Glacial Lake Aitkin EarthCache
-
Difficulty:
-
-
Terrain:
-
Size: 
(not chosen)
Please note Use of geocaching.com services is subject to the terms and conditions
in our disclaimer.
he proglacial lakes of Minnesota were lakes created in what is now the U.S. state of Minnesota in central North America in the waning years of the last glacial period. As the Laurentide ice sheet decayed at the end of the Wisconsin glaciation, lakes were created in depressions or behind moraines left by the glaciers. Evidence for these lakes is provided by low relief topography and glaciolacustrine sedimentary deposits. Not all contemporaneous, these glacial lakes drained after the retreat of the lobes of the ice sheets that blocked their outlets, or whose meltwaters fed them. There were a number of large lakes, one of which, Glacial Lake Agassiz, was the largest body of freshwater known to have existed on the North American continent; there were also dozens of smaller and more transitory lakes filled from glacial meltwater, which shrank or dried as the ice sheet retreated north.
Glacial Lake Aitkin was also a product of the recession of the St. Louis Sublobe, and for significant portions of its history was contiguous with Glacial Lake Upham. It occupied a broad lowland along the valley of the present-day Mississippi River between Grand Rapids and Aitkin in north central Minnesota. The lake bed is now a sandy and clayey plain.
Glacial Lake Upham was formed in the wake of the retreat of the St. Louis Sublobe of the Des Moines Lobe. It drained through a series of successively lower outlets to Glacial Lake Duluth, culminating in the Saint Louis River. Its former lake bed is now a broad boggy area comprising much of the watershed of the latter stream.
The Wisconsin glacial episode, the most recent glacial period, has been subdivided into four substages, each representing an advance and retreat of the ice. The substages, named from the oldest to the youngest, are the Iowan, Tazewell, Cary and Mankato. Only the Iowan, Cary and Mankato are recognized in Minnesota, but studies indicate that the Tazewell drift may be present in southwestern Minnesota.
The Iowan drift occurs extensively at the surface only in southwestern and southeastern Minnesota, and contains few, if any, lakes because of the relatively mature surface drainage. The Tazewell drift in the southwestern Minnesota is devoid of lakes; in fact, the criterion of drainage was used by Robert Ruhe to distinguish Tazewell from Cary drifts.
Nearly all of the lakes in Minnesota are found within the borders of the Cary and Mankato drifts. For this reason, it is necessary to consider in some detail the nature and distribution of these two drift sheets.
With the retreat of the Patrician ice, the stage was set for the final phase of the Wisconsinan glaciation in Minnesota. The last major advance of the continental glacier in Minnesota culminated in a lobe that reached as far south as Des Moines, Iowa. The glacial movement from the northwest was from a more distant source than ice from the northwest. The subsequent glacier that moved into Minnesota was quite thin and unable to cause much erosion. The Des Moines lobe produced a northeast-moving projection known as the Grantsburg sublobe. Also protruding from the main Keewatin ice sheet was the St. Louis sublobe. The drift of these ice lobes is generally in late Wisconsinan time. The sediment transported by the Mankato glacier is colored tan to buff and is clay-rich and calcareous because of shale and limestone source rocks to the northwest. The Superior lobe also developed during Mankato time and advanced as far west as Aitkin County, Minnesota.
The Grantsburg sublobe effectively blocked the drainage of the Mississippi River from north of St. Cloud southeastward through the Twin Cities. The outwash carrying large quantities of sand was diverted overland to the east around the sublobe. No true drainage valley was produced; instead, multiple small streams flowed toward the northeast depositing their overloads of sand as they went. This produced a roughly triangular sandy outwash region called the Anoka Sand Plain, reaching from St. Cloud to the Twin Cities up to the northeast to Grantsburg, Wisconsin.
Submit these answers to receive created for the Earthcache:
1) What is your current elevation?
2) The Elevation of Lake Superior is approx 602 ft above sea level. How much of a descent does the water need to make from this location before it reaches Lake Superior?
3) How long ago did the first glacier cover Minnesota?
4) When did the Wisconsin glacier begin to melt?
5) According to Carbon 14 dating how old are the peat bogs left by Glacial Lake Aitkin?
Additional Hints
(No hints available.)