| Summary: | Ice-Thrusting In North Dakota: North Dakota's Champagne Geology5 The landforms resulting from thrusting of geologic material by the glaciers are among the most unusual and interesting found in North Dakota. These are places where the glacier extracted (or plucked) a large chunk of rock and sediment and moved it a short distance before setting it down again. Generally, the result was a hole, from the spot where the block was taken, along with a nearby hill, where the material was dropped. The ice-thrust features are most common in central North Dakota. For example, Steele Lake at the town of Anamoose occupies a depression formed about 12,000 years ago when the glacier extracted (lifted or thrust) a chunk of material, moved it as a single piece, and then set it down a tenth of a mile to the southeast. The lake is straight south of Anamoose and the materials that were once where the lake is today form a hill on the southeast edge of the lake (fig. 8 and 9). Areas of ice-thrust topography are found in many places on the plains of North Dakota and throughout the prairie provinces of southern Canada. Several other kinds of ice-thrust landforms have now been recognized in North Dakota, but the hill-hole combinations, roughly equidimensional hills containing thrust masses located downglacier from a source depression of similar size and shape, are the most striking. NDGS geologists developed a theory about 30 years ago to explain features like the one at Anamoose, The features tend to be found overlying buried aquifers, rock or sediment layers that contain water. An aquifer might be contained in a bed of permeable sandstone, or it might be developed in gravel beds that had been deposited by water flowing from the melting ice ("permeable" means that the rock or sediment has interconnected holes so that water can seep or flow through it). In many cases, the aquifers are deeply buried, beneath impermeable layers of rock (impermeable means water can't flow through it). When the glacier advanced over such an aquifer, its weight sometimes caused elevated pore-water pressures to build up in the beds of sediment overlying the aquifer. When this happened, the pressure pushed or forced the materials upward, into the path of the advancing ice. As a result, large, intact blocks of material were incorporated into the advancing glacier. The situation is analogous to the action of a hydraulic jack. The "jack" pushed the material upward into the path of the glacier. These large blocks of material were moved a short distance (they were "thrust") by the glacier. The result was a hill (the ice-thrust block) and a hole (the place the block came from). Some of the blocks that were moved by this process are truly huge, on the order of a few hundred feet thick and several square miles in area. Pressurized groundwater is necessary for thrusting to occur and in order to build up pressure, something had to confine the water, ff the groundwater was able to escape ahead of an advancing glacier, thrusting did not occur. If it could not escape, for whatever reason, thrusting was possible. In some cases, it's likely that a surface layer of 480 Chapter Ten - Natural History
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