https://ir.uiowa.edu/igs_ofm/1167/thumbnail.jp

Clark, Ryan

Kerr, P

English

Iowa Research Online

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QUATERNARY
THICKNESS
fe
et
 b
el
ow
 su
rf
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≤25
≤50
≤75
≤100
≤125
≤150
≤175
≤200
≤225
≤250
≤275
≤300
≤325
≤350
Bedrock Elevation and Quaternary Thickness Maps of the West Point 7.5’ Quadrangle, Lee County, Iowa
MN
0°54´
16 MILS 1°1´
18 MILS
GN
Keith Schilling, State Geologist
Published October, 2020
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 West Point 7.5’ Quadrangle 
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 97 m (320 ft) thick based 
on well data in the southeastern portion of the mapping area. In contrast, bedrock exposures are 
common in the western 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 West Point 7.5’ 
Quadrangle (OFM-20-4). 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 western part of the map area. Bedrock topographic detail is muted where bore-
holes intersecting the bedrock surface are scarce or at greater depth, such as the bedrock valley in 
the southeastern part of the map. The thickness of Quaternary materials was generated by subtract-
ing the elevation of the bedrock surface from the land surface elevation. The Surficial Geologic 
Map of the West Point 7.5’ Quadrangle (OFM-20-5) provides further information regarding the 
nature and extent of Quaternary deposits within the mapping area.
Methodology
The Bedrock Elevation and Quaternary Thickness Maps of the West Point 7.5’ Quadrangle 
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 companies and quarry operators. Field reconnaissance of seven bedrock 
outcrops and one abandoned quarry was completed as well as the collection of 36 passive seismic 
data points.  
More than 200 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 West Point 
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 
from the surficial DEM raster. The resulting surface was rounded to the nearest integer and con-
tours were generated from this result then then smoothed. 
Ryan Clark and Phil Kerr
Iowa Geological Survey, IIHR-Hydroscience & Engineering, University of Iowa, Iowa City, Iowa
Open File Map: OFM-20-6 
BEDROCK
ELEVATION
feet above sea-level
≥700
≥675
≥650
≥625
≥600
≥575
≥550
≥525
≥500
≥475
≥450
≥425
≥400
≥350
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 the landowners who allowed us to explore their properties: Carol and Marvin Newton, 
Jim Bair, and Mike Tweedy. 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 Bie-
besheimer, and Allison Kusick helped with various aspects of data collection and management including 
producing descriptive logs of water well cutting 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 Nat-
ural Resources provided GIS technical help. Administrative support was provided by Suzanne Doershuk, 
Melissa Eckrich, Teresa Gaffey, Carmen Langel, and Rosemary Tiwari.
Base map from the USGS West Point 7.5' Quadrangle map, published by the US Geological Survey in 2018. Map projection and coor-
dinate 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.
ADJOINING 
QUADRANGLES
1 Salem, IA
2 Lowell, IA
3 Danville, IA
4 Donnellson, IA
5 Fort Madison, IA-IL
6 Argyle, IA-MO
7 Nauvoo, IA-IL
8 Niota, IA-IL
1 2
4
6 7
5
3
8


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

https://ir.uiowa.edu/cgi/viewcontent.cgi?article=1167&context=igs_ofm

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

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