Sediment supply to eskers

Eskers form where the sediment load delivered to a subglacial stream exceeds the transport capacity of the stream. The debris-laden basal ice

Figure 8.26. (a) Map of the Penobscot River and a section of the Katahdin esker near Medway, Maine. Near the middle of the map, the esker leaves the valley of the river and trends south-southwestward up a small tributary valley. (b) Map of equipotential contours beneath a glacier with a southward surface slope of 0.0048. The esker generally follows a trough in the potential surface. (After Shreve, 1985a. Reproduced with permission of the author and the Geological Society of America.)

Figure 8.26. (a) Map of the Penobscot River and a section of the Katahdin esker near Medway, Maine. Near the middle of the map, the esker leaves the valley of the river and trends south-southwestward up a small tributary valley. (b) Map of equipotential contours beneath a glacier with a southward surface slope of 0.0048. The esker generally follows a trough in the potential surface. (After Shreve, 1985a. Reproduced with permission of the author and the Geological Society of America.)

of the glacier is one source of such sediment. As the energy dissipated by the flowing water melts this ice, debris is released and an inward flow of ice toward the tunnel is induced.

A nice demonstration of this process is provided by lithologic pebble counts from the Great Pond section of the Katahdin esker, down-flow from a point where the esker crosses bedrock units of distinctive lithology (Van Beaver, 1971). The concentration of pebbles of these lithologies in the esker reaches a maximum about 3 km down-flow from the point where the esker crosses the units (Figure 8.27). Had the stream been acquiring the pebbles directly from the bedrock, a difficult task at best once the esker began to develop on top of the rock, the concentration should have peaked at the down-flow edge of the unit. Rather, we infer that it was the glacier that eroded the pebbles from the bed and carried them along arcuate paths, as shown, until they were released into the stream (Shreve, 1985a).

Calculations suggest that this source of sediment is quite adequate to overload a subglacial stream, leading to deposition. For example, suppose the median (by weight) grain size in an esker is 0.03 m, and that

Figure 8.27. Schematic sketch showing variation in concentration of lithology A in an esker down-flow from the point where the esker crosses the outcrop of this lithology. Rocks eroded by the glacier are carried along arcuate paths downglacier and inward toward the tunnel.

Figure 8.27. Schematic sketch showing variation in concentration of lithology A in an esker down-flow from the point where the esker crosses the outcrop of this lithology. Rocks eroded by the glacier are carried along arcuate paths downglacier and inward toward the tunnel.

Distance

the esker is forming in a tunnel beneath a glacier with a surface slope of 0.005. Then, based on equations for sediment transport in gravel-bedded streams (Parker, 1979), a conduit ~0.7 m high with a discharge of 1.0 m3 s-1 per meter of conduit width would be required to transport this material, and the sediment load would be about 0.04 m3 d-1 per meter width. Under these conditions, the energy available for melting would be ~50 J m-2 s-1, and the melt rate on the conduit roof would be ~0.014 m d-1 (Equations (8.9) and (8.12)). If the basal ice contained 10% debris by volume, the debris released by this melting would overload the stream after it flowed along the conduit for only 30 m (0.014 x 0.10 x 30 = 0.04 m3 d-1 per meter width).

In some eskers, however, some of the water and sediment load may have been derived from the glacier surface by way of moulins. For example, Mooers (1990a) found eskers in central Minnesota that headed in conical hills of glaciofluvial gravel. He inferred that the hills were formed by sediment-laden supraglacial streams that reached the glacier bed through moulins and deposited a significant fraction of their load there before continuing to the margin through the subglacial conduits in which the esker formed.

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