Bluegrass Land and Life

Land Character, Plants, and Animals of the Inner Bluegrass Region of Kentucky: Past, Present, and Future

Mary E. Wharton, Roger W. Barbour

• Publisher: The University Press of Kentucky
• Year: 2021
• Print ISBN:
  9780813155593
• eBook ISBN:
  9780813186795
• Topic: Social Sciences
• Subtopic: North American History
• Index: Social Sciences

Part I
Geology and Environmental History

1. Geological Background

NATURE’S ENDOWMENT of beauty and fertility in the Inner Bluegrass has a background of many millions of years. It results from a combination of rock type, geological structure, and geological history that accounts for the gently rolling, rich, and fertile upland into which the Kentucky River has cut a spectacular gorge. Geology is fundamental in determining all aspects of life. The type of rock and the regional topography it creates, together with climate, determine what vegetation will flourish, how much animal life the land will support, and whether a human community established there will be rich or poor.

The Inner Bluegrass is a portion of the Bluegrass Section¹ of the Interior Low Plateaus Province of eastern United States, according to Fenneman’s physiographic classification (1938). Fenneman’s Bluegrass Section includes the Inner Bluegrass, the Outer Bluegrass, and the intervening Eden Hills.

STRUCTURAL GEOLOGY, STRATIGRAPHY, AND SOILS

The Inner Bluegrass region encompasses approximately 2,400 square miles, occurring in portions of fourteen counties (Davis 1927). Its occurrence is determined by the outcrop of Middle Ordovician limestones deposited about 400 million years ago, the oldest in the state.² These limestones appear here and not elsewhere because the area is situated on the Jessamine Dome of the Cincinnati Arch, or Geanticline, which is the main axis of the uplift between northwest Alabama and Lake Erie. The dome has been beveled by erosion through long geologic ages, exposing the oldest strata at the point of greatest uplift. From the center of the dome, the beds dip gently away in all directions. Hence in a somewhat concentric pattern successively younger rock is encountered as one moves outward in any direction (Fig. 1, Structure Section).

The Inner Bluegrass is immediately surrounded by the “Eden shale belt” or “Eden hills,” an area of shaly hills of Upper Ordovician age. Outward from this occurs the Outer Bluegrass, also of Upper Ordovician age, containing more limestone than the Eden and hence more closely resembling the Inner Bluegrass.³ The term Bluegrass may refer to the total area of Ordovician outcrop in Kentucky: the Inner and Outer Bluegrass sections and the intervening Eden shale belt. The relation of the Inner Bluegrass to surrounding areas is shown in Map 5. The Outer Bluegrass is locally somewhat similar in topography to the Inner but has slightly more relief, less pronounced underground drainage, fewer sinks, and, with less phosphorus in the soil, a fertility less distinctive.

Figure 1. Structure Section across Kentucky from West to East.
Adapted from McFarlan (1958), 15.

The major faults are in the Kentucky River Fault Zone (discussed below, pp. 15-18). Two other significant zones of normal faulting are the West Hickman Creek Fault Zone and the Bryan Station Fault Zone, narrow belts in Jessamine and Fayette counties. Here younger (Upper Ordovician) shales dropped down millions of years ago between fractures in the rock formations and now lie alongside Middle Ordovician limestone. This has resulted in less fertile farmland than in the surrounding areas and in differences in present natural vegetation. It should be noted that Woodford is the only county that is virtually all Inner Bluegrass in the strict sense of the term: that is, with its surface and soils derived from Middle Ordovician limestones. Fayette and Jessamine counties would have been were it not for the Kentucky River faults, the West Hickman fault, and the Bryan Station fault, all of which brought down some of the Clay’s Ferry Formation. To see the exact extent and boundaries of the Inner Bluegrass, see Maps 1 and 2 (front and back endsheets).

Map 5. The Inner Bluegrass in Relation to Surrounding Physiographic Areas and to the Cincinnati Arch. From Palmquist and Hall, 5.

The oldest of the Middle Ordovician limestones are the Camp Nelson, Oregon, and Tyrone formations, composing the High Bridge group, all of which outcrop only in the vicinity of the Kentucky River and its immediate tributaries (Map 6). They are predominantly massive-bedded, cliff-forming, dolomitic limestones. The Camp Nelson, which is the oldest formation in the state, is a fine-grained, mottled limestone, 200-350 feet thick, containing some dolomitic beds; the Oregon, a calcareous dolomite, is 10-65 feet thick; and the Tyrone, the uppermost of the three, is a lithographic limestone 60-90 feet thick. In the early literature, the Tyrone was called the Birdseye limestone from the dark facets of calcite on the white surfaces of the weathered stone. The Oregon is less resistant than the other two, principally because of exfoliation in weathering; for this reason, creeks passing from the Tyrone to the Camp Nelson have waterfalls at the Oregon, and on the river bluffs there is usually a shelf at the top of the Camp Nelson.

The High Bridge group of formations are of Black River and lower Trenton age (McFarlan, 1943; Nosow and McFarlan). At the time of their deposition, Kentucky would have been a complex of warm carbonate tidal flats and intervening shallow marine lagoons, similar to the area around the Bahamas today, with the tidal flats shifting position from time to time. The deposition of fine lime sediment would have been at the rate of approximately 4 cm per 1,000 years; the High Bridge group, therefore, would represent about 5 million years (Cressman and Nager).

Map 6. Outcrop Area of the High Bridge Group.
From Cressman and Noger, 2.

