Page 80 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soils That Are Sediments
Soils That Are Sediments 75

Figure 4.6
Aerial photograph
of a braided river
carrying glacial
outwash in Alaska.

Outwash plains are created where outwash streams spill out over their banks to
deposit a broad deposit of sand and gravel. Outwash plains tend to be fan-shaped
as they spread out from a glacial front. Long Island and Cape Cod contain a series
of such fans.

Outwash terraces are alluvial terraces that are elevated remnants of former
floodplains that were abandoned when the river entrenched or cut downward.
Outwash terraces are common along rivers that carried glacial outwash because,
as the nature of the sediment being carried by the river changed, its downslope
gradient changed. Generally the downhill gradient of a river carrying a full load of
coarse granular outwash is steeper than that of the modern floodplain, so outwash
terraces are high close to a glacial front and decrease in relative elevation with
distance downstream, eventually merging into and plunging below the modern
floodplain.

As a glacial front does its final retreat and the river starting point moves
northward, the river normally will entrench and leave a series of step-like terraces,
the oldest being the highest and the most extensively dissected by later stream
erosion.

Outwash terraces are prime sources for sand and gravel. Intelligent prospecting
for gravel in these areas requires an appreciation of the landforms and recognition
of stream terraces. These generally appear lighter on airphotos and are confirmed
with test borings.

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