Thursday, March 3, 2016

Hike across ice age sand drift in Minnesota

Trailhead for Spirit River Nature Area Loop. Courtesy City of Cambridge.
Map of Spirit River Nature Area and trail.
Courtesy City of Cambridge.

Spirit River Nature
Area Loop
runs 6.2 miles
in Cambridge

Day hikers can walk through a landscape deeply shaped by the most recent ice age at eastern Minnesota’s Spirit River Nature Area.

The 6.2-mile Spirit River Nature Area Loop crosses a sand plain and passes a kame left behind some 14,000 to 10,000 years ago when a glacier covered this area and then retreated. The nature area is located in the growing city of Cambridge, north of the Minneapolis-St. Paul metro area.

To reach the trailhead, in Cambridge from South Fern Street take Second Avenue SW west across the Rum River. Parking is available immediately after the bridge.

From the grassy area next to the parking lot, a primitive trail runs south through a woods. This is the dryer section of the nature area.

Among the largest glacier advances during the last ice age was the Des Moines lobe, which crossed from Canada and the Dakotas southwestward into Minnesota and Iowa. About 14,000 years ago, part of that lobe advanced northwest, covering the very area this trail strikes through. That advance is known today as the Grantsburg sublobe, because it ended at the small Wisconsin town of the same name to the northeast.

At about 0.22 miles from the trailhead, you’ll reach a junction with a mowed trail on land that once sat below that glacier. Continue south on the primitive trail.

Oxbow and kame
A few miles north of modern Cambridge, the Grantsburg sublobe held back a long body of meltwater, Glacial Lake Grantsburg, which stretched from northeast of Grantsburg to St. Cloud around 12,500 years ago. The lake lasted for about only a century before draining away as the glaciers melted.

At 1.6 miles and then again at 1.8 miles, the primitive path crosses a mowed trail. Continue straight on the primitive trail. After the second crossing is a floodplain forest to the northwest.

The trail reaches an oxbow lake at 2 miles. An oxbow forms when sediment on a river bend chokes off the water flow, and the river straightens its course. The water left behind usually becomes a horseshoe-shaped lake.

Circling around the oxbow, at 2.25 miles, the loop reaches a mowed trail that runs alongside the lake. Turn left/southeast onto it.

You’ll approach a kame that sits southwest of the trail. A kame is a small hill created when glacial meltwater laid down sand and gravel across an area.

At 2.61 miles, the trail comes to a four-way intersection at the oxbow’s southeast end. Go left/northeast onto the primitive trail. At the 2.84 mile mark, the trail crosses a narrow bridge of land where the oxbow lake was cut off from the Rum River.

Anoka Sand Plain
As the Grantsburg sublobe melted and retreated, it left fine-textured gray soil from limestone and calcareous shale. That soil is still there but buried under the current sandy surface.

At 2.95 miles, the loop reaches a mowed grass trail. Go right/east onto it and hike alongside the Rum River, which cuts through that sandy cover.

While the Grantsburg sublobe was in retreat about 12,000 to 10,000 years ago, it blocked the drainage of water to the east, resulting in the formation of Glacial Lake Anoka west of Cambridge. The shallow lake filled with fine sediment in an ancient depression. Eventually the sublobe melted enough to allow the lake to drain, leaving behind a loamy fine sand. This sand then blew east, including across the Cambridge area, creating what today is known as the Anoka Sand Plain.

The loop at 4.55 miles intersects a mowed trail. Go right/northeast onto it and continue following river.

The sand plain ranges from a few inches to several feet deep. As walking alongside the Rum River, see if you can spot any change in the bank’s texture between sand and the gray drift.

At 6.2 miles, the loop reaches the grassy area where you started hike on the primitive trail.

Final note: The nature area used to be known as West Park and so may appear with that moniker in older guides and maps.

Learn about trail guidebooks available in the Hittin’ the Trail series.