Sherburne County Heritage Center Interpretive Plan Final Report 2005

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Exhibit Station: 4 Topic: Sherburne County is a Glacial Beach Story Glaciers covered Sherburne County off and on during the Pleistocene Epoch, which started 2 '/2 million years ago. The most recent glacial era was the Wisconsin glaciation, which lasted from about 35,000 to 10,000 years ago. The Superior Lobe was the first ice sheet to cover Sherburne County during this period, reaching out of the Lake Superior basin to just south and west of Sherburne County before retreating about 13,500 years ago. As this glacier advanced and retreated, it bulldozed and deposited mixed rock, gravel, and sand creating rolling hills. Soil in the Santiago area formed in the reddish brown till deposited by the Superior Lobe. The next and most recent glacier to cover Sherburne County was the Grantsburg Sublobe of the huge Des Moines Lobe of the glacier that swept in from the northwest, then turned and entered Sherburne County from the southwest. It buried most of Sherburne County in a half -mile thick layer of ice. The glaciers gouged and remade the land as the earth's climate periodically cooled and warmed. Surface bedrock boulders were reduced to rock, gravel, sand, and silt by the grinding action of the gigantic glacier. As the glaciers receded, the ice stagnated for a period in northern and eastern parts of the county. A large crevasse formed in the ice and mixed sand and gravel from both the Superior Lobe and the Grantsburg Sublobe washed into the broad, long gash. As the glaciers continued to melt and recede, the deposits in the crevasse remained in high relief, forming the glacial till hills north of Elk River. These hills, one of the most abundant sources of gravel in the county, still rise above the level plains of most of Sherburne County. Sherburne County was on the edge of the Anoka Sandplain. As the glaciers slowly receded between 12,500 and 10,000 years ago, glacial meltwater sorted and sifted debris. Sand -laden streams formed from the meltwater and shifted across the surface of the county. These streams deposited sand and gravel in broad, level plains. A tongue of Glacial Lake Anoka licked down from the northeast of the county, leaving flat sands around Zimmerman. As the ice continued to melt, large streams cut a wide channel from St. Cloud to Minneapolis. The glacial meltwater gradually cut a deeper channel that later became the Mississippi River. Former wide channels became floodplain benches that can be clearly seen today by their flat surfaces that stretch from the river to higher crests that indicate previous channel shorelines. As the glacier retreated, huge chunks of rotting ice were broken off and buried beneath insulating debris. When the ice finally melted hundreds of years later, the debris collapsed into a huge hole, creating kettle lakes. As the weather continued to warm and become drier about 8,000 years ago, wind scooped exposed sand and piled it into unusual dunes in central Sherburne County. Examples of the dunes are visible on the Heritage Center trail. Sherburne County Historical Society Heritage Center Interpretive Plan, April 21, 2005, page 24