I got a call the other day from a friend in St. Louis. She had seen television reports about the awful flooding in Minnesota and was concerned about us. I assured her we were fine, but I couldn’t explain why. Why is most of Anoka County dry only a few hours after a heavy rain?
The answer lies in the Anoka Isanti Sand Plain. It’s one of those places where history and geology intersect. It all started during the Pleistocene period about a million years ago. During this time, the entire state was covered by a glacier. Glaciers, of course, still exist, and may be several hundred yards across. Still, I have a hard time imagining a sheet of ice that covered all of Minnesota and most of the continent.
Apparently there were two distinct glaciers, both originating in southern Canada. One came from the northwest and is called the Keewatin Ice Sheet, and the other from the northeast is called the Labradorean Ice Sheet. Both flowed silently over anything in their paths, imprisoning rocks and debris in their icy underbellies and scraping the earth’s surface as they moved on. This happened very slowly over thousands of years of moving, melting, growing, receding, and advancing again.
Glacial action gouged out the St. Croix River Valley. Initially, the St. Croix moraine was a solid core of ice, still too frozen to allow ice melt from the receding glacier to escape through it, until later when sufficient melting had occurred. The last advance of the glacier entirely covered what is now Anoka, Isanti, Sherburne, and Chisago counties. As it receded, the glacier left fan shaped deposits of rock, gravel and sand. Prominent hills in northwestern to southernmost Anoka County were formed in this way. More significantly, the deposits created a dam separating the area from the moraine.
Eventually, the glacier receded to the north, across land that was low and flat — probably because the area had been scraped by the same glacier a few hundred years prior. At any rate, the melt water could not escape through the frozen St. Croix moraine, so it became a huge frozen lake — a single giant ice cube that covered most of Anoka, southern Isanti and central Chisago counties, and lasted for hundreds of years. As the Ice Age waned, the glaciers branched into smaller lobes, including the Grantsburg lobe, which was unique in that it was a northeast flowing offshoot of a south flowing lobe. It flowed around the lake, over its top, and through channels in the decaying ice, bringing silt, sand, and finely ground gravel formerly trapped in the glaciers. The cold of the existing ice chilled and slowed the melt water, causing it to deposit its cargo of sand and finely ground particles before refreezing. This happened slowly, melting a little more each year for hundreds of years, melt water seeking channels, deepening them where the flow was rapid and depositing silt where the water slowed. Eventually the dominant channel grew to become the Mississippi River.
“Geomorphic evidence in southeastern Chisago County indicates that a new outlet south of the blocking fan opened and allowed the catastrophic draining of the lake.” It left behind the same fine sand that is typical of local lake bottoms today, but on a giant scale, and to a great depth.
The glaciers are why Anoka County is rich with water, both on the surface and underground. Melting and freezing continue to sculpt the terrain even today, but one would need to wait a few thousand years to see where this process is heading.
*Taken from Anoka Sand Plain Regional Hydrogeologic Assessment: Surfical Geology, by Gary N. Meyer, Carrie J. Patterson and Howard C. Hobbs of the Minnesota Geological Survey and J. D. Lehr of the Minnesota Department of Natural Resources.
Maria King is a volunteer with the Anoka County Historical Society.