Fossils, glaciers, and the water we drink

Poke along the trail at the Lilydale brickyards in St. Paul, and you’re likely to find fossilized remains of prehistoric sea creatures that lived 450 million years ago during the Ordovician period – bryozoans, crinoids, brachiopods, and trilobites. How did they end up here, more than 1000 miles from the nearest ocean?

Looking out over the Mississippi River and St. Paul from Cherokee Park, above the Lilydale brickyards.

If you climb the trail, you’ll find yourself at the top of a steep bluff, gazing out onto a broad river valley below. The Mississippi River, mighty though it may be, is but a thin blue line through the center of the valley, like a rivulet streaming down of a bucket that has just been emptied. Where did the rest of the water go?

I used to think that rocks were boring, but now I am fascinated by the geologic stories in the rocks, cliffs, rivers, and lakes around our state.

The St. Croix River Valley was formed by a “failed continental rift” 1.1 billion years ago.

Take the St. Croix River as an example. According to Justin Tweet, a paleontologist with the American Geosciences Institute, the river valley was formed by a failed continental rift that almost split the land apart 1.1 billion years ago. In other words, if things had gone differently, there would be more dividing Minnesota and Wisconsin than just a Vikings-Packers rivalry.

During the Cambrian and Ordovician periods, 540 to 443 million years ago, the continents were arranged differently and an ocean covered most of what is now Minnesota. Over time, the sand at the bottom of this prehistoric ocean compacted to form layers of sedimentary rock called sandstone. The shells, coral, algae, fecal matter, and other organic debris on top of the sand eventually solidified into limestone. This limestone layer is where we find marine fossils today.

Image from Minnesota Geological Survey: Simplified map showing the extent and flow directions of ice lobes that covered Minnesota during the Ice Age (Lusardi, 1994, fig. 2).

Much, much more recently in geologic time, a series of glaciations swept through Minnesota in several waves from 2 million to 10,000 years ago. As the glaciers melted and receded, they created most of the lakes, rivers and groundwater aquifers that we see in our present landscape. Over time, the rivers carved through layers of soft sandstone to create steep bluffs like what we see along the St. Croix River and Mississippi River Gorge in Minneapolis. Downstream of Fort Snelling and along the Minnesota River, however, the bluffs are far from the water’s edge. That is because a much larger river called the Great River Warren once flowed through this valley as it drained glacial Lake Agassiz. In comparison, the modern-day Minnesota and Mississippi Rivers are but trickles. 

Sunrise at Paul Hugo Farms Wildlife Management Area, a popular location for duck and waterfowl hunting. Photo by Aaron DeRusha.

Glaciers also carved small dimples in the landscape that later filled with water to form lakes and wetlands. We call these prairie potholes. The prairie pothole region of the upper Midwest is known as the “duck factory” and supports more than 50 percent of our nation’s migratory waterfowl.

When the glaciers melted, some of the water soaked down into the ground and filled empty pore space in the sandstone layers deep below; these are the aquifers that we use for drinking water today.  Groundwater provides 100 percent of the drinking water for people in Washington County and 70 percent of the drinking water in Minnesota.

Groundwater aquifers beneath Washington County provide drinking water for residents.

The deepest aquifer in Washington County is the Mt. Simon aquifer, which exists in a layer of sandstone 160-255 feet thick that was laid down at the beginning of the Cambrian period. Forest Lake, Lake Elmo Park Reserve, and private wells along the St. Croix River draw water from the Mt. Simon aquifer. Above this, the Eau Claire, Wonewoc, Tunnel City, St. Lawrence, and Jordan aquifers also exist in sandstone from the Cambrian period. Numerous communities, including Cottage Grove, Lake Elmo, Oakdale, Oak Park Heights, Newport, St. Paul Park, and Woodbury, draw water from the Jordan aquifer.

The next layer, formed by sandstone from the Ordovician period, is the Prairie-Du Chien aquifer. This layer is fragile and there are portions of Washington County, especially in the southeast, where karst features such as sinkholes, fractures, and caves are quick to form. Hugo, Stillwater, and Mahtomedi draw their community water supplies from the Prairie-Du Chien and Jordan aquifers.

Fossils in limestone from Lilydale brickyards
A piece of Limestone found at the Lilydale brickyards contains brachipod, bryzoan, and crinoid fossils.

Along the Mississippi River, sandstone, shale, and limestone can be found above the Prairie-Du Chien aquifer. Closer to the St. Croix, however, these topmost layers have been worn away over time. That’s why you won’t find fossils at the St. Croix Boomsite the way you do at the Lilydale brickyards.

So, thank a glacier for the water we drink and enjoy a day at the ancient beach!