Kettle lakes As glaciers advanced and retreated through Minnesota, some of the ice that stagnated was more difficult to melt than other areas. The glaciers continued to deposit sediments around and sometimes on top of these isolated ice blocks. As the ice blocks melted, they left behind depressions in the landscape. The depressions filled with snowmelt and rainwater producing
kettle lakes. Kettle lakes may be formed within the
ground moraine region behind the
terminal moraines. They can be of any size and their shorelines can be composed of anything from clay to sand to boulders. In a terminal moraine region, the kettles are fairly small but deep, to fit between the moraine's steep and hilly ridges. If the ice had advanced outward and then retreated leaving behind an outwash, kettles may have formed. Outwash kettle lakes are usually shallow and their numbers are much smaller than in other glaciated regions. The abundant sand quickly can fill in the depressions and composes most of the beaches of these lakes. Because Minnesota has had glacial movements into the state from both the northeast and northwest, the landscape has been modified by overlapping glacial regions. For example, an outwash plain from the Cary glacier may have a newer cover of ground moraine from the Mankato glacier, or a Cary ground moraine may have been subsequently covered over by Mankato outwash. The majority of lakes in the world are kettle lakes produced by glacial activity. In Minnesota, the majority of kettles lakes reside in ground moraine and terminal moraine areas.
Bedrock erosion lakes In northeastern Minnesota, the glaciers were thousands of feet thick. As the glaciers moved through the area they eroded large quantities of rock away. Ice itself is not very abrasive, but by picking up and moving pieces of rock it was able to scrape away softer underlying materials. Volcanic rocks underlie the area. Along the
Rove region of the
Arrowhead Region, there are multiple tilted layers of volcanic rocks interspersed with layers of shale; the shalelayers are softer than the volcanic basalt. As the glaciers eroded the materials, the shales were removed. The topography in the Rove region alternates hills (composed of diabase) and valleys (former shale layers); these hills and valley orient in a west to east direction. Many of these valleys are now lakes. Because the lakes are oriented west-east, visitors to the Rove region think that the ice moved in an east-west direction. Glacial striations (scratches) show that the ice moved from north-to-south, perpendicular to the orientation of the lakebeds themselves. Adjacent to the Rove area
Lake Superior's basin resides in a billion-year-old trough which was caused by the Midcontinent Rift. Preglacially the depression had been filled with eroded shales. The thousands of feet of glacial ice eroded away a large amount of the shale. The ice was so thick that it scoured the sandstone down to depths of below sea level. The present Lake Superior is the single largest freshwater lake in area in the world.
Glacial lakes About 18,000 years ago, the
Laurentide Ice Sheet began to melt and retreat. As the Mankato ice shrank meltwaters became ponded in several places along the margin of the glacier. Some of these lakes covered several hundred thousand square miles and have left a definite imprint on the
topography. All of them have since been drained by natural force or have been shrunk considerably from their original size.
Glacial Lake Duluth Glacial Lake Duluth is the body of water that formed at the southwestern margin of the Superior lobe and occupied a much larger area than the present
Lake Superior. Its shorelines stood nearly 148 meters above the level of its modern successor, Lake Superior. During its early history Lake Duluth drained into the Mississippi River down the
St. Croix River Valley. There were two outlets, one along the
Kettle and
Nemadji Rivers in Minnesota and another to the east along the
Bois Brule River in Wisconsin. Later, however, when the Superior lobe had retreated farther to the northeast, the waters of Lake Duluth merged with those in the Michigan and Huron basins, and the southern outlets were abandoned in favor of a lower one on the east end of Lake Superior. The Kettle River no longer drains Lake Superior, but resides in a large valley, which itself could not have produced with its present discharge. The Nemadji and Bois Brule Rivers actually flow northward toward Lake Superior through the eastern proglacial outlet. Even though vast amounts of water flowed over the southern rim of Lake Superior, the Bois Brule River outlet was never scoured deep enough to remove a
continental divide at the Bois Brule River's
headwaters.
Proglacial lakes Glacial Lake Agassiz The largest of all the
proglacial lakes was
Lake Agassiz, a small part of which occupied the present
Red River Valley of Minnesota and
North Dakota. Glaciers to the north blocked the natural northward drainage of the areas. As the ice melted, a proglacial lake developed southward of the ice. The water overflowed the continental divide at
Browns Valley, Minnesota; drained through the
Traverse Gap and cut the present
Minnesota River valley. The amount of discharge was staggering. It helped the adjacent Mississippi River to form a very large valley in the southeastern Minnesota. The river that drained from Lake Agassiz is called the
Glacial River Warren. It flowed over the top of a
recessional moraine at Browns Valley. As the water eroded away the glacial deposits, the level in the lake dropped. Eventually enough large boulders were left behind that a boulder pavement was produced, which inhibited further downward cutting. The lake level was thus stabilized for a while. During the few decades when the level was constant, waves on the lake produced noticeable beaches along the shoreline. Glacial outwash was also being deposited on the bottom of the lake. Eventually the boulders at the lake outlet were eroded downstream and the river then could erode downward through a mix of sediment sizes. Again, a boulder pavement formed and, as before, the lake level stabilized at a lower level, again forming another set of beaches. After further retreat of the ice into
Canada, lower outlets were uncovered to Hudson Bay, and the Minnesota Valley outlet was abandoned. The continental divide at Browns Valley become the headwaters for the north flowing
Red River of the North and southeast flowing tributary of the Mississippi River, the
Minnesota River. During its existence, Lake Agassiz may have been the largest freshwater lake to ever have existed. The lakebed composed of lake muds and silts is one of the flattest regions of Earth and is extremely fertile. No bedrock erosion lakes exist there because the ice was too thin to erode. No kettle lakes are found on the lakebed because lakebed deposits would have filled their depressions. ==See also==