| Summary: | stagnant ice over the Turtle Mountains melted, the debris it contained gradually became concentrated at the surface of the ice, resulting in an increasingly thick insulating layer that greatly retarded the rate of melting. Thus, even though the glacier stopped moving and stagnated over the Turtle Mountains about 13,000 years ago, the layer of insulation that built up on top of the stagnant glacial ice kept it from melting for about 3,000 years. It was not until about 10,000 years ago that the last ice on the Turtle Mountains melted. The covering of glacial sediment on the stagnant glacier on the Turtle Mountains was irregularly distributed and, as a result, the ice there melted unevenly. This uneven melting caused the upper surface of the stagnant ice to become hilly and pitted with irregular depressions. The glacial sediment on and within the ice was saturated with water from the melting ice and it was highly fluid. It slid down the ice slopes as debris flows and filled in the depressions. Thick accumulations of debris in the depressions on the stagnant glacier caused the ice in those places to melt more slowly, whereas newly exposed ice, from which the insulating cover had recently slid, melted more rapidly, resulting in continued reshaping of the surface of the stagnant, sediment-covered glacier. The environment over the Turtle Mountains gradually stabilized and the lakes Hooding the sediment-lined depressions on the stagnant glacier became more temperate. Most of the water in the lakes came from runoff from local precipitation, rather than water from melting glacial ice. Precipitation at the time was much greater than it is today, probably 50 inches or more a year, and the mean annual temperature was a few-degrees cooler than it is today. Fish and clams and other animals and plants thrived in the lakes that developed on top of the sediment-covered glacier. Surrounding the lakes and streams, the covered glacier was forested by spruce, tamarack, birch, poplar, aquatic mosses, and other vegetation, much like parts of northern Minnesota today. The stagnant-ice environment in the Turtle Mountains about 10,000 years ago was similar, in many ways, to stagnant, sediment-covered parts of certain glaciers in south-central Alaska today. Eventually, all the stagnant ice over the Turtle Mountains melted, and all of the material on top of the glacier was distributed in its present position, forming the hilly "collapse" topography that is found in the area today. These landforms are sometimes referred to by geologists as "hummocky collapsed glacial topography," or "dead-ice moraine." The modern landscape in the Turtle Mountains is marked by hundreds of lakes and ponds, hummocky topography, but with some fairly broad, flat areas that stand above the surrounding rougher land and some flat, lowland areas. Many of the flat areas are old lake plains of silt and clay that were once surrounded by glacial ice ("elevated lake plains"). Other flat areas are covered by stream deposits of gravel and sand. Chapter Ten - Natural History 541
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