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Surficial Geologic Map of Allegan County Mapping Project Update
Surficial geologic mapping of Allegan County was completed by the Michigan Geological Survey because the county is a high priority area for mapping within the State of Michigan due to its high population growth rate and existing transportation corridors of I-96 and US-131. Allegan County contains complex glacial landforms and sediments that formed during advance and retreat of the Lake Michigan Lobe of the Laurentide Ice Sheet during the last glaciation (~32,000 to ~11,700 years ago). Since the ice sheet retreated out of Allegan County ~16-17,000 years ago, wind, water, and weathering have eroded and modified glacial sediments and formed eolian dunes (e.g., coastal and inland), stream sediments (e.g., alluvium), lake sediments, and productive soils for agriculture. Uses of Surficial Geologic Maps: A surficial geology map shows the geological materials such as diamicton, sand and gravel, or clay found within ~5 feet of the ground surface. In some cases, these surface units extend to much greater depths, and in other cases a different sediment may lie just below the surface sediment. The map shows the areal distribution of the different types of sediment and landforms as described in the map explanation. Surficial geology maps can assist anyone wanting to know what lies at or beneath the land surface. MGS staff collecting samples in the field
Example core description and gamma-log from a borehole in Allegan County
For example, these maps may aid in the search for water supplies, or economically important deposits such as sand and gravel for aggregate or clay for bricks or clay tile. The maps, along with water-well logs, can provide an indication as to whether an aquifer at depth is connected to a surface stream. This information is critically important in assessing whether or not a water well near a stream could cause an excessive depletion of surface water. Foundation conditions determined by the surficial geological materials are critical inputs to any type of development. Environmental issues such as the location of a suitable landfill site or the possible spread of contaminants are directly related to surficial geology. Construction projects such as locating new roads, excavating foundations, or building homes may be better planned with a knowledge of the surficial geology of the site. Once a surficial geologic map is constructed, it can be used for a variety of derivative maps, such as aquifer thickness and extent, range of transmissivity values, sensitivity to surface and near surface derived contaminants, and reserves of sand and gravel.
Methods: Field mapping of Allegan County was completed by Prof. Patrick Colgan (MGS contract mapper and professor at Grand Valley State University) and Nathan Erber (MGS senior research associate) with support from John Esch (MGS contract mapper). Their mapping was informed by existing county maps constructed by Gephart and Larson (1989), and quadrangle maps by Alan Kehew and his students. Light Detection and Ranging (LiDAR)-derived 1-m DEMs, which include high-resolution elevations of the land surface (<4 in), are available for Allegan County. The LiDAR data was a critical tool to interpreting the subtle features like sand dunes, terraces, and beach ridges. Without the LiDAR data many of these features would not have been recognized. Test borings using the rotary wireline, mud rotary, and roto-sonic coring methods were drilled as part of this mapping project and are plotted on the maps. Cores are archived at the Michigan Geological Repository for Research and Education (MGRRE) facility at the Michigan Geological Survey. These borings are essential to an understanding of the subsurface geology of the glacial deposits. Gamma-ray logs are made of the borehole at the time of drilling.
