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Anoka Sandplain:

Geologic History of the Anoka Sandplain

The formation of the landscape that we see as the Anoka Sandplain today is primarily due to a series of glaciers that scoured the terrain and left deposits of sediment as they retreated. To fully understand the geology of the sand plain, resource managers must look back over a billion years ago when the crust of the earth split, from Lake Superior to Kansas. Lava poured out and hardened into basalt. The lava accumulated to such an extent that the earth's crust sagged, forming a depression in the basalt. Water carried sediments into the depression for millions of years, covering the basalt and compacting into layers of sedimentary rock such as sandstone, siltstone, and shale.

About 550 million years ago, a series of shallow seas covered the area depositing more sedimentary rocks over the area. After the seas withdrew, about 450 million years ago, millions of years of weathering ensued. Beginning about 1.6 million years ago, a series of glaciers advanced and retreated across the landscape. They left rock, silt, clay, sand, and gravel as they moved and rearranged the landscape. Most of the landscape features we see today were created by the last episode of glaciers, the Wisconsin glaciation.

The Wisconsin glaciation started about 35,000 years ago and lasted until about 10,000 years ago. It consisted of two major glacial ice lobes. The first was the Superior lobe, which advanced southward from eastern Ontario through Lake Superior to encompass the Anoka Sandplain (nearly level to undulation, excessively drained, somewhat poorly drained, and very poorly drained soils that are dominated by fine sands throughout. When it retreated, it left behind reddish brown sandy loam.

The second major glacial ice lobe was the Des Moines Lobe. It came from Manitoba in the northwest and traveled south into Iowa. On the eastern side of the Des Moines Lobe, a sub-lobe called the Grantsburg Sublobe formed and pushed north into the Anoka Sandplain area. It carried "gray" till with it, a light brown sandy loam derived from the limestone bedrock around Winnipeg, Manitoba in Canada.

As the glaciers receded, large amounts of water laden with sediment flowed from the ice. Streams formed and shifted leaving mostly sand and some gravel in their path. The streams generally flowed into the St. Croix river valley. However, the stream channels were dammed several times and a series of large lakes were formed. Each lake may have lasted for hundreds of years until water levels rose high enough to break through the dams.

Lake currents spread the sand sediment over the glacial till. Areas of higher elevation of glacial till did not get covered by the sand. Depressions were created where large blocks of ice were buried and gradually melted. Peat began to form in many of these depressions and the low areas of the lake beds beginning about 10,000 years ago. During a warm dry period 4,000 to 8,000 years ago, the sands were exposed to strong winds and dunes were formed. This combination of factors led to the diversity of landscape types within the Anoka Sandplain.

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