks of younger, resistant sandstone stand as linear ridges above valleys eroded in older carbonates of upthrown blocks.
Most of the Mississippian Plateau is drained by the Green River and its tributaries. The river valley and karst drainage network are subtly controlled by the regional dip of the limestone beds away from the Cincinnati arch and into the Western Coal Field region. The rugged topography of the boundary has developed by headward erosion by the Green River and its tributaries, updip through the westward-dipping strata of the karst area of the plateau. Collapsed topography, sinking streams, and extensive cave networks are common. Mammoth Cave occurs along the Green River and is part of the most extensive cave development in Kentucky. It occurs in strata that form an intensely dissected cuesta. The steep outward facing slopes of the cuesta are known as the Dripping Springs Escarpment.
The cuesta forming the Dripping Springs Escarpment (fig. 18) is composed of the St. Louis, Ste. Genevieve, and Girkin Limestones and is capped by the Big Clifty Sandstone Member of the Golconda Formation. The Dripping Springs Escarpment has been described by Dicken (1935) as a "solution cuesta" because it has formed by solution weathering of the limestone strata.
Western Coal Field.--The Western Coal Field region (Burroughs, 1924) is a hilly upland of low to moderately high relief dissected by streams occupying wide, poorly drained, and often swampy valleys. The hills and valleys define a system of cuestas and fault blocks in a structural basin filled with Pennsylvanian sandstones, shales, and coal beds cut by numerous eastwest-trending normal faults. The boundary with the Mississippian Plateau, locally known as "clifty" areas, is a narrow belt of sandstone ridges which form a relatively high and irregular rim about a lower interior. The border area is particularly rugged along the southern part of the Pottsville Escarpment, where the basal Pennsylvanian sandstones and conglomerates of the Caseyville Formation are thickest. The northern boundary of this physiographic region in Kentucky is the alluvial valley bottom of the Ohio River.
Rocks of varying resistance to weathering have been juxtaposed by many high-angle faults; these structures have had only a small effect on the drainage pattern. Major streams cross the region without apparent structural or stratigraphic control. The Western Coal Field is drained by the lower reaches of the Green River and its tributaries and by the Tradewater River. These rivers have generally eroded broad valley bottoms in shale. Flood plains are built upon alluvium and Quaternary lake sediments, deposited when the river mouths were dammed by glacial outwash in the Ohio Valley.
Rocks underlying the uplands are generally deeply leached and weathered. The weathered overburden is easily excavated, and on both hilltops and valley bottoms, the surface mining of thick, persistent coal beds has reshaped the topography of vast areas.
Jackson Purchase.--The Jackson Purchase (Davis, 1923) is a region of flat, low plains dissected by a dendritic network of low-gradient creeks and small rivers, situated at the head of the Mississippi Embayment. The region is bounded by the Mississippi River on the west, the lowermost reaches of the Ohio River on the north, and the Tennessee River on the east. Wide, terraced valley bottoms of these rivers are flanked by scarps cut into the low plains of the area.
The uplands are underlain by unconsolidated Mesozoic and Cenozoic sand, gravel, silt, and clay. Valley bottoms of the tributaries and major rivers are underlain by Quaternary alluvium. Erosional topography of the uplands is subdued by a mantle of windblown silt (loess) which decreases in thickness eastward. Oldest, highest elevations of the uplands are underlain by deeply weathered gravel. Extensive gully erosion has reworked much of the loess blanket and upland gravel into colluvium and alluvium which are deposited in and along the sides of smaller streams.
Valley bottom terraces of the Mississippi and Ohio Rivers are typically engraved with traces of old meander loops and oxbow lakes which resulted from the continuous migration of the river channels across their flood plains. Extensive floods are common, especially in the spring. Slumps along steep bluffs, sand blows on flood plains and low terraces, and Reelfoot Lake in nearby Tennessee created by the New Madrid earthquake (1811-12) are the products of historic seismicity (Russ, 1982; Obermeier, 1984).
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