Page 79 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soils That Are Sediments
74 Geotechnical Engineering

4.5 GLACIO-FLUVIAL DEPOSITS

4.5.1 A Mixed Breed

Glacio-fluvial deposits are sediments deposited by water from melting glaciers.
They extend beyond the margins of glaciers, but also are deposited within
moraine areas as the glacier retreats.

Although glacial melting normally proceeds from the glacier surface down-
ward, the water released by melting readily infiltrates downward through
cracks and flows as a river underneath the ice. After the ice has melted the
alluvial deposit remains as a ridge of sand and gravel, called an esker, as shown
in Fig. 4.5.

Kames are sand-gravel mounds that accumulated in pockets in the ice. They
often occur in association with kettle lakes. Kame terraces are deposited along
edges of glaciers confined in valleys, and show evidence of collapse after the ice
in the valley melted. Eskers and kames may be used as local sources for sand and
gravel, and appear as light areas on airphotos because of good drainage.

4.5.2 Outwash

Glacial sediment carried down river valleys is referred to as outwash and typi-
cally consists of a wide range of coarse particle sizes, from sand up to gravel
and even boulders. Outwash-carrying streams do not meander lazily down
their floodplains, but race downhill in a wild series of interconnecting, rapidly
shifting channels called a braided stream, as shown in Fig. 4.6. The rapid
current in a braided stream leaves deposits of sand and gravel that may be
covered with silt during waning stages of the river.

Figure 4.5
Subglacial stream
actively forming
an esker and
emerging at the
terminus of the
Matanuska
glacier, Alaska.
Note the heavy
concentration of
sediment in the
basal ice.

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