| Summary: | north than in the south, the amount of crustal depression (and subsequent rebound) was more in the north. It is theorized that the earth's crust was depressed about one foot for every three feet of ice and the ice was several thousand feet thick in northeastern North Dakota. Crustal rebound is still taking place to the north of North Dakota; in the Hudson Bay area, near Churchill, where the ice was much thicker and melted more recently, the land is still rising at the rate of 4 feet per century. North Dakota's modern landscape bears the legacy of the late Pleistocene glaciations, which eroded and reshaped the land surface. Broad areas of hummocky moraine formed when thick layers of stagnant glacial ice melted over the Turtle Mountains and the Missouri Coteau. Hilly areas of intensely thrust topography such as Sully's Hill are found in parts of eastern North Dakota along with ranges of rugged hills ("end moraines"). Eskers like those at Dahlen and Benedict occur where rivers once flowed in tunnels and cracks in the glacier. The Missouri and Sheyenne valleys are just a few of the routes used by water flowing from the melting water from the melting ice. Melrwater lakes like Lake Souris and Lake Agassiz were also the result of glaciation. Broad areas of sand dunes found on old river deltas in the lake plains were shaped by the wind in the time since the end of the glacial epoch. Even in southwestern North Dakota, which was never reached by the glaciers, a tundra climate during the Pleistocene resulted in frost polygons and other frozen-ground features, many of which persist today. Climate North Dakota's location at the center of North America results in a typical sub-humid, continental climate. The climate of the state is characterized by large annual, and day-to-day temperature changes, light to moderate precipitation which tends to be irregular in time and coverage, low relative humidity, plentiful sunshine, and nearly continuous air movement. Annual mean precipitation in the state ranges from 13 inches in the northwest to more than 20 inches in the extreme east and southeast. The Rocky Mountains act as a barrier to the prevailing westerly flow of air across the continent. They modify the temperature and moisture characteristics of air masses originating in the Pacific Ocean in ways that reinforce the continental characteristics of the climate. The lack of mountain barriers in the polar areas or the Gulf of Mexico, north and south of North Dakota, means that air masses originating in these regions easily overflow the state, sometimes with only minor changes in their basic characteristics. In every year and every season, North Dakota is visited several times by cold and dry air masses which originate in the polar regions, warm and moist air masses from tropical regions, or mild and dry air from the northern Pacific. The usually rapid progression of these air masses over North Dakota from the different source regions results in frequent and rapid changes of weather. 1997-1999 North Dakota Blue Book 497
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