The Tyrone formation contains several thin layers of bentonite (volcanic ash). The Pencil Cave layer, found 14-20 feet below the top, is present throughout and is composed of two or more ash falls; other layers occur locally. The volcanoes would have been located east of Kentucky in the Land of Appalachia.

The High Bridge group is overlain unconformably by the Lexington limestones deposited later in the Middle Ordovician. These bioclastic limestones, totalling about 300 feet in thickness, underlie most of the Inner Bluegrass. At the time of their deposition, the former mud flats were covered by a slightly deeper sea laying down coarsely crystalline limestone. The resulting formations contain more fossil shells and some thin layers of shale; all are thinner bedded than the High Bridge group. The Lexington limestone is correlated in age with the Trenton (McFarlan, 1943; Nosow and McFarlan).

The Lexington limestone is composed of eight members: Curdsville, Logana, Grier, Brannon, Tanglewood, Devil’s Hollow, Millersburg, and Nicholas. The Curdsville, Grier, and Tanglewood members, which are the highly phosphatic limestones, occur throughout the region. The others are local in extent; with the exception of the Nicholas limestone, these are argillaceous limestones or they contain some interbedded shale. Approximately two-thirds of the Lexington limestone is composed of the Grier and Tanglewood members. As now defined, the Lexington includes the former Cynthiana limestone (Black et al.). Its uppermost beds span the transition between the Middle and Upper Ordovician, and the Clay’s Ferry formation sometimes intertongues with the upper member of the Lexington.

Map 1 shows the extent of the Lexington limestone in the counties involved and hence delineates the boundary of the Inner Bluegrass. Some authors and some maps in the past have differed as to whether to include the “Cynthiana limestone” in the Inner Bluegrass. McFarlan (1943) so includes it, however, and since the present interpretation is to treat the “Cynthiana” as the upper two members of the Lexington, it is indeed a part of the Inner Bluegrass.

The soils derived from these Middle Ordovician limestones are dark brown silt loams. In extensive interstream areas with minimum soil erosion, deep residual soils of great fertility have formed. On the Bluegrass Plain, which is the area of Lexington limestone, the prevailing soil is the Maury silt loam, deep and well drained, on slopes of 2-12%; on slopes of 12-30% the soil is the McAfee. Lowell soils, with less permeability than the Maury, occur especially near the periphery of the Inner Bluegrass. In the vicinity of the Kentucky River gorge the excessively steep slopes above the river and its tributaries have a shallow and rocky soil, the Fairmont, on 6-50% slopes with rock outcrops. The deep soil of the river floodplain is the Huntington (USDA 1968 A and B). The Inner Bluegrass soils are essentially residual; the most extensive transported soils are on the flood-plains. The alluvium of the Kentucky River has had its source in sandstone and shale of the Cumberland Plateau as well as Ordovician limestone in the watershed. This should be noted when considering it as a botanical habitat, in terms of both geographical affinities and soil chemistry. The South Fork of the Licking River arises in the Inner Bluegrass and hence does not introduce extraneous material except a small amount from the Clay’s Ferry Formation on its sides downstream.

It should be emphasized that “Inner Bluegrass” as here defined refers only to the area of Middle Ordovician limestone outcrop and soils derived largely from the weathering of these rocks. Contained within the boundary of the area are pockets of other materials. The largest of these are in the Hickman Creek and Bryan Station fault zones. Examples of smaller tracts are Trumbo Bottom and the lowland surrounding Devil’s Backbone in Franklin County and Alton swamp in Anderson County, which are thick deposits of Pliocene alluvium, acid and poorly drained, in abandoned meanders or oxbow lakes of the ancient Kentucky River. From the standpoint of human geography, the people living in all of these areas can consider themselves living in the Inner Bluegrass since they are surrounded by Inner Bluegrass. But as Inner Bluegrass botanical habitats, these areas should be excluded, being islands of chemically and physically different materials producing growing conditions for plants different from those produced by the rock formations responsible for determining the Inner Bluegrass.

The fact that the soils are derived from limestone excludes acid-requiring plants, includes many lime-requiring plants as well as species having a wide pH range, and adapts most of the land to profitable agriculture. Filson (1784) reported that the land produced 100 bushels of corn per acre, and both Filson and Michaux (1802) reported that the land was too rich for wheat until it had been reduced by four or five years of corn cultivation. (We can add that the process also involved leaching from uncovered land through the winters.)

These formations in Kentucky are similar to others laid down in Middle Ordovician seas, but one significant difference is a much higher phosphate content, a feature that makes this area outstandingly adapted for livestock production. Late in the nineteenth century Dr. Robert Peter studied the influence of soil and underlying rock strata, and wrote that bluegrass grown in other regions does not yield the results it gives in this section. “The peculiar richness of our bluegrass pastures is not in the bluegrass per se but is dependent on the soil, which is abundantly supplied with the indispensable mineral elements of vegetable and animal nutrition” (Peter, 1882, 25). This statement was based on both observation and chemical analysis of the ash of several plant species grown here compared with the...

Table of contents

• Cover
• Half title
• Title
• Copyright
• Dedication
• Contents
• List of Maps, Figure, and Tables
• Foreword by John O. Simonds
• Acknowledgments
• Introduction
• Part I. Geology and Environmental History
• Part II. Habitats and Natural Community Organization
• Part III. Annotated Lists
• Part IV. The Future of the Bluegrass
• Appendixes
• Bibliography
• Index of Species
• General Index