The landforms resulting from thrusting by the glaciers are among the most unusual and interesting found in North Dakota. These are places where the glacier extracted — 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 (Fig. 1).
Figure 1. Map of part of central North Dakota showing area where large numbers of ice-thrust hills and associated depressions occur. An area of long drumlins occurs behind (northwest of) the ice thrust blocks.
Figure 2. View from the air over the town of Anamoose in southeastern McHenry County. Steele Lake, on the south edge of town, is adjacent to an ice-thrust hill immediately to its southeast. The hill consists of material that was moved by the glacier, southeastward from where the lake is now.
Figure 3. Map of the same area shown on Figure 2.
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 (as at Anamoose) are the most striking.
Briefly, the theory NDGS geologists developed about 30 years ago to explain features like the one at Anamoose is as follows:
Figure 4. Extent of the Prophets Mountains in western Sheridan County, a few miles northwest of McClusky. The Prophets Mountains occur over a large aquifer which is contained within the valley of an ancient river.
Figure 5. Air view of the Prophets Mountains. These ice-thrust hills consist of slabs of material that were pushed up by the ice from east to west. The snow marks troughs between ice-thrust ridges.
Pressurized groundwater is necessary for thrusting to occur and in order to build up pressure, something had to confine the water. If 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 frozen ground — permafrost — contributed to thrusting by acting as a confining layer for the groundwater.
Figure 6. View over a lake, toward the Binford Hills in Griggs County. The lake is contained in the hole that formed when the Binford Hills were thrust by the glacier into their present location.
Figure 7. Glacially deformed bedrock in a road cut in the Prophets Mountains.
Just a few more examples of thrust features like the one at Anamoose: 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; Blue Mountain and the adjacent depression west of Stump Lake in Nelson County — dozens of the features are found in North Dakota. The granddaddy of all ice thrusts is Devils Lake. The materials 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.
I suppose it's appropriate that the best champagne topography in the United States is found in North Dakota, the home of Lawrence Welk and his "champagne music!"
Photos by John Bluemle