Natural
Resource Mapping of Linn County
by Robert D. Libra
In 1994, the Geological Survey Bureau (GSB) began producing detailed geologic maps of portions of Linn County. These maps depict the types of geological materials - rock strata, glacial deposits, stream alluvium, and wind-blown sand and silt - that occur within about 18 feet of the land surface. Compiled at a scale familiar to users of U.S. Geological Survey topographic maps, 1:24,000 (1 inch = 2,000 ft), they incorporate a wide array of information, including water well records, observations of rock and soil exposures, borings taken along road and power line rights-of-way, and the Linn County Soil Survey. Existing data sources are assembled and studied, and then augmented with research borings obtained by GSB staff. Bore holes are targeted to provide needed information about the relationship between the terrain and its underlying materials within local areas. This research-based understanding is the key to constructing a geologic map, as it allows the mapper to project information beyond the point-specific data provided by individual drill holes.
Geological maps produced at this scale and by this process are quite detailed, and are rich in information about the near-surface environment where society conducts its day-to-day business. This information, which can aid the decisions of commercial, industrial, and housing developers; quarrying and mining industries; and local departments of health, engineering, economic development, planning and zoning, and solid waste, is most useable when processed from its raw geologic form into a variety of applied, "derivative" maps depicting more specific natural resource conditions. Because the data contained in the geologic map are stored as Geographic Information System "coverages," or electronic map layers, the derivative maps can be tailor-made to meet local needs. In addition, information from other coverages can be added to the geologic data, further enhancing the utility of the derivative maps. Six such applied resource maps were derived from the surficial geologic maps of the Cedar Rapids North and Marion quadrangles. Three are shown here. These maps are a spin-off of the federal STATEMAP project, funded initially by the U.S. Geological Survey, and later joined by several county and city agencies, as well as private industry in Linn County.
The Groundwater Vulnerability map (above) shows the varying susceptibility of aquifers to contamination from near-surface sources. The most vulnerable areas are shown in red and include areas underlain by sandy alluvial aquifers, areas where bedrock aquifers lie near the land surface, and areas characterized by karst features such as sinkholes. In contrast, aquifers in areas shown in green are overlain by 100 feet or more of slowly permeable glacial deposits, and are largely protected from surface-related contamination. These areas with greater natural protection are better suited for developments that have the potential to adversely impact groundwater, such as waste-generating industries or agri-businesses, housing developments with a high density of septic systems, or landfills.
The Aggregate Resources map (above) shows where shallow, readily extractable deposits of sand, sand and gravel, and rock are present and therefore identifies potential locations for sand pits and quarries. These materials are in demand in areas where "suburbanization" of rural areas is occurring. Having to transport aggregate long distances adds to the cost of development, making the use of local deposits attractive. At the same time, people living in developing areas may view quarries and sand pits as less-than-desirable neighbors, and subdivisions may be planned right on top of the very resources needed to build them. Knowledge of where potentially economic aggregate deposits occur will hopefully add to the discussions that planning and zoning officials, pit and quarry operators, developers, and home buyers are having on these issues.
The Potential Hazards map (above) indicates areas where thick deposits of wind-blown silts and sands are piled upon the less permeable underlying glacial deposits. These conditions often result in seeps, where shallow groundwater discharges to the surface near the base of the sand and silt deposits, resulting in slope instability and construction difficulties. Also shown are areas where rocks that are susceptible to karst formation lie near the surface. The potential for sinkhole formation in these areas needs to be taken into account when development is planned. The map incorporates other coverages as well, showing the locations of known potential sources of contamination, such as landfills and underground storage tanks.
Geologic field research - from drilling soil borings to examining outcrops - yields the understanding needed to accurately map geologic materials in the shallow subsurface. Applied resource maps can then be generated to answer questions, from both the private sector and county or city agencies, concerning land use, development, and zoning, and hopefully will form a basic data source for all interested parties as they wrestle with the thorny issues involving economic development, natural resource availability, and environmental protection.