| Summary: | frozen ground, permafrost, contributed to thrusting by acting as a confining layer for the groundwater. We have identified several kinds of ice-thrust features, but the most striking and obvious of those found in North Dakota are the hill-hole combinations like the one at Anamoose. Discrete hills of ice-thrust material, often slightly crumpled, are situated a short distance downglacier from source depressions of similar size and shape. The source depressions commonly contain lakes or ponds. A typical ice-thrust hill of this type is about 100 feet high and less than a mile across. It may be located immediately adjacent to the hole from which it came or it may be as far as three miles away from it. If the glacier continued to advance over an area after it had thrust the material, the feature tended to become smeared out. Many ice-thrust features are buried beneath younger glacial sediment. The best ice-thrust hills are found in places where the glacier stopped advancing almost as soon as it completed the job of thrusting the materials. Dozens of ice thrust features are found in North Dakota. Other examples of thrust features like the one at Anamoose are Egg Lake and Egg Lake Hill southeast of Harvey in Wells County, Medicine Lake and the adjacent Grasshopper Hills north of Jamestown in Stutsman County, Rugh Lake and the adjacent hill in eastern Nelson County, and Blue Mountain and the adjacent depression west of Stump Lake in Nelson County. The largest of all the North Dakota ice thrusts is Devils Lake. The material from the ice-thrust hole that today contains Devils Lake are all piled up along the south side of the lake. A lot of these kinds of ice thrust features are found in North Dakota, but the features are not common in other states. Why? A partial answer to this question relates to the fact that regional drainage in North Dakota was northward and the glaciers were advancing southward. Thus, the glaciers were advancing against the direction of groundwater flow. The rivers were simply diverted east and south, around the edge of the glacier, but the groundwater couldn't go anywhere. In any situation in which the groundwater could not easily escape, there was the potential for pressures to build up. For example, if the surface of the ground ahead of an advancing glacier was frozen, this could form an impermeable seal on the surface of the ground. In many parts of central and northern North Dakota, water probably simply became trapped in the bedrock or glacial sediments beneath the ice, increasing the likelihood of thrusting. Thrusting also occurred in northern Europe, where glaciers advanced southward against the regional groundwater flow systems. On the other hand, in areas where the glaciers advanced in the same direction as the regional groundwater flow systems, as in Scandinavia and the British Isles, very little thrusting occurred. The groundwater was able to escape ahead of the advancing ice. The formation of thrust features like the one near Anamoose might be likened to popping the cork from a bottle of champagne. After the initial release when the thrusting occurred, the groundwater gushed out of the resulting hole and the pressure dissipated. To draw an analogy, the pressure in the bottle (aquifer) was released when the cork (hill) was removed from the bottle. Thus, "North Dakota champagne topography!" 482 Chapter Ten - Natural History
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