fl d027-02 1st pgs page 19 The Geological Society of America Field Guide 27 2012 Bourbon and springs in the Inner Bluegrass region of Kentucky Ashley M. Barton Cory W. Black Eagle Alan E. Fryar Department of Earth and Environmental Sciences, University of Kentucky, 101 Slone Building, Lexington, Kentucky 40506-0053, USA ABSTRACT This fi eld trip explores the role of geology in the origins and production of a dis- tinctly American distilled spirit. Bourbon whiskey originated in the late 1700s and early 1800s in the Bluegrass region of north-central Kentucky. The Inner Bluegrass is marked by fertile, residual soils developed on karstifi ed Ordovician limestones. Corn was grown, ground, fermented, and distilled to yield a high-value product that would not spoil. The chemistry of limestone water (dilute calcium-bicarbonate type with near-neutral pH) limits dissolved iron and promotes fermentation. Many farms and settlements were located near perennial springs, whose relatively cool tempera- tures (~13–15 °C) facilitated condensation of steam during distillation. We will visit three historically signifi cant springs. Royal Spring in Georgetown was an early site of whiskey production and is one of the few springs in Kentucky still used for municipal water supply. McConnell Springs was the purported site of Lexington’s founding and occupies a karst window in which distilleries once operated. Cove Spring in Frank- fort was the site of the fi rst public water supply west of the Allegheny Mountains. We will also tour two distilleries: Woodford Reserve (among the oldest and smallest in the state, and a National Historic Landmark) and Four Roses (listed on the National Register of Historic Places). INTRODUCTION the geologic and hydrologic setting of the Inner Bluegrass and the history of bourbon production. A case study illustrates how Karst terrain, which is formed by the dissolution of soluble the stable isotope oxygen-18 differs among various bourbons bedrock (primarily limestone), is characterized by sinkholes, and how bourbon becomes systematically enriched in the isotope sinking streams, caves, and springs. Approximately 15% of the during aging. Finally, we summarize each fi eld-trip stop. contiguous United States (Aley, 1984) and 55% of Kentucky (Currens, 2002) are underlain by soluble rocks. Many communi- REGIONAL GEOLOGY ties in karst terrain formed around springs, which provided water for various purposes, including potable use, milling, and cooling. The Bluegrass region of Kentucky (Fig. 1) is bounded to These uses were integral in the development of bourbon whis- the west and south by the Mississippian Plateau, to the north by key in the Bluegrass region of Kentucky. This chapter reviews the Ohio River, and to the east by the Eastern Kentucky Coal Barton, A.M., Black Eagle, C.W., and Fryar, A.E., 2012, Bourbon and springs in the Inner Bluegrass region of Kentucky, in Sandy, M.R., and Goldman, D., eds., On and around the Cincinnati Arch and Niagara Escarpment: Geological fi eld trips in Ohio and Kentucky for the GSA North-Central Section Meeting, Dayton, Ohio, 2012: Geological Society of America Field Guide 27, p. 19–31, doi:10.1130/2012.0027(02). For permission to copy, contact [email protected]. ©2012 The Geological Society of America. All rights reserved. 19 fl d027-02 1st pgs page 20 20 Barton et al. Field of the Appalachian basin. The Inner Bluegrass is defi ned est elevations, at ~1100 ft (330 m) above sea level (asl), occur as the 30% of the Bluegrass region that is underlain by the rela- in the central portion of the area. The lowest elevations, which tively fl at-lying Lexington Limestone (McFarlan, 1943), which is occur along the Kentucky River in the southern and western Upper Ordovician in age (Clepper, 2011). The Inner Bluegrass is extremes of the area, range from 469 to 549 ft (143 to 167 m) separated from the Outer Bluegrass by the Bluegrass Hills, for- asl (McGrain and Currens, 1978). Locally, topography varies merly known as the Eden Shale belt. Fertile, residual soils (e.g., according to the rock units exposed at or just below the surface. phosphatic silt loams) and numerous karst features have formed Topography formed on the Clays Ferry Formation consists of on the Lexington Limestone (Hendrickson and Krieger, 1964; narrow, steep-sided ridges with narrow, V-shaped valleys and McDonald et al., 1983; Newell, 1986). In the Inner Bluegrass, the dendritic drainage patterns. Thin limestone slabs cover steep Lexington Limestone rests on the underlying limestones of the slopes formed in shales in many places. In the lower part of High Bridge Group. Above the Lexington Limestone is the Clays the Clays Ferry Formation, more gently to moderately rolling Ferry Formation, which is composed of interbedded shales and uplands prevail, with small sinkholes and localized underground thin limestone layers. Table 1 shows the generalized stratigraphy drainage in the limestone units. The topography formed on the of the Lexington Limestone and subjacent and superjacent units. upper