6.1. SR 03-17-2003 - Laserfiche WebLink

Home Browse Search

specific geomorphic features such as the gravel rich esker complex which comprises the surficial geology of the gravel mining district. Eskers or more generally, ice contact deposits are the result of deposition by subglacial fluvial processes such as meltwater streams and rivers. These deposits are essentially reworked till fi.om which the fines were removed by fluvial activity. The lack of fines provides for the relatively high permeability of these deposits." "An alternating sequence of till and glacio-fluvial deposits are present beneath the esker deposit. The uppermost unit beneath the esker appears to be a relatively continuous till unit that is variable in thickness. A sand and gravel unit is generally present below the till. Other sand and gravel units are interbedded within or between successively deeper till units. These sand and gravel units are important water supplies for local residential use. A majority of the residential developments west of the mining district utilize the first sand and gravel unit present below the esker deposit." "Beneath the glacial deposits the first bedrock encountered in the immediate vicinity of the mining district is the Eau Claire formation. This formation consists of alternating sandstone and shale, and is generally recognized as a regional confining layer. Beneath the Eau Claire formation are the Mount Simon and Hinckley formations. These two formations are generally described as fine to coarse grained quartzose sandstones which combine to form the Mt. Simon-Hinckley Aquifer. The Mt. Simon-Hinckley is an important water supply locally and throughout the Twin City Basin, as well. According to USGS Hydrologic Atlases, the basement rock beneath the Mt. Simon-Hinckley consist of undifferentiated Pre- Cambrian igneous and metamorphic rocks including quartz, diorite, granodiorite and quartz m, onzonite." "The bedrock elevation beneath the mining district and over much of the area to the west and south appears to be at an elevation of 800 to 850 feet National Geodetic Vertical Datum (NGVD). Slightly east of the mining district and extending along an approximate north- south trend is a reported major bedrock valley incised over 100 feet deeper than the bedrock elevation beneath the mining district. No major bedrock structures are documented west of the district and the bedrock elevation averages 800 feet NGVD." "The uppermost aquifer within the mining district is a water table aquifer present within the gravel deposits. Several glacial aquifer sequences are present beneath the water table aquifer and do not appear to be in complete hydraulic connection with the water table aquifer. These confined aquifer units are important water supplies to residential housing developments around the perimeter of the mining district. The glacial aquifer systems are separated from deeper bedrock aquifer units by the confining properties of the Eau Claire formation and till units overlying the Eau Claire Formation. The Eau Claire formation overlies the Mount Simon-Hinckley aquifer which is the lowermost aquifer in the vicinity of the mining district. Bedrock aquifers beneath the Eau Claire formation may be under artisan pressure where confined by the Eau Claire formation and/or till units." "Shallow groundwater in the mining district is present as an unconfined water table aquifer. Water table conditions are manifested within the esker complex at elevations LmSCH ASSOCI^TES, INC. Hydrogeologists · Engineers · Environmental Scientists · Jan-2003 · · Page 13 ·