![]() Geologists think this blanket of sediment postponed the buried ice around the Turtle Mountains from melting for about 3,000 years. Over time, a layer of sediment and other debris built up on top of these stalled glaciers and insulated the ice. As the climate warmed between 15,000 and 13,000 years ago, that ice began to thin until glaciers on the hills stagnated and stopped flowing. About 25,000 years ago, ice sheets flowed southward over the Turtle Mountains. ![]() But in the eyes of geologists, they are clues pointing to a cooler period when the hills were blanketed by ice. The lakes make for picturesque canoeing and camping destinations for travelers. In the lower image, a detailed view of a largely undeveloped part of Turtle Mountain Provincial Park, hundreds of ponds and lakes pockmark the landscape. The surrounding lowlands are a patchwork of grasslands and farms. As shown by this image from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on NASA’s Terra satellite, enough moisture collects on the hills to support forests. ![]() Whether hills or mountains, the hummocky highlands that straddle the border between North Dakota and southern Manitoba have enough elevation that they receive significantly more precipitation than the surrounding plains. Board of Geographic Names argued that mountains should have at least 1,000 feet (300 meters) of local relief to earn the designation, but the group abandoned the argument for linguistic consistency in the 1970s.) But in North Dakota, one of the flattest states, people have a habit of calling even relatively modest rises mountains. In most American states, the Turtle Mountains-which rise 600 to 800 feet (180 to 240 meters) above the surrounding plain-would be called hills. ![]()
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