Geologic Map Series

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    Geologic Map Series 3. Geology of the Cloquet Quadrangle Carlton County, Minnesota
    (Minnesota Geological Survey, 1970) Wright, H.E. Jr; Mattson, L.A.; Thomas, J.A.
    The Cloquet quadrangle is bisected by the St. Louis River, a major stream that flows into the western end of Lake Superior. Precambrian rocks are extensively exposed in the river valley where overlying Pleistocene glacial deposits have been eroded. The Precambrian rocks are dominated by the Thomson Formation, which consists of interbedded slates, slaty graywackes, and graywackes. Small-scale cross-bedding, graded bedding, flute casts, load casts, clastic dikes, and other primary and penecontemporaneous structures are common, as are calcareous and siliceous concretions. The formation probably was deposited in a relatively deep-water basin, in part by turbidity currents. It has only one conspicuous marker bed, the Otter Creek unit, so the stratigraphic thickness across the intricate folds of the region is difficult to determine, but it probably is about 20,000 feet. Normal and reverse faults with displacements of a few tens of feet are common, as are steeply dipping conjugate joints of northwest and northeast trends. Cleavage is well developed in slaty units. The Thomson Formation is correlated with the Animikie Group. Abundant microgabbro dikes were intruded during subsidence of the Lake Superior syncline. The Pleistocene glacial history was marked by three phases of advance and retreat of the Superior Lobe, preceded by a phase of the Rainy Lobe. Drumlins, moraines, outwash plains, eskers, lake plains, and diversion channels constitute distinctive landforms. During final withdrawal of the Superior Lobe from the area, the St. Louis River, which carried the outflow from Glacial Lake Upham, was diverted to form prominent erosional channels leading to the St. Croix River at progressively lower elevations, until it finally flowed into the proglacial Lake Nemadji, whose outlet formed a final channel in the sequence.
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    Geologic Map Series 2. Surficial Geology of the New Brighton Quadrangle, Minnesota
    (Minnesota Geological Survey, 1966) Stone, John E.
    The New Brighton quadrangle is underlain by as much as 500 feet of glacial drift, which lies directly on Paleozoic sedimentary rocks. The bedrock, which is part of the northern flank of the Twin Cities artesian basin, is traversed by a complex network of buried valleys as much as 400 feet deep that were cut during several cycles of erosion. Most of the surface morphology and the related deposits were formed about 12,500 years ago, during the advance and retreat of the Grantsburg Sublobe, which produced the following formations in the approximate order of their deposition: (1) the Hillside Sand, a proglacial outwash deposit; (2) the Twin Cities Formation, a complex mixture of gray and red till; (3) the Falcon Heights Sand, a retreatal outwash deposit; (4) the Turtle Lake Sand, a lake deposit; (5) the Arsenal Sand, a kame deposit; (6) the New Brighton Formation, a lake deposit; and (7) the Fridley Formation, another lake deposit. The units have been mapped at the surface and traced in the subsurface. After the active ice retreated from the immediate area, the West Campus Sand, a valley-train deposit, was deposited. Later, ice in the drift melted to produce many hundreds of kettle holes, some of which are now occupied by lake or swamp deposits; the valley train was dissected, leaving a terrace along the Mississippi River; and a poorly integrated drainage system was produced. In late-glacial and postglacial time, thin deposits of eolian sand, loess, and colluvium were laid down. The most significant changes in the landscape in postglacial time have been made by man, who settled the area in the 1830's and 1840's. Preliminary data are given on the engineering properties of the surficial deposits. These data in conjunction with the geology are useful for engineering and hydrologic studies, land-use planning, and resource evaluation.
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    Geologic Map Series 1. Bedrock Geology of Duluth and Vicinity St. Louis County, Minnesota
    (Minnesota Geological Survey, 1963) Taylor, Richard B.
    Duluth, Minnesota, is on the northwest limb of the Lake Superior syncline, a northeast-trending structure of Precambrian age. The north- west limb of the syncline dips 10-20 degrees S.E. toward Lake Superior and is dominated by the Duluth Gabbro Complex, a huge sill-like mass with crescentic outcrop that extends almost 150 miles from Duluth to near Hovland. At Duluth the gabbro complex lies on the Thomson Formation, and apparently was intruded along the surface of unconformity below the Keweenawan rocks. The gabbro complex was formed by multiple intrusion, and consists of an older anorthositic gabbro and a younger layered gabbro and related intrusions. Keweenawan flows above the gabbro mass are cut by diabase sills. The basalt flows at one locality currently are being quarried as a source of crushed rock for concrete aggregate.