Geology Matters

Editor's Note: This post was written in summer 2011, but with the surge of construction projects in the area, may prove an interesting read this year as well.

We live on some very interesting and challenging geology. To see what is deep underfoot walk along Big Rivers Trail from Lilydale Rd. to Mendota. Those living along the bluff, and others that could affect its use, need to understand the dynamics of this geology.

We seem to be coming out of a drought, so it is a good time to consider what challenges a wet cycle have had and could have on this area. Even in dry years, tremendous amounts of water move through these formations. On your walk you will see several places where water flows continually from the bluff, becoming beautiful ice falls in the winter. Also check out Hwy.13 before it winds into Mendota where the road collapsed last year and where this year water blew out some of the bluff repair. There is a storm grate on the north side of the highway located before the road slopes down toward Mendota. One continually hears water running at this drain.

The geologic formations include the St. Peter Sandstone, the Glenwood Shale, the Platteville Limestone and the Decorah Shale.  According to U. of MN Geology Professor E. Calvin Alexander Jr., “most of the bedrock formations are relatively stable.” However, “the two shales are very erosive due to surface and ground water both aided by the frost shattering associated with Minnesota's climate. The Glenwood shale is deeply, actively undercutting the Platteville Limestone and the
Platteville bluff itself is currently retreating southward and is falling out from under structures built close to the edge.”

Alexander also cautioned about the Decorah Shale on the bluff south of Hwy 13. “As groundwater penetrates the slumped Decorah it oxidizes the abundant sulfides present in the original Decorah and the resulting sulfuric acid further weakens the shale and starts dissolving the underlying Platteville.”

According to Ralph Graham, a long time resident of Mendota Heights, years ago a contractor trying to build a residence on Hwy. 13 and Lexington had his construction equipment swallowed by a what he called “blue gumbo” (Decorah Shale). The project was abandoned. In 1982, a house was built on the lot. The current owner is continually working to manage the water from springs and seeps.

In the late 1980s, a family living along the south side of Hwy. 13 had to relocate for several months because of the bluff collapsing causing damage to its home. Mendota Heights built a storm water project to protect this area.

A recent article in the St. Paul Pioneer Press discussed cost overruns in massive infrastructure projects. Bent Flyvbjerg, University of Oxford Said Business School stated, “We know that geology is difficult. No matter. It’s ignored in project after project. Therefore, the problem is not geology itself but the fact that we disregard geology.”

If one takes the geology into account, a builder can prevent some mishaps. To accommodate water and unstable soils, an engineer designed his house on the south side of Hwy. 13 so water from seeps and springs would flow under the building. He built his house on concrete pillars extending down into solid rock. His house sits on stilts with only a small garage and driveway on the ground.

Construction in dynamic geologic environments is risky, dangerous, and costly. We don’t have the luxury to ignore our geology. It needs to be addressed in both small and large projects. Anyone intensifying the use of the bluff needs to understand the implications.  If we are entering a wet cycle, nature could make us pay attention.

The views expressed in this blog are the views of Marilyn Lundberg and do not necessarily reflect the views of the Lilydale City Council.