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In order to count this Earthcache as a find, you must complete the following tasks and email the answers to me. 1. What does the present area look like? 2. What do you think this area would have looked like just after the last glacier melted? Use your imagination and be as specific as possible. 3. Estimate the width of the present river channel. 4. Estimate the height of the ridge you are standing on in comparison with the river below. This should give you a rough approximation of how deep the ancient river channel was at this location. 5. In what direction is the river flowing? This earthcache is located at a little pull-off just off of highway 10. A National Natural plaque for Lake Agassiz and Glacial River Warren is located at this site as is a brief history of the ancient lake. Vistas of Lake Traverse can also be seen from this area. Enjoy! Eons ago, Lake Traverse in front of you, Big Stone Lake just to the south, and the eleven-mile stretch of water in northeastern South Dakota known as Cottonwood Slough, just to the north of here, was part of Lake Agassiz, the world’s largest glacial lake. The Wisconsin glaciation occurred somewhere between 30,000 and 10,000 years ago. During this last ice age, the northern part of North America was covered by a glacier. As the ice sheet started to break apart and melt, it sent strong and fast-moving streams of water southward. This water dammed up behind a barrier of glacial debris known as the Big Stone Moraine and pooled into a lake larger than all the Great Lakes combined. About 13,000 years ago, this gigantic lake, once known as Lake Agassiz, covered present-day Manitoba, western Ontario, eastern Saskatchewan, northern Minnesota, and eastern North Dakota. At its greatest extent, the lake covered up to 440,000 square kilometers--roughly the size of Uzbekistan or Iraq. Lake Agassiz was much larger than any current lake in the world, including the Caspian Sea, and held more water than contained by all lakes in the world today. At its maximum, Lake Agassiz was over 300 feet deep--where downtown Fargo and Moorhead sit today. The lake drained at various times and in all directions. It drained south through the Traverse Gap into the Glacial River Warren, east through present-day Lake Nipigon (then Lake Kelvin) to present-day Lake Superior, and west through the Mackenzie River through the Yukon Territory and Alaska. It is believed that around 13,000 BP there was a major flood of the Lake, causing it to drain through the Great Lakes and St. Lawrence River into the Atlantic Ocean. Climatologists believe this flood and draining may have been the cause of the Younger Dryas stadial. The Younger Dryas stadial, also known as the Big Freeze, was a geologically brief period of time (about 1300 years) in which the climate returned to ice age conditions. The return of ice for sometime offered a bit of a reprieve for the Lake. About 9900 years ago, after the ice sheet retreated north of the Canadian border and slowly melted, Lake Agassiz refilled. These events of Lake Agassiz had a significant impact on climate, sea level, and possible early human civilization. The majority of the final drainage of Lake Agassiz occurred within a very short amount of time--about a year. A recent study by Turney and Brown links this rapid drainage (and subsequent global seal level rise of about one meter) to the expansion of agriculture in Europe. This study also suggests that this drainage and previous flooding may also account for various flood myths of prehistoric cultures, including the Biblical flood. The last major shift in the drainage of Lake Agassiz took place about 8400 years before present. During this time, the lake took up its current watershed and slowly drained into the Hudson Bay. Over the next 1000 years, the Lake drained completely. Cottonwood Slough is a glaciated finger of the world’s largest glacial lake. Cottonwood Slough begins just east of the little town of Victor, near the North Dakota border, and meanders below farmsteads and cattle pastures before narrowing into the crick and then into Lake Traverse. The Cottonwood valley is about a half-mile wide and constitutes 5400 acres in total. Cottonwood Slough constitutes the southern tip of the massive Hudson Bay watershed. All the rest of South Dakota drains into the Gulf of Mexico, via the Missouri and Mississippi rivers. The two watersheds are separated by one of North America’s five great continental divides--ranges of hills and mountains that send the continent’s waters to the Pacific, Arctic, and Atlantic Oceans, and to the Hudson Bay and the Gulf of Mexico. The continental divide that passes through South Dakota is located just south of Cottonwood Slough. Along with its geological history, the wetlands of Cottonwood Slough act as a buffer zone that divides the tall grass prairie from the eastern deciduous forest country. A group of scientists from the University of Colorado who visited Cottonwood Slough to study the site concluded, “It is among the top natural areas in this part of the country for concentrating and production of waterfowl and one of the best natural riverine wetland complexes remaining.” Lake Traverse and Big Stone Lake inadequately fill the Glacial River Warren channel that was created when Lake Agassiz spilled out of its banks and carved a deep and wide channel. These two lakes also mark the division between the Hudson Bay watershed and the Gulf of Mexico watershed at the continental divide. In 1975, the National Park Service declared the lakes a Natural National Landmark. The landmarks program identifies unique geological and biological lands owned by either public or private parties. Less than 600 have been found in the entire United States. Cottonwood Slough is one of a dozen such sites in South Dakota. Resources: Hunhoff, Bernie. “Cottonwood Slough: A Natural Treasure.” South Dakota Magazine, March/April 2008: 26-30. Print. NOT A LOGGING REQUIREMENT: Feel free to post pictures of your group at the area or the area itself - I love looking at the pictures.
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