Near Two Harbors in Lake County, Minnesota — The American Midwest (Upper Plains)
Geology Complicates Bridge Foundation
Many years before construction began, a team of geologists and engineers began studying this site. They were searching for a subsurface material that would support the enormous forces exerted by the bridge and its traffic. Initially, it seemed as if the basalt rock exposed in the gorge would be sufficiently strong for this task. Lurking beneath the hard, resistant surface, however, lie weaker layers. Each lava flow contains a solid, strong middle section, with much weaker, porous zones above and below. These porous zones are the result of gases that were trapped in the rapidly cooling outer portions of each flow. Groundwater later percolated through these zones for a long time, and they weathered to a near-soil condition. Complicating
Subsurface core drilling and the detailed mapping of outcrop, or exposed rock, helped geologists predict the three-dimensional extent of the solid portions of the lava flows. This study was made more difficult by the inability to drill at some of the foundation locations, because of the adverse terrain and the team's desire to preserve the natural beauty of the area. After the solid and weak layers in the rock were carefully mapped, construction plans were drawn up and the bridge was built. Predictions of the subsurface layers proved reliable, and only minor adjustments were required during construction.
Erected 2009 by the Geological Society of Minnesota in partnership with the Minnesota Department of Transportation, the Minnesota Geological Survey, and the Minnesota Department of Natural Resources.
Topics and series. This historical marker is listed in these topic lists: Bridges & Viaducts • Environment • Parks & Recreational Areas. In addition, it is included in the Minnesota: Geological Society of Minnesota series list.
Location. Touch for map. Marker is at or near this postal address: 3206 Highway 61 East, Two Harbors MN 55616, United States of America. Touch for directions.
Other nearby markers. At least 8 other markers are within walking distance of this marker. Having Fun on the North Shore / Cruisin' on the Shore (a few steps from this marker); Cooperation Helps Build a Park / We Built this Park. (a few steps from this marker); Ancient Rivers & Lakes (within shouting distance of this marker); Building the Wall (within shouting distance of this marker); Gooseberry Falls CCC Historic District (within shouting distance of this marker); "C.C.C. Worker" (within shouting distance of this marker); A Castle in the Park (within shouting distance of this marker); CCC Camp Site (within shouting distance of this marker). Touch for a list and map of all markers in Two Harbors.
Credits. This page was last revised on August 21, 2022. It was originally submitted on August 20, 2022. This page has been viewed 70 times since then and 6 times this year. Photos: 1, 2, 3. submitted on August 20, 2022.