The Surficial and Environmental Geology of the French River Quadrangle, St. Louis County, Minnesota
1977-05-31
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The Surficial and Environmental Geology of the French River Quadrangle, St. Louis County, Minnesota
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1977-05-31
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The French River quadrangle is located just northeast of the City of Duluth, Minnesota. It is bordered on its southeastern margin by Lake Superior, and except for a residential zone along the lake shore, the remainder of the quadrangle is predominantly forested. Local areas were cleared for agricultural purposes in the earlier part of the century, but most of these are now abandoned or used for hay or pasturage. The present landscape consists of three distinct elements: a hummocky ice disintegration complex in the northwestern part of the quadrangle, a northeast-trending belt of ground moraine paralleling the southern edge of the ice disintegration complex, and an extensive area of exposed lake plain with associated strandline features representing the highest level of Glacial Lake Duluth, a predecessor to Lake Superior. Through the 1940's, only limited reconnaissance work related to surficial geology had been done on a regional scale. Detailed stratigraphic or lithologic studies of the complex Quaternary history of the area had not been attempted. Later studies differentiated glacial sediments and their relationships to advances of distinct ice lobes. This helped provide a more complete picture of relationships between Late Wisconsin events on a statewide basis. This study provides detailed areal, stratigraphic and lithologic descriptions of the Quaternary sediments in the French River region. In addition to the typical geological descriptions of the sediments such as color, texture, stone content and sorting, physical characteristics of each unit have been described in terms of potential constraints or favorable properties related to land-use planning. Compaction, permeability, shrink-swell potential, cohesion and frost action potential are among the most important of these soil properties. Lodgement till related to two distinct advances of Superior Lobe ice are recognized in the French River quadrangle. The lower sandy, silt-rich till has a textural composition of 36% sand, 46% silt and 18% clay (average of 9 samples). The upper, more silt-rich till is composed of 30% sand, 50% silt and 20% clay (average of 28 samples). Both contain abundant rock fragments characteristic of the Lake Superior region bedrock; diabase, gabbro, basalt, red rock differentiates and minor red sandstone. These tills have very low plasticity indices. Permeability is slow to moderate. Shrink-swell potential is usually low, while shear strength and compressibility are moderate. Soils of this type may be suitable for septic systems or sanitary landfill sites provided on-site sampling is conducted. The rolling, hummocky terrain in the northwestern part of the quadrangle represents the southernmost edge of the Highland moraine, and is attributed to the Superior Lobe advance which deposited the upper silt-rich till. Due to the small, local depositional environments which may develop in ice disintegration complexes, texture of the sediment is quite variable, and on-site sampling is imperative for almost any proposed land use. As the Superior Lobe began its final retreat, proglacial lakes formed around its margin, eventually coalescing to form Glacial Lake Duluth which reached an elevation of 1150' in the French River quadrangle. At this time the red lacustrine clays which cover much of the southern part of the quadrangle were deposited. Sand content is typically low in these sediments; an average of 22 samples shows a typical textural distribution to be 5% sand, 26% silt and 69% clay. The clays are very plastic, highly impermeable, and susceptible to large amounts of runoff. They are generally unsuitable for septic systems, show poor workability for excavation and building, and in general do not support a thriving agriculture. Where exposed along stream banks and the Lake Superior shoreline, slumping and accelerated erosion are common. Contemporaneous with the deposition of these clays was the formation of large deltaic sand and gravel deposits along the 1150' strandline. Their chief importance is their present and potential economic value for use as road building and construction materials. A depth to bedrock map was prepared as an aid in land-use planning. Ease of excavation, suitable slopes for placement of foundations, and likely difficulties in use of soil absorption septic systems may be estimated from this map. Except for shoreline erosion and variable seasonal flooding there are few geologic hazards in the area. The region lies in an aseismic area, and although it was the scene of active volcanisn in the Late Precambrian, no volcanic activity has occurred here in the last billion years.
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A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Carol Marie Moss in partial fulfillment of the requirements for the degree of Master of Science, May 1977. There are 4 supplementary files also attached to this record, which contain Plates 1-3 referenced in the thesis as well as the legend for Plates 1-2.
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Moss, Carol Marie. (1977). The Surficial and Environmental Geology of the French River Quadrangle, St. Louis County, Minnesota. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/212411.
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