Mapping
for the Next Century
by Greg A. Ludvigson, E. Arthur Bettis III, and Bernard E. Hoyer
Passage of the National Geologic Mapping Act of 1992 provided a stimulus for detailed geologic mapping of the U.S. directed towards the resolution of environmental problems. This decade-long mapping program is administered by the U.S. Geological Survey and includes a "STATEMAP" component which offers financial support for geologic mapping to state geological surveys through a competitive grant process. During the first year of the program in 1993-94, the Iowa Geological Survey (IGS) mapped a segment of the Mississippi River valley covered by the Blanchard Island and Letts 7.5 minute quadrangles in southern Muscatine and northern Louisa counties. Tools unavailable in the earlier years of traditional geologic mapping were applied. These include satellite images to assist with mapping boundaries between different geologic deposits, and computer technology to convert maps to digital databases, which then can be transferred in electronic form, printed as colored maps, combined with other geographic information, and be easily updated.
SATELLITE IMAGE: Taken on May 27, 1989, this color-infrared Landsat TM image of the Mississippi River valley shows the distribution of vegetation (red), bare soil (light blue-greens), and wet soil (dark blue-greens). Sandy, better drained materials (purple) and open water (black) also can be distinguished. These patterns help to differentiate deposits representing thousands of years of floodplain history. Circular features are center-pivot irrigation plots. This image was used to confirm and adjust geologic contacts mapped below.
Geologic maps (of Iowa) display the location and distribution of
various rock types, faults, and sediments deposited by marine
seaways, glaciers, wind, streams, and hillslope processes. They
also convey information about the three-dimensional geometry of
these deposits and their relative age relationships. Such
qualities make geologic maps valuable to scientists in
understanding the Earth's composition and structure, processes
and history. They also are valuable to society, which lives and
depends on geologic materials. People need answers to questions
such as the best place to locate a landfill, the extent of
groundwater contamination problems, sources of road-building
aggregate, and geologic hazards affecting subdivision
development.
Geographic information systems (GIS) combine computer mapping and assorted databases. This technology can be used to prepare customized maps derived from geologic maps and accompanying data bases. Features such as water wells and core holes may be selected for display based on their depth, construction, capacity, or their use -- whatever is recorded in the database about them. Similarly, streams, sinkholes, geologic contacts, or faults could be selected by a database criterion. Lakes, sand dunes, or bedrock units may also be selected based on attributes that describe them, such as water quality, thickness, or permeability. The result is that maps may be constructed for one general purpose but can be converted quickly into another, more specialized purpose if appropriate attributes are available in a database.
The completed Letts and Blanchard Island quadrangles encompass about 110 square miles of the Mississippi Valley and adjoining uplands south of Muscatine in eastern Iowa and adjacent western Illinois. The goal of the project was to map geologic materials to a depth of five meters (about 18 ft) at 1:24,000 scale (1 in = 2,000 ft) in order to provide baseline geologic information for a host of environmental and resource issues. The mapped area includes Muscatine Island, a portion of the Mississippi Valley under competitive pressures from agriculture and industry for land and groundwater. A portion of the Upper Mississippi River navigation system and several wildlife refuges and game management areas also occur in this area and present a series of contrasting resource management issues. Land degradation from soil erosion and headward advance of Mississippi tributary valleys, as well as landfill siting are important issues on the upland.
Several sources of subsurface information, including water well records, engineering boring records obtained from public utilities and the Iowa Dept. of Transportation, monitoring well records of the U.S. Geological Survey, borings made by the IGS, and published soil surveys, were used to construct the geologic maps. This data was compared to landscape patterns on high-altitude air photos and satellite imagery to formulate and draw the map units. Thirty-two map units, each depicting a unique succession of geologic materials to a depth of five meters, were developed.
COMPUTERIZED MAPS: Geologic maps of the Letts and Blanchard Island quadrangles (along the Mississippi Valley) are combined in this graphic and represent the first detailed mapping of the shallow (upper 18 ft) geologic materials in Iowa done under the STATEMAP program. Two major groups of (Quaternary) deposits are present -- those left by glacial ice and wind compose the uplands (yellow and green areas), and those left by streams occupy the valleys (blue, brown and rose areas). These maps can be updated as more subsurface information is acquired.
The pattern of map units (above) shows many elliptical,
smooth-edged units that characterize river deposits in the
Mississippi Valley, while the bordering uplands contain mainly
two units of loess-mantled glacial drift that are dissected by
branching units of younger stream deposits along drainage ways.
The map and related cross-sections (not shown) provide a detailed
view of the geologic materials that most affect day-to-day
activities in this area and can give planners a sound base of
geologic information from which to make resource decisions.
In July 1994, the IGS began work on its second STATEMAP project, with the inauguration of a three-year program to map the shallow geology of Linn County. In compliance with national program directives, the IGS assembled a statewide geologic mapping advisory panel that consisted of individuals from government, academia, professional societies, engineering firms, and mineral producing firms. This panel, representing potential users of geologic information, selected Linn County as the area where surficial geologic mapping could be most usefully applied to recognized environmental problems. During the 1994-95 project year, the Cedar Rapids North and Marion 7.5 minute quadrangles were mapped, and the bedrock geology of the entire county was updated. Specific environmental problems addressed in these quadrangles included drainage and groundwater contamination, suburban expansion in areas of sinkholes (karst), and long-range plans for the county sanitary landfill. In addition to the federal award, financial support for the mapping was provided by the Linn County Engineering, Planning and Zoning, Regional Planning, and Solid Waste agencies, as well as the cities of Cedar Rapids, Hiawatha, and Marion, all of whom were anticipated end users of the map information. All maps were compiled using GIS technology and are stored in DNR's Geographic Information System Library, procedures that facilitated digital access for many users and will provide flexible use of geologic mapping for various applied purposes in the future.
Today there is a recognized need for more rapid access to more detailed information. GIS techniques provide an effective means to develop specific information, tailored to specific needs, in a timely manner. The use of this technology enables geologists to develop map information in ways that better assist society in understanding and resolving its environmental and resource problems.