part of the Lexington Limestone consists of broad, fl at Most of the Inner Bluegrass consists of a gently rolling, valleys, while that formed on the lower part tends to consist of karst landscape dominated by fl uvial landforms. Relief is typi- rolling to dissected uplands. Sinkholes and well-developed sub- cally less than 160 ft (50 m) (Thrailkill et al., 1982). The high- surface drainage (solution conduits) are common. Bench-like Figure 1. Inner Bluegrass region of Ken- tucky (from Paylor and Currens, 2002). map area Kentucky fl d027-02 1st pgs page 21 Bourbon and springs in the Inner Bluegrass region of Kentucky 21 topography is formed on the more resistant shale, bentonite, and and a few meters above it. The Tyrone Formation is exposed in limestone layers along hillsides and valleys. the upper elevations of the cliffs and along the lower elevations The Kentucky River fl ows along the southern and western of tributaries. The upper few meters of the Tyrone and the over- edges of the Inner Bluegrass through a deep gorge known as the lying Curdsville Member of the Lexington Limestone form more Palisades, which represents incision by the river of ~330 ft (100 m) gentle slopes extending into the uplands. At least three levels of the below the adjacent land surface (Currens and Paylor, 2009). The Kentucky River have been identifi ed near its current location. An topography formed on the High Bridge Group typically consists age of ~1.5 million years for this stage of the river is suggested by of cliffs and steep slopes along the Kentucky River and its tributar- radiometric dating, whereas gravel deposits and terraces indicate ies. Numerous grikes (sub-vertical to vertical joints that have been both a relatively young stage of the river at the lowest levels of its enhanced by dissolution) occur within the walls of the gorge. The current valley, as well as an earlier, sub-upland stage of the river Oregon and Camp Nelson Formations only crop out at the river before incision into the plateau (Andrews, 2005). TABLE 1. GENERALIZED STRATIGRAPHY OF THE LEXINGTON LIMESTONE SHOWING APPROXIMATE EQUIVALENCES WITH MAJOR LATE ORDOVICIAN CHRONOSTRATIGRAPHIC UNITS Clays Ferry Formation Upper Tongue of the Tanglewood Member Strodes Creek Edenian Millersburg Member Millersburg Tongue of the Tanglewood Member discontinuity Devils Hollow Member Middle Tongue of Sulphur Well Member the Tanglewood Member Stamping Ground Member Lexington Limestone discontinuity Brannon Member Brannon Tongue of the Tanglewood Member Shermanian discontinuity Perryville Member Lower Tongue of the Chatfieldian Caradocian Tanglewood Member Upper Ordovician Grier Member Logana Member Kirkfeldian Curdsville Member Rock Trenton-Maquoketa Unconformity landian Tyrone Formation Oregon Formation High Bridge Group Turinian Stage Camp Nelson Limestone Blackriverian Blackriverian Note: F.R. Ettensohn, Department of Earth and Environmental Sciences, University of Kentucky, personal communication, 2011. fl d027-02 1st pgs page 22 22 Barton et al. REGIONAL HYDROLOGY higher gradient. Flow directions in the groundwater basins often have little or no relationship to fl ow in the surface streams above. The majority of the Inner Bluegrass is drained by the Ken- The potentiometric surface in the Lexington Limestone is rep- tucky River and its tributaries. All surface drainage in the region is resented by the water surface in the larger conduits where fl ow ultimately confl uent with the Ohio River. The climate is temperate, occurs under equilibrium conditions (Thrailkill, 1985). As such, with warm, humid summers, moderately cold winters, and no dis- it is very irregular in both lateral and vertical extent. tinct wet or dry seasons. The Kentucky River basin receives 46 in Although subsurface overfl ows between groundwater basins (117 cm) of precipitation on average annually, of which ~18 in are known to exist (Currens et al., 2002), interbasin subsurface (46 cm) contributes to surface water (Andrews, 2005). fl ow generally appears to be limited to small conduits in the epi- Surface and subsurface drainages are integrated through karst zone that are shallow and oriented parallel to land surface. sinkholes, conduit networks, and springs (Fig. 2). Dye-trace stud- Flow follows the topographic slope to emerge at small, high- ies by numerous workers in the Inner Bluegrass have delineated level springs that tend to be ephemeral. These interbasin areas groundwater drainage basins that discharge at springs, as mapped form part of the catchment areas for large springs draining the by Currens et al. (2002, 2003). These basins are typically <6 mi2 groundwater basins. Surface streams and high-level springs are (15 km2) in area, but can be as large as 25 mi2 (65 km2) (Thrailkill commonly perched on shaly and argillaceous facies within the et al., 1982; Currens et al., 2002, 2003). Dye-trace results sug- Millersburg and Brannon Members of the Lexington Limestone gest geologic control of the groundwater basins, with overall fl ow (Thrailkill et al., 1982).