MODELING GLACIATED TERRAINS

ABSTRACT

Glaciation has been one of the most significant geologic
processes to shape the landscape in much of the northern United
States.  Glaciers repeatedly covered Illinois throughout the
Pleistocene Epoch (approximately 2,000,000 to 8,000 years ago). 
Glacial erosion and deposition has changed the landscape of 
Illinois by filling preglacial river valleys, changing the course 
of rivers, and creating new landforms.   Some of the most prominent 
landforms created by glacial deposition  in Illinois are end 
moraines.  End moraines are broad, low-relief ridges that mark 
the advances and readvances of the glaciers. The morphology of the 
moraines is variable and can be dependent on the length of time 
that the glacial margin occupied a given position on the landscape, 
the amount of sediment carried by the glacier, and/or the age of 
the moraine.  

Traditionally, geologists have used field observations and analysis
of paper topographic maps to study landform morphology.  Recently, 
however, geologists have turned to readily available digital surface 
elevation data (United States Geological Survey Digital Elevation 
Model data) and ArcInfo analytical tools to visualize, analyze, and 
model the landscape.  For this study, functions within GRID, TIN, ARC, 
and ARCPLOT  were used to characterize differences in moraine forms.  
Surface profiles were used to identify differences between previously 
mapped glacial moraines, and to find previously unidentified features 
on the landscape that may be mapped as end moraines.

Introduction

The two main types of glacial depositional landforms found in 
Illinois are ground moraine and end moraine.  Ground moraine formed 
of diamicton and is generally characterized by a gently rolling to 
flat surface (Ritter, 1986).  End moraines are arcuate ridges that 
mark ice-marginal positions of glacial advances, readvances, or 
stillstands.  Identification of these features on the landscape is 
important for determination of unit boundaries and the dynamics of 
a glacial lobe (Embleton and King, 1975).  In Illinois, the 
topographic relief of end moraines of the Wisconsin glacial episode 
can range from 70 meters to as low as 3 meters of relief.

Recent geologic history and landforms in Illinois
Preglacial landforms
Illinois' topography probably has been relatively flat through 
much of its recent geologic history.  Before glaciation, the 
low-relief landscape was due to the generally flat-lying Paleozoic 
(greater than 245 million years old) rocks that fill the Illinois 
Basin.  Structural uplift, folding, and faulting were responsible for 
some landforms, especially in the southern part of the state.  The 
major landforms in central Illinois were largely the result of 
well-developed drainage systems (Herzog et al, 1994).  

Pre-Illinoian glaciation
These early glacial advances, which took place approximately 
2,000,000 to 500,000 years ago, were responsible for reducing the 
relief of Illinois' already flat topography both by erosion and 
deposition.  Many preglacial bedrock valleys were partially filled 
with thick deposits of outwash sand and gravel (Willman and Frye, 
1970).  The extent of the pre-Illinoian glaciation is not exactly 
known as most of these deposits were subsequently buried and/or 
eroded by the later Illinois glacial advance.

Illinois glacial episode
The Illinois glacial advance, which took place approximately 200,000 
to 120,000 years before present, created new landforms found in much 
of western and southern Illinois.  During this advance, the maximum 
reach of the ice in Illinois marked the southernmost extension of 
continental glaciation in North America (Willman and Frye, 1981).  
At one time, ice of this glacial episode covered nearly 90% of the 
state and today its deposits comprise approximately 55% of the 
surficial deposits in Illinois.  The Illinois glacial advances 
left behind mostly low-relief, rolling ground moraine.  End moraines 
may have formed, but have lost their prominence due to subsequent 
erosion, particularly mass wasting under paraglacial/periglacial 
conditions (Johnson, 1992), and the deposition of a blanket of 
wind-blown silt (loess) across most of the landscape during the 
Wisconsin glacial episode.  Drainage networks developed in the 
ground moraine are responsible for most of the modern topography. 

Wisconsin glacial episode
The most recent glaciation in Illinois was during the latter part of
the Wisconsin glacial episode between about 25,000 and 14,000
radiocarbon years before present.  At one time, ice of this glacial
episode covered 33% of the state's surface, primarily in the
northeastern quarter of Illinois.  This glacial episode produced
many landforms, including end moraines, ground moraine, and flat
lake beds.  The most visible of these landforms are the end
moraines.  Figure 1 shows the locations of these moraines, the
glaciated areas of Illinois, and the location of the study area for
this paper.

New Mapping and Modeling
Input data
USGS 1:250,000 DEM data
The United States Geological Survey provides 1:250,000 digital
elevation models (DEMs) as one of their available standard
products.  These DEMs cover 1-degree by 1-degree areas.  For
complete coverage of Illinois, all or parts of 29 DEMs are
needed to create a contiguous  grid.  These DEM files were
converted to ArcInfo lattices with the ARC DEMLATTICE
command.  In their original form, the DEMs have a coordinate
system based in decimal seconds of latitude and longitude.  The
29 individual lattices were merged into one lattice with the ARC 
LATTICEMERGE command.  During this process, the composite
lattice was clipped with a boundary polygon.  This composite
lattice was then projected into a Lambert Conformal Conic
projection.

