Engineering Geology

Chapter 3
Most melt water streams that deposit outwash fans do not originate at the snout of a glacier
but from within or upon the ice. Many of the streams that flow through a glacier have steep
gradients and are, therefore, efficient transporting agents, but when they emerge at the
snout, they do so on to a shallower incline, and deposition results. Outwash deposits typically
are cross-bedded and range in size from boulders to coarse sand. When first deposited, the
porosity of these sediments varies from 25 to 50%. Therefore, they are very permeable and
so can resist erosion by local run-off. The finer silt–clay fraction is transported further downstream.
Also, in this direction, an increasing amount of stream alluvium is contributed by tributaries,
so that eventually the fluvio-glacial deposits cannot be distinguished. Many outwash
masses are terraced.
Five different types of stratified drift deposited in glacial lakes have been recognized, namely,
terminal moraines, deltas, bottom deposits, ice-rafted erratics and beach deposits. Terminal
moraