Geology

Today, hills and wetlands cover the landscape of the City of Maplewood. This landscape was formed as the last glacier that covered this area melted, about 10,000 years ago.

But the record in rock in Maplewood goes much farther back. The bedrock under Maplewood is from the Ordovician period (500 to 435 million years ago). During this long period, the Twin Cities area was on the southern edge of a land mass and was covered by a shallow sea. Sandstone was formed from ancient beach, limestone was formed from the sea and its creatures, and shale was formed from mud. Evidence of the ancient sea is exposed along the Mississippi River bluffs near south Maplewood as outcrops of Decorah Shale, Platteville Limestone, Glenwood Shale, and St. Peter Sandstone.

Chain Lakes

Of course, many more rock layers were added since the Ordovician, but most of that geological record was erased by glacial ice building, moving, and melting over the last 2 million years. One pre-Ice Age remnant we still see is the chain of lakes of Kohlman-Gervais-Spoon-Keller-Round-Phalen. These lakes outline the St. Croix River valley from before the last glacier, when it flowed west of its current course.

Last Glacier

The last glacier to cover Maplewood was during the Wisconsin Ice Age (100,000 to 10,000 years ago), the Superior Lobe and later the Grantsburg Lobe extended to Maplewood. A glacier nearly a mile thick covered Maplewood. As the glacier melted, the landscape was sculpted by the retreating glacier and its meltwater, which resulted in the hills and wetlands found throughout the city. The area of I-694 and White Bear Avenues is the approximate southern boundary of the Grantsburg Lobe. The relatively flat area around I-694 and U.S. 10 is the bottom of a glacial lake formed by water from ice melting from the Grantsburg Lobe.