https://ir.uiowa.edu/igs_ofm/1164/thumbnail.jp

Clark, Ryan

Kerr, Phil

English

Iowa Research Online

QUATERNARY
THICKNESS
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≤25
≤50
≤75
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≤125
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≤200
≤225
≤250
MN
0°51´
15 MILS 0°56´
17 MILS
GN
Keith Schilling, State Geologist
Published October, 2020
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40.6250°
40.7500°
-91.6250° -91.5000°
40.7500°
40.6250°
-91.5000°-91.6250°
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D
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1:24,000
0 1 20.5
0 1 20.5
CONTOUR INTERVAL 25 FEET
Kilometers
Miles
1:24,000
0 1 20.5
0 1 20.5
CONTOUR INTERVAL 25 FEET
Kilometers
Miles
Bedrock Elevation Quaternary Thickness
Introduction
The Bedrock Elevation and Quaternary Thickness Maps of the Donnellson 7.5’ Quad-
rangle were produced in conjunction with the surficial and bedrock geologic maps. The bedrock 
surface within the quadrangle, like much of Iowa, is mostly concealed by glacial deposits. The 
boundary between Paleozoic bedrock and unconsolidated Quaternary materials is likely just as 
irregular as the land surface itself. Therefore, the thickness of Quaternary deposits varies widely 
across the quadrangle, generally ranging from 0 to 18 m (0-60 ft) and reaching over 68 m (225 ft) 
thick in the southwestern portion of the mapping area. In contrast, bedrock exposures are common 
in the eastern portion of the quadrangle along Sugar Creek and its tributaries.
Bedrock topographic lines were drawn based on borehole data coupled with bedrock out-
crop occurrences identified while constructing the Bedrock Geologic Map of the Donnellson 7.5’ 
Quadrangle (OFM-20-1). Where the bedrock surface is exposed or lies within a few feet of the 
land surface, the bedrock topography essentially mirrors the land surface topography, such as along 
Sugar Creek in the northeastern part of the map area. Bedrock topographic detail is muted where 
bedrock is deeper and borehole data is scarce, such as within the bedrock valley in the western part 
of the map. The thickness of Quaternary materials was generated by subtracting the elevation of 
the bedrock surface from the land surface elevation. The Surficial Geologic Map of the Donnellson 
7.5’ Quadrangle (OFM-20-2) provides further information regarding the nature and extent of Qua-
ternary deposits within the mapping area. 
Methodology
The Bedrock Elevation and Quaternary Thickness Maps of the Donnellson 7.5’ Quadran-
gle were constructed using the same datasets as the surficial and bedrock geologic maps. Geologic 
information utilized included drilling records housed in the Iowa Geological Survey (IGS) Geo-
Sam database, existing maps and technical reports, Iowa Department of Transportation data, and 
reports from engineering projects and quarry operators. Field reconnaissance of six bedrock out-
crops and five quarries was completed as well as the collection of 14 passive seismic data points.  
More than 280 boring records from the IGS GeoSam database, including both driller’s logs 
and lithologic descriptions of well cutting samples (strip logs), were evaluated for the Donnellson 
Quadrangle. Each record was checked for locational accuracy using information from the driller’s 
logs, historic plat books, county assessor information, and direct communication with landowners. 
The depth to the surficial-bedrock contact was determined for each well and assigned an elevation 
value by subtracting it from the surface digital elevation model (DEM). These data points provided 
the framework for the Bedrock Elevation Map. Additional information was gained from an assess-
ment of the Natural Resources Conservation Service county soil survey by identifying soil series 
that indicate shallow bedrock. 
Bedrock elevation contours were digitized manually on screen using ArcGIS software at 
a 25-foot contour interval. The bedrock elevation raster was generated using interpolations of the 
bedrock surface created with the ‘Topo to Raster’ and ‘Empirical Bayesian Kriging’ tools in Arc-
Map 10.6. The Quaternary Thickness Map was created by subtracting the bedrock elevation raster 
values from the surficial DEM raster. The resulting surface was rounded to the nearest integer and 
contours were generated from this result and then smoothed. 
Ryan Clark and Phil Kerr
Iowa Geological Survey, IIHR-Hydroscience & Engineering, University of Iowa, Iowa City, Iowa
Open File Map: OFM-20-3 
≥675
≥625
≥650
≥600
≥550
≥575
≥525
≥475
≥500
≥425
≥450
BEDROCK
ELEVATION
feet above sea-level
MAP SYMBOLS
ROAD CLASSIFICATION
Local road
U.S. Route
State Route
bedrock outcrop
GeoSam point
topographic line
geophysics
collection point
water body
quarry or pit
river/stream
UTM GRID AND 2020 MAGNETIC NORTH
DECLINATION AT CENTER OF SHEET
Acknowledgments
Special thanks to Clem and Jolisa Krogmeier for allowing us to explore their property and to all the nice 
folks that stopped by to ask if we needed assistance when they saw us parked on the side of the road. Thanks 
also to Don Smith of Cessford Construction, Co. (Oldcastle Materials) for allowing us to access quarries in 
and around the mapping area. University of Iowa (UI) Department of Earth and Environmental Sciences 
(EES) student Karin Johnson and Cornell College student Gabi Hiatt prepared well cutting samples for 
stratigraphic logging. UI-EES students Megan Koch, Ellie Biebesheimer, and Allison Kusick helped with 
various aspects of data collection and management including producing descriptive logs of water well cut-
ting samples, updating well locations, and geophysical data acquisition. Thanks to Rick Langel of the Iowa 
Geological Survey (IGS) for managing the Iowa geologic sampling database (GeoSam). Brian Witzke (IGS 
- retired), Associate Professor Brad Cramer (UI-EES), and Illinois State University Professor James “Jed” 
Day are thanked for their help with the stratigraphic research components stemming from this mapping 
project as well as many fruitful discussions about the geologic complexities of the tristate region. Casey 
Kohrt and Chris Kahle of the Iowa Department of Natural Resources provided GIS technical help. Admin-
istrative support was provided by Suzanne Doershuk, Melissa Eckrich, Teresa Gaffey, Carmen Langel, and 
Rosemary Tiwari.
Base map from the USGS Donnellson 7.5' Quadrangle map, published by the US Geological Survey in 2018. Map projection and 
coordinate system based on Universal Transverse Mercator (UTM) Zone 15N, datum NAD83.
The maps are based on interpretations of the best available information at the time of mapping. Map interpretations are not a substi-
tute for detailed site-specific studies. The views and conclusions contained in this document are those of the authors and should not 
be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government.
Supported in part by the U.S. Geological Survey Cooperative Agreement Number G19AC00243 National Cooperative Geologic Map-
ping Program (STATEMAP).  This work was partially supported by a National Science Foundation Award: Improving Undergraduate 
STEM Education Grant GP-IMPACT-1600429.
Bedrock Elevation and Quaternary Thickness Maps of the Donnellson 7.5’ Quadrangle, Lee County, Iowa
ADJOINING 
QUADRANGLES
1 Hillsboro, IA
2 Salem, IA
3 Lowell, IA
4 Farmington, IA
5 West Point, IA
6 Croton, IA-MO
7 Argyle, IA-MO
8 Nauvoo, IA-IL
1 2
4
6 7
5
3
8


Bedrock Elevation and Quaternary Thickness Maps of the Donnellson 7.5\u27 Quadrangle, Lee County, Iowa

https://ir.uiowa.edu/cgi/viewcontent.cgi?article=1164&amp;context=igs_ofm

Bedrock Elevation and Quaternary Thickness Maps of the Donnellson 7.5\u27 Quadrangle, Lee County, Iowa

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