In over a thousand places in the State we find long narrow hills that look like abandoned railway embankments and locally are called "hogsbacks" and "Indian trails." Geologically these hills are eskers --- long narrow winding ridges of stratified gravel, sand and silt. They formed under stagnant rather than moving ice. Often not recognized as being glacial features, are the low, sinuous hills that can stretch for miles and often resemble snakes when seen from the air. These are eskers.
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    An esker is an ancient river bed that has been formed inside or on top of a glacier. As meltwater seeks to escape from the lower levels in the glacier, it forms channels along zones of weakness and eventually emerges from under the ice at the glacial margins. In its passage, the water transports silt, sand and rocks and lays them down along its narrow course. The water is unable to escape laterally because of the confining walls of ice, and the stream bed is gradually built up above the level of the ground on which the glacier rests.
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    Eskers were formed by deposition of gravel and sand in subsurface river tunnels in or under the glacier. The mouths of the tunnels became choked with debris, the melt water was ponded back and dumped its load of sediments in the channel. The most famous and longest esker in Michigan is the Mason Esker, which runs from Dewitt through Holt and Lansing, and ends at Mason. Most of the Mason esker has been removed, its sand and sorted gravel used to make concrete highway construction.
    Most eskers are on till plains although some are known to cut through moraines and even cross drumlins. They vary in length from a fraction of a mile to scores of miles, and in height from a few feet up to several hundred and more feet. More than a thousand eskers have been found in Michigan.
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Just as rivers on land carry and deposit sediment, meltwater that flows in the openings beneath, above and within a glacier also carries and deposits sediment. Tunnels near the base of retreating glaciers fill with transported sediments, which remain as
sandy or gravelly ridges that look like upside-down stream beds after the glacier melts away. The ice that formed the sides and roof of the tunnel subsequently disappears, leaving behind sand and gravel deposits in ridges with long and sinuous shapes. 
The shape of an esker (in cross-section) is shown in the cut below.
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The image below shows the stratified sand and gravel within the Osmun Lake esker, in Presque Isle County, Michigan.
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Eskers can be 500 to 600 kilometers long and, depending on the pattern of the glacier's inner tunnels, can interconnect
in a pattern of central ridges and tributaries, just like a branching river system.   As you can see, the esker is being mined as a source of aggregate.
    Since eskers are made up of highly porous sand and gravel, they are frequently excavated for construction. They are considered an endangered geomorphological species since they have been used either to develop roadways -- offering natural elevated, dry terrain -- or they have been ripped up for the gravel to build nearby roads. For centuries in northern Canada,
Inuit and wildlife have typically used eskers for high and dry travel routes. More recently, eskers have been used in the hunt for diamonds. Since they lie in the direction of glacial flow, prospectors have used eskers to trace where minerals glacially eroded from diamond-bearing formations have been transported. They trace these "indicator" minerals "upstream" in an esker until they abruptly disappear: this indicates the diamond source is nearby.

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