Existing mapping
Willman and Frye (1970) published a map showing end moraines of 
the Wisconsin glacial episode.  This map was used as a base 
information overlay on the composite DEM to identify the morphology 
of end moraines.  The positions of the moraines on Willman and 
Frye's map were used to identify similarly oriented ridges as 
potential moraines.  

Mapping Procedures
Raw elevation data
The composite statewide DEM for Illinois was displayed in ARCPLOT 
with the GRIDSHADES command.  This drawing command was also used 
in ARCEDIT with the AP command.  When displaying this grid, ridges 
are indicated by bands of lighter shades.  This grid proved to be 
very helpful in moraine identification.  Figure 2 shows the 
elevation grid, with higher elevations shown in white (and lighter 
shades of gray) and lower elevations shown in black (and darker 
shades of gray).  Locations of the surface profiles are also shown.


Shaded relief grid
A shaded relief portrayal of the landscape was created from the 
statewide DEM with the ARC HILLSHADE command.  The HILLSHADE command 
allows the user to change the sun's azimuth and inclination angle, 
either of which can alter the appearance of the output grid.  The 
shaded relief grid is displayed in ARCPLOT with the GRIDSHADES 
drawing command.  GRIDSHADES allows a linear stretch of the values 
in the shaded relief grid to be applied to enhance the appearance 
of relief.  This exaggeration of the shading increases the visibility 
of subtle features of the landscape.  This ARCPLOT command was also 
used in an ARCEDIT AP AML so that new moraine locations could be 
digitized from the shaded relief basemap.  Several versions of the 
shaded relief grid were developed for various sun angles and 
inclinations, as well as vertical exaggerations.

Cross section profiles
A series of surface elevation profiles was generated from the 
statewide DEM with the ARCPLOT SURFACEPROFILE command.  This 
command generates profiles from either XY coordinate pairs, or 
from an existing line coverage.  Profile information can be saved 
into a user-specified INFO file.  For this study, surface profile 
lines were digitized based on the existing moraine map.  Among the 
items in the info file are DISTANCE and SPOT.  Item DISTANCE refers 
to the distance along the surface profile for each sample point.  
Item SPOT refers to the interpolated elevation for each point.  
These two items can be treated as X and Y coordinates and written 
to a file that is used to generate line coverages of the profiles.  
Figure 3 shows six land surface profiles.  The previously mapped 
moraines are clearly visible, as are the features identified by 
this study.  In general, the moraines identified by Willman and 
Frye (1970) have more relief than the features newly identified 
in this study.


Results and Future Research
Figure 4 shows several newly identified ridges that may be moraines.  
These ridges are generally parallel to previously mapped moraines.  
At this time, the newly mapped features have not been field verified 
as moraines.  Field verification may include such tasks as shallow 
drilling and sampling to record material characteristics and location/
elevation verification with a Global Positioning System.  Other 
verifications may include comparisons between the soils and vegetation 
of the newly identified features and previously identified moraines.  


Additional digital revisions to the moraine map may be possible when 
more detailed (larger scale) digital elevation data are available.  
Currently, larger-scale DEMs are generally not available for Illinois.  
However, other forms of larger-scale data may soon be available.  
With the introduction of USGS Digital Raster Graphics (DRG) files, 
contour line information at a scale of 1:24,000 may soon be available.  
Contour vectors can be converted to in-house produced digital elevation 
models with the ARC command TOPOGRID.  At the scale of 1:24,000, even 
more subtle features of the landscape may be identified.

References
Embleton, C., and King, C. A. M., 1975, Glacial Geomorphology: 
     Halsted Press, New York, pp 430 - 460.

Johnson, W. H.  1992, Periglacial Landforms and Landscape
     Modification During the Last Glacial Maximum in East-
     Central Illinois:  Geological Society of America Abstracts
     with Programs of the North-Central Section meeting,
     Geological Society of America, Inc. Boulder, Co., p. 24.

Herzog, B. L., B. S. Stiff, C. A. Chenoweth, K. L. Warner, J. B.
     Sieverling, C. Avery, 1994, Buried Bedrock Surface of
     Illinois:  Illinois State Geological Survey Illinois Map 5, 
     scale 1:500,000.

Ritter, D. F., 1986, Process Geomorphology:  Wm. C. Brown
     Publishers, College Division, Dubuque, Iowa, pp 379 - 402.

Willman, H. B., and J. C. Frye, 1970, Pleistocene Stratigraphy of
     Illinois:  Illinois State Geological Survey Bulletin 94, plate 1,
     scale 1:500,000.

Willman,  H. B., and J. C. Frye, 1981, The Glacial Boundary in
     Southern Illinois:  Illinois State Geological Survey Circular
     511.


Curtis Abert, Associate Geologist/GIS specialist
Illinois State Geological Survey
615 East Peabody Drive
Champaign, IL  61820
Phone:  (217) 244-2188
FAX:  (217) 333-2830
email:  abert@flanders.isgs.uiuc.edu