During the last Ice Age, northern North America was covered by a glacier, which alternately advanced and deteriorated with variations in the climate. This continental ice sheet formed during the period now known as the Wisconsin glaciation, and covered much of central North America between 30,000 and 10,000 years ago. As the ice sheet disintegrated, it created at its front an immense proglacial lake, formed from its meltwaters.
Quaternary Geology
The term Quaternary refers to the last 5 million years of geologic time. Geologists subdivide it into the Pleisocene or the "Great Ice Age", when nearly all of Minnesota was covered by glacial ice, and the Holocene or "Recent", a time since the end of the Great Ice Age.
In Pleistocene time, glacial ice that flowed south into Minnesota came from an ice sheet that covered much of Canada. In some places, that ice was more than a mile thick. Glacial ice was much thicker in areas where precipitation was heavy and the climate cold. The areas of thicker ice are called ice domes, or ice centers.
Ice in the domes flowed in response to the pressure and gravity on thick masses of ice. The direction or movement was away from the ice centers. Ice streams, or ice lobes, advanced southward from the Keewatin and Labradorean Domes. As climactic conditions waxed and waned over time, the strength of the flow away from those ice centers also changed over time. Thus, as ice lobes flowed south, they battled one another for space as their masses and relative velocities changed. When a milder climate and other factors reduced the thickness of glacial ice, a lobe ceased to flow, and it stagnated. Ultimately, these lobes melted back to a more northern location. Places where the ice stopped and stagnated are marked my end moraines or complicated landforms having a rugged topography and numerous small lakes and swamps.
Glacial Lake Agassiz
As the Wisconsin Glacier slowly began to melt some 12,000 years ago, it created Glacial Lake Agassiz, the largest known glacial lake in North America. At its maximum the lake covered an area of over 110,000 square miles -- more than the area of the combined Great Lakes -- spread over parts of areas known today as Minnesota, North and South Dakota, Manitoba, Saskatchewan, and Ontario. Over 400 feet deep in places it extended over 700 miles from its southern tip near Browns Valley to its northern tip near Hudson Bay.
While the glacier moved south during its advance, it built up a moraine on its southern end which ponded the melt waters when the ice began its retreat. Eventually the water over flowed, cutting an outlet which created the present Minnesota River valley. Over approximately 3,000 years, as the glacier continued to melt, numerous beaches were created along the lake's shores.
As the glacial ice thinned and melted back, the deployment of meltwater and runoff from precipitation was influenced more and more by the emerging drift-covered bedrock geography. When the Des Moines Lobe melted back across a topographic divide near Brown's Valley, which now separates north-flowing drainage in the Red River Basin from the south-flowing Minnesota River. Even before that, water became ponded in back of the Bemis Moraine as the south-western side of the Des Moines Lobe melted down into the Minnesota River Lowland. Expansion of the lake began by at least 12,000 years ago, when the Des Moines Lobe retreated northward into the Red River Basin. Meltwater was trapped between the ice margin and a higher topographic rim, a recessional moraine, forming the southern boundary of the Red River Lowland. Some of the stages of lake level are recorded in a series of beaches, some of which can be traced continuously for hundreds of kilometers. They indicate that Lake Agassiz fluctuated significantly in size and depth, with a surface area at any one time not exceeding 128,000 square kilometers. In places it was more than 120 meters deep.
The Red River flows across the flat lakebed of the ancient glacial Lake Agassiz, an enormous glacial lake created at the end of the Wisconsin glaciation from meltwaters of the Laurentide ice sheet. As this continental glacier decayed, its meltwaters formed the lake, and over thousands of years sediments precipitated to the bottom of the lakebed. These lacustrine soils are the parent soils of today's Red River Valley. The river itself is very young; it began only after Lake Agassiz drained, about 9,500 years ago.
The word "valley" is a misnomer. While the Red River drains the region, it did not create a valley wider than a few hundred feet, and the much-wider floodplain is the lakebed of the glacial lake. It is remarkably flat; from its origin near Breckenridge, Minnesota to the international border near Emerson, Manitoba, its gradient is only about 1:5000 (1 metre per 5 kilometres), or approximately 1 foot per mile. The river, slow and small in most seasons, does not have the energy to cut a gorge. Instead it meanders across the silty bottomlands in its progress north. In consequence, high water has nowhere to go, except to spread across the old lakebed in "overland flooding". Heavy snows or rains, on saturated or frozen soil, have caused a number of catastophic floods, which often are made worse by the fact that snowmelt starts in the warmer south, and waters flowing northward are often dammed or slowed by ice. These periodic floods have the effect of refilling, in part, the ancient lake.
To log this Earthcache, please email me the answers to these questions:
1. What are the listed names of the beaches referenced above?
2. From the same source as the information above (and in the hint): What is this region sometimes called?
3. What evidence can you find at this site of glacial activity?(You may have to do some homework to know how to answer this question! Relate what you know or learn about the affects of glaciers on the landscape to what you see around this area.)
4. What would the elevation of the surface of glacial Lake Agassiz be at this site, using a reading of the site's elevation and adding the above depth of 120m?
Please post photos of you at the site!
Sources: Morey and Dahlberg. Geology of Minnesota. (1995) Minnesota Department of Natural Resources. St. Paul, MN.
Ojakangas and Matsch. Minnesota's Geology. (1982) University of Minnesota Press. Minneapolis, MN.
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