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Carbonates and Evaporites ISSN 0891-2556Volume 28Combined 1-2 Carbonates Evaporites (2013) 28:13-21DOI 10.1007/s13146-013-0157-2

Hydrogeologic and topographic controlson evolution of karst features in Illinois’sinkhole plain

Samuel V. Panno, Walton R. Kelly, JulieC. Angel & Donald E. Luman

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ORIGINAL ARTICLE

Hydrogeologic and topographic controls on evolution of karstfeatures in Illinois’ sinkhole plain

Samuel V. Panno • Walton R. Kelly •

Julie C. Angel • Donald E. Luman

Accepted: 8 February 2013 / Published online: 9 March 2013

� Springer-Verlag Berlin Heidelberg 2013

Abstract In the sinkhole plain of southwestern Illinois,

the size and morphology of cover-collapse sinkholes can be

used as an indicator of the size of the associated underlying

crevice/conduit system. Sinkholes that lead to relatively

small crevices and small conduit systems (sinkhole vol-

umes [103 m3, but \105 m3) typically are found in areas

with relatively high water tables and, based on storage

capacity of conduits and secondary porosity of adjacent

bedrock, are incapable of accepting large, rapidly inflowing

volumes of surface-water runoff without backflooding.

Once flooded, sinkholes become low-energy environments

and sites of deposition of fine sediments; consequently,

their growth potential (via erosion by runoff within and at

the margins of the sinkhole) is limited. Conversely, in those

areas where sinkholes lead to relatively large, branchwork-

type cave systems, water tables are typically deeper and the

storage capacities of the cave system and adjacent bedrock

can be one or more orders of magnitude greater. Because

these large cave systems can accept greater volumes of

rapidly inflowing runoff without backflooding, the

sinkholes associated with large cave systems typically

become sites of extensive erosion. The aerial extent of a

sinkhole draining to large cave systems may continue to

increase and incorporate other sinkholes within its drainage

area, thereby creating large, compound sinkholes up to

30 ha in area. These sinkholes can have direct or indirect

connections to cave passages and have volumes that can be

as small as 104 m3 and as large as 106 m3. Beneath the

glacial till and loess cover, and in the vicinity of the largest

cover-collapse sinkholes, irregularities in bedrock topogra-

phy appear to have had an effect on the location of the ini-

tiation of large caves in the sinkhole plain. The coincidence

of bedrock depressions and swales in close proximity to the

initiation point of the large caves suggests that focused

recharge to fractures leading to bedding-plane partings in

bedrock played a major role in the initiation and evolution of

the large branchwork-type caves of Illinois’ sinkhole plain.

Keywords Cover-collapse sinkhole � Cave formation �Solution-enlarged crevices � Erosion

Introduction

A systematic study of the size and morphology of sinkholes

with respect to openings in the underlying geology (i.e.,

crevices and conduits) has been conducted by few

researchers (Palmer 1969). In the course of mapping and

characterizing cover-collapse sinkholes in southwestern

Illinois’ sinkhole plain, sinkholes in this area were

observed to exhibit distinct characteristics that reflect the

size and character of the underlying conduit systems within

the St. Louis Limestone. Similar observations were made

by Palmer (1969, 2007) in Indiana in the vicinity of Blue

Springs Cave, also in the St. Louis Limestone. Palmer

S. V. Panno (&) � D. E. Luman

Illinois State Geological Survey, 615 E. Peabody Drive,

Champaign, IL 61820, USA

e-mail: s-panno@illinois.edu

D. E. Luman

e-mail: dluman@illinois.edu

W. R. Kelly

Illinois State Water Survey, 2204 Griffith Drive,

Champaign, IL 61820, USA

e-mail: wkelly@illinois.edu

J. C. Angel

Parkland College, 2400 W. Bradley Ave.,

Champaign, IL 61821, USA

e-mail: jangel@parkland.edu

123

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DOI 10.1007/s13146-013-0157-2

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examined the depth of sinkholes overlying and in close

proximity to the cave and found that the deepest sinkholes

were located over the main stream passage of Blue Springs

Cave.

Similarly, the bedrock topography of Illinois’ sinkhole

plain may have played a major role in the formation of

large branchwork-type cave systems along bedding planes

in southwestern Illinois. Branchwork caves typically begin

forming where fractured, soluble bedrock is exposed or lies

beneath soil cover where recharge waters are most

aggressive (Palmer 2007). Recharge waters enter the

fractures, infiltrate the bedrock, and slowly enlarge

numerous fractures (initially and most intensely near their

point of entry). Solutionally enlarged fractures tend to

become narrower with depth and along flow paths. At some

point in their enlargement and subsequent cave develop-

ment, the largest pathways grow the fastest and ultimately

became the major flow paths (Palmer 2007). Preliminary

work by the authors suggested that large-scale swales and

depressions in the bedrock may have been focal points for

recharge that were responsible for the initiation and growth

of the large caves.

For this investigation, the authors took a holistic view of

Illinois’ sinkhole plain and examined the relations among

the size and morphology of sinkholes (including sinkhole

axis orientation); sinkhole distribution and densities; the

distribution of ponded sinkholes, springs, and swallow

holes; the degree of localized limestone dissolution; and

bedrock topography. The objectives of this investigation

were to characterize the relations among these observations

and explore the possible mechanisms responsible for such

relations. In addition, the underlying bedrock within the

groundwater basins of four of the largest caves in the

sinkhole plain was examined to explore the relations

among sinkholes and bedrock surface depressions and

swales near initiation points of the caves.

Geology and hydrogeology

Illinois’ sinkhole plain is on the western margin of the

Illinois Basin where Mississippian-age carbonate bedrock

dips gently several degrees to the east toward the center of

the basin (Fig. 1). This region is referred to as Illinois’

sinkhole plain because of its high density of cover-collapse

sinkholes (Panno 1996) and is part of the Salem Plateau

Section of the physiographic provinces of Leighton et al.

(1948). Over 10,000 sinkholes with densities as high as

95/km2, numerous large springs and the largest caves in the

state are all located in the sinkhole plain (Angel et al. 2004;

Panno et al. 2008b, c, d). The upland area is covered, for

the most part, by a relatively thin layer of loess and Illi-

noisan glacial till and residuum that is typically 0–15 m

thick with an average thickness of about 10 m (Piskin and

Bergstrom 1975). This material, especially loess, is easily

eroded and forms steep embankments and gullies.

Most karst bedrock and caves in the study area occur in

the Mississippian-age St. Louis, Ste. Genevieve, and Salem

Limestones. Cover-collapse sinkholes overlie these rocks,

producing the widespread karst terrain in southwestern

Illinois (Panno et al. 1997; Weibel and Panno 1997).

Geologic structures also appear to play a role in cave

development in the sinkhole plain. Specifically, the

Waterloo-Dupo Anticline, Columbia Syncline, Valmeyer

Anticline, and Monroe City Syncline occur in the study

area and trend northwest-southeast (Nelson 1995), and

three of the largest known caves in Illinois developed

parallel to and just northeast of the axis of the Valmeyer

Anticline (Panno et al. 2008b, d).

Methods

Cover-collapse sinkholes were identified on 7.5 min, US

Geological Survey (USGS) topographic maps and on aerial

photographs from 1940 and 2005. All data were uploaded into

an ARC-GIS� format and examined (Panno and Luman

2012). The morphology, size, and distribution of cover-col-

lapse sinkholes were compared to existing cave maps for four

major caves in southwestern Illinois (Stemler Cave, Krue-

ger’s Cave, Illinois Caverns, and Fogelpole Cave). Bedrock

topography in the vicinity of the cave systems (Panno et al.

2008a, e, f) was examined and cross sections were drawn

along the axes of the groundwater basins using well logs from

the Illinois State Geological Survey’s database. In addition,

water table maps of the sinkhole plain were prepared in the

regions of the large caves and compared to those in areas with

no known caves. It was found that water tables were from 5 to

15 m or more deeper in the vicinity of large cave systems than

in areas of no known cave systems (Panno, Illinois State

Geological Survey, unpublished data).

Characterization of sinkhole morphology, crevice and cave

size and morphology was done by inspection and exploration

of sinkholes between the early 1990s and 2010. Some crevices

and caves beneath sinkholes were accessible and others were

exposed during excavations by local landowners and following

major recharge events. In the sinkhole plain, over 1,000 sink-

holes and sinkhole drains and over 250 crevices and caves were

inspected in the field, and over 13,000 sinkholes were identi-

fied, examined, and cataloged using 1940 and 2005 aerial

photographs (Panno and Luman 2012).

A cumulative probability plot was made of the sinkhole

volume for 254 sinkholes throughout the state of Illinois to

study the distribution of sinkhole size, as it relates to

underlying conduit size. The technique was developed by

Sinclair (1991) for identifying thresholds between back-

ground and anomalies in searching for ore deposits. Panno

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et al. (2006) adapted the same technique for groundwater

contaminants (such as nitrate), and it was also used here to

separate the different populations of sinkholes based on

their volume. Only sinkholes in karst areas of Illinois with

similar soil types and soil thicknesses were considered in

the calculations.

Results and discussion

Cover-collapse sinkholes are larger and more abundant in

Illinois’ sinkhole plain than anywhere else in the state

(Panno et al. 1997). The largest cover-collapse sinkholes

are found within the groundwater basins of the largest

known cave systems in southwestern Illinois. One sinkhole

may cover as many as 30 ha (Panno and Luman 2012) and

the watershed may contain complex ephemeral stream

systems (active only during large recharge events) that

converge at sinkhole drains (Fig. 2). The coincident loca-

tions of the large sinkholes and the large caves suggest a

possible causal relationship.

During most of the year, the surface of the water table in

the sinkhole plain may be seen in small and large caves and

within bedrock at the bottom of sinkholes that have been

Fig. 1 Map of Illinois’ sinkhole

plain showing groundwater

basins for four large caves and

cross-section locations

(modified from Panno et al.

2001). A–A0 Stemler Cave, B–B0

Krueger’s Cave, C–C0 Illinois

Caverns, D–D0 Fogelpole Cave

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excavated for stabilization purposes. Groundwater path-

ways or conduits typically develop in a portion of the

vertical fractures and intersected bedding planes within the

Mississippian-age limestone. The water table is shallowest

at the headwaters of groundwater basin and along

groundwater divides, where sinkhole ponds are most

plentiful. The water table is deeper, as would be expected,

in the vicinity of the larger caves (Panno, Illinois State

Geological Survey, unpublished data). However, because

the location and size of all cave passages in the study area

are not known, a more rigorous statistical analysis is not

possible at this time.

It is hypothesized that the drainage area of a sinkhole is

roughly proportional to the size and storage capacity of the

underlying conduit system and solution-enlarged crevices

of adjacent bedrock. That is, the conduit system underlying

large, compound sinkholes must be relatively large to

accommodate the volume of runoff that would flush

through the sinkholes during intense rainfalls over

relatively short periods (\24 h). This ability to move large

volumes of water rapidly is, in part, due to the deeper water

tables created by caves. Conversely, the conduit system

and associated crevices of the underlying smaller, simple

cover-collapse sinkholes typically have a relatively shallow

water table. These crevices can accommodate only rela-

tively small volumes of rapidly inflowing recharge (Fig. 3).

The net effect is to produce a ponded, depositional envi-

ronment in flooded sinkholes, thus preventing them from

enlarging. The deeper water table of the large cave systems

rises less quickly during recharge events and sinkholes

connected to large caves do not often flood (sinkholes

associated with Fogelpole Cave and Illinois Caverns never

flood) due to the greater storage of the caves systems. The

rapid movement of water through sinkholes draining into

large caves produces an erosional environment promoting

expansion of the sinkholes.

The volume of water that can pass through a conduit

system is controlled by hydrologic factors that include

Fig. 2 Cover-collapse

sinkholes in the vicinity of

mapped cave passages of one of

the largest caves within the

sinkhole plain (from USGS

7.5 min quadrangle map and

Panno et al. 2004) are

exceptionally large in aerial

extent and have the appearance

of stream valleys. These may be

pirated streams that have since

disappeared following

enlargement of the underlying

cave. Sinkholes farther away

from the cave passages,

particularly to the south, tend to

be smaller and circular to

elliptical in plan view

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hydraulic gradient, hydraulic conductivity, matrix storage

of adjacent bedrock, cross-sectional area of the conduit,

and factors associated with turbulent flow regimes. The

size of a cover-collapse sinkhole is controlled by erosion of

the loess and glacial till mantle. As a sinkhole grows, the

increasingly larger capture zone causes it to receive more

runoff with greater energy. High-energy runoff into a

sinkhole can cause erosion of the margins and interior of

the sinkhole via gullying, resulting in over-steepening of

the walls that will eventually collapse (Fig. 4). This ero-

sional growth is limited by the rate at which the conduit

system can dispose of the incoming sediment-laden water

discharged through the conduits systems. In addition, the

larger influx of surface water into the cave system can

result in a more rapid enlargement of the underlying cave

by dissolution and abrasion (Palmer 2007); this combina-

tion appears to be responsible for the concomitant

enlargement of both the overlying sinkholes and the

underlying cave systems in the sinkhole plain. For exam-

ple, the volume of Illinois Caverns, located in the study

area, was estimated based on length and ceiling height and

passage width, to be approximately 2.83 9 105 m3. For

sinkholes feeding a solution-enlarged crevice system,

pathways may flood completely during a recharge event

slowing down recharge of the sediment-laden water to the

underlying aquifer. This lower energy environment within

the sinkhole (an ephemeral sinkhole pond) would more

likely be depositional than erosional and be less likely to

promote sinkhole growth. In fact, if deposition is great

enough, the sinkhole drain and portions of the underlying

crevice, or both, may plug with sediment and debris and

create a temporary or permanent sinkhole pond; these

ponds are most commonly found along margins of

groundwater basins within the sinkhole plain. For example,

crevice-controlled sinkholes draining an area similar to that

drained by Illinois Caverns and assuming three, 20 cm

wide crevices about 3 m apart and 9.65 km long, and 5 m

of vertical unsaturated space from the base of the sinkholes

to the top of the water table could accommodate

2.7 9 104 m3 of water. This is an order of magnitude lower

than the volume of water that could be accommodated by

Illinois Caverns.

A number of sinkholes in the sinkhole plain that had

been excavated prior to their stabilization were physically

examined with a standpipe. Sinkholes 60–120 m in diam-

eter tended to be nearly circular and typically had an

underlying crevice ranging from 15 to 45 cm wide with a

localized cavity roughly a meter across and 1–2 m deep.

Very large irregularly shaped sinkholes ([120 m in

diameter) often had more continuous crevices large enough

to crawl into leading to small to large caves with active

cave streams.

Measurements of sinkhole volume were conducted in

the field along transects following roadways that crossed

karst topography in 11 counties in Illinois including the

sinkhole plain using a range finder and an inclinometer in

1998 (Panno and Weibel, Illinois State Geological Survey,

unpublished data). The volumes of each of 254 sinkholes

were calculated and used in cumulative probability plots to

identify threshold values separating different sinkhole

Fig. 3 Idealized cross section

of two types of cover-collapse

sinkhole drainage in a covered-

karst terrain. The sinkhole

draining into a large cave (left)remains open during recharge

resulting in continuous erosion,

whereas the sinkhole draining

into a small cave or crevice

(right) can backflood during

recharge events resulting in

deposition of sediments within

the sinkhole

Carbonates Evaporites (2013) 28:13–21 17

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populations (Panno et al. 2006). Threshold values between

separate populations were found at approximately 102, 104,

and 105 m3. Sinkholes with the smallest volumes

(\102 m3) were found in northern Illinois in sediment

overlying crevice-dominated Silurian-age dolomite. Sink-

holes with the greatest volumes ([105 m3) were found in

Monroe and St. Clair Counties in the heart of the sinkhole

plain in sediment overlying Mississippian-age limestone.

Within this population, the sinkholes with the largest vol-

umes ([106 m3) were found within the groundwater basins

of Stemler Cave and Fogelpole Cave. The volumes of

sinkholes in groundwater basins of caves in Illinois were at

least 103 m and up to 106 m3 for areas overlying the largest

caves and up to 105 m3 for areas overlying smaller caves.

Sinkhole volumes for crevice-dominated sinkholes in the

sinkhole plain are typically [104 m3, but \105 m3. From

this, it was concluded that the thresholds indicate that,

while there is considerable overlap, crevice-dominated

sinkholes of the sinkhole plain have volumes[103 m3, but

\105 m3, whereas cave-dominated sinkholes (with direct

or indirect connections to cave passages) have volumes that

can be as small as 104 m3 and as large as 106 m3.

During our field work, we observed an example of a

sinkhole whose growth was limited in size by a rela-

tively narrow underlying crevice. A stream that origi-

nally flowed into the entrance of a large cave system

within the sinkhole plain was pirated by a circular, 60 m

diameter, 5 m deep sinkhole after being diverted by a

fallen tree. Changes to the sinkhole were monitored for

2 years, during which time both its sudden growth and

subsequent sedimentation within the sinkhole were

observed. Within 2 months, following the diversion of

the stream into the sinkhole, the sinkhole deepened from

5 to 10 m and its boundaries widened by about 10 m as

a result of slumping of the side walls (Fig. 4). The width

of the crevice draining the sinkhole ranged from 15 to

60 cm (based on visual inspection at times when the

crevice was exposed). The water table appeared to be

located about 1.5–2 m below the sediment–bedrock

interface at the base of the sinkhole during low-flow

conditions. While the sinkhole was able to drain the

incoming stream water during low-flow conditions, it

flooded during moderate rainfall events and, over the

course of a year, was partially filled with silt and

Fig. 4 Evolution of a sinkhole that pirated a stream in southwestern

Illinois. Left to right increased recharge to the sinkhole initiated

erosion and deepening of the sinkhole. Ultimately, the sinkhole could

not accommodate enhanced recharge from moderate rainfall events,

ponded and filled with fine sediments, and reverted back to its original

depth. Photographs by S.V. Panno

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reverted back to its original depth of 5 m (Fig. 4). The

low-energy nature of this sinkhole, in spite of the rela-

tively high-energy input of the stream during flooding

conditions, is an example of the limiting conditions of

relatively high water tables and drainage controlled by

crevices and relatively small conduit size or both.

Bedrock topography and cave formation

Bedrock topography can play a major role in the formation

of caves, their groundwater basins, and associated karst

terrain. Palmer (2007) stated ‘‘cave origin is enhanced

where surface runoff is concentrated into small areas of

infiltration.’’ It has been the authors’ observation that large

caves in the sinkhole plain are frequently initiated at pla-

teaus or swales where rainwater and snowmelt can focus

recharge into the Ste. Genevieve and St. Louis Limestones.

Cross sections of each of four large caves (Fogelpole Cave,

Illinois Caverns, Stemler Cave and Krueger’s Cave) in

southwestern Illinois show that there is a distinct and rel-

atively large topographic depression in the bedrock near

the entrances or beginning of each cave (Fig. 5). These

swales or depressions in bedrock would likely collect

surface drainage directly and by movement of recharge

along the soil–bedrock interface allow it to recharge into

the underlying fractured bedrock. Surface water would

eventually enter fractures in the bedrock and flow along

dissolution-enhanced bedding-plane partings. Subtle

irregularities in the bedding-plane surface, as seen in

exposures in the field, probably control the sinuosity of the

bedding-plane channels (Panno et al. 2011). With time,

some of these bedding-plane channels (or proto-caves) and

their initial sinuous patterns increase in size to become

caves where active streams continue to flow through and

enlarge their passages. As a result, the sinuosity of these

caves in plan view resembles that of surface stream

channels.

Because the large caves of southwestern Illinois are

located at the Illinoisan glacial maximum, and based on

unpublished data on stalagmite ages by Panno (Illinois

State Geological Survey) and cave sediment ages (Panno

et al. 2004), the authors suggest that meltwaters of the

Illinoisan glacier (ca. 135,000 years BP) infiltrated and

enlarged bedrock crevices and bedding-plane partings and

conduits. During this time, the water table would have been

lower than today’s because of the cold, dry conditions that

prevailed at that time. Recharge would have migrated

deeper into bedrock before encountering the water table

and flowing horizontally along bedding-plane partings. The

bedrock depressions at the sediment–bedrock interface

would have created focal points and pathways down near-

vertical crevices to bedding-plane partings for meltwaters

and later meteoric waters to migrate (Figs. 1, 5).

Conclusions

The mechanisms that control the initiation, growth and

evolution of caves and cover-collapse sinkholes in Illinois’

sinkhole plain are governed by the glacial geology, local

hydrology, and the geology and hydrogeology of the

Mississippian-age limestone bedrock. Caves in the sink-

hole plain can be up to 15 km in length and 10 m in

diameter, are branchwork type with active cave streams,

and usually form along bedding planes. Cross sections

extending the length of each of the four relatively large

cave systems studied revealed depressions in the bedrock

surface at the farthest updip and upgradient portions of the

caves where surface-water recharge could concentrate and

flow into bedrock crevices and along bedding planes.

The sinkholes of this region are abundant, typically

circular to oval in plan view, range from centimeters to tens

of meters in diameter and depth, and form in loess and fine

glacial sediment deposits. Clusters of the largest and often

compound sinkholes are found overlying and in the vicinity

of the state’s largest known cave systems. Cover-collapse

sinkholes near some of these large cave systems appear to

have fluvial drainage and can approach 0.5 km2 in aerial

extent where relatively thick Quaternary-age deposits are

present. The size and morphology of these sinkholes are

distinctly different than those in the vicinity of smaller

crevice-karst bedrock features. Sinkholes associated with

solution-enlarged crevices and smaller caves and conduits

are typically circular to elliptical depressions from 60 to

120 m in diameter.

Surficial glacial till and loess deposits are easily eroded

by running water and their thicknesses in western Illinois

(10 m or greater) allow for the development of well-

defined cover-collapse sinkholes. The authors suggest that

the higher the energy of surface runoff flowing into a

sinkhole, during and following a relatively large rainfall,

the more erosion that will occur at its margins and its

interior. Because caves and conduits in Illinois’ sinkhole

plain are formed along bedding planes, and because these

conduits are morphologically and hydrologically similar,

but at different scales, the erosive growth of a sinkhole is

limited primarily by the underlying conduit system’s abil-

ity to channel, store, and discharge incoming water. If a

conduit system is too small, the conduit drainage system

may be overwhelmed during and following a large rain

event resulting in flooding of the sinkhole or sinkholes

feeding it. Sinkhole flooding, in turn, creates a low-energy

environment that is more depositional than erosive, and

sinkhole growth is inhibited or stabilized. In fact, the

sinkhole may be partially filled with fine sediments. Con-

versely, if the conduit system is large and the inflow of

water into and through the sinkhole drain is uninhibited at

even the highest recharge rate, the energy of inflowing

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Fig. 5 Cross sections of four

cave systems in the sinkhole

plain showing low-lying

incision points for cave

development. A–A0 Stemler

Cave, B–B0 Krueger’s Cave,

C–C0 Illinois Caverns, D–D0

Fogelpole Cave

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water can become and remain very high. Incoming runoff

water will continue to erode the margins and interiors of

sinkholes resulting in internal gully formation, concomitant

slumping, and cover-collapse sinkhole enlargement.

Threshold calculations indicate that crevice-dominated

sinkholes of the sinkhole plain have volumes[103 m3, but

\105 m3, whereas cave-dominated sinkholes (with direct

or indirect connections to cave passages) have volumes that

can be as small as 104 m3 and as large as 106 m3.

Finally, longitudinal sections of the four largest caves in

the sinkhole plain suggest that these caves were initiated

along depressions and swales in the bedrock surface. The

authors suggest that these bedrock depressions similarly

focused recharge to the underlying bedrock pathways

(fractures and bedding planes), thereby initiating the caves

along bedding-plane partings.

Acknowledgments The authors acknowledge the Illinois State

Geological Survey (ISGS) for their support of this work. We thank

Cheryl Nimz and Mike Knapp (ISGS) for their generous help with the

graphics and editing, and Keith Hackley and Don Keefer for their

insightful comments on the manuscript. Publication of this article has

been authorized by the Director of the Illinois State Geological Sur-

vey and by the Institute of Natural Resource Sustainability, University

of Illinois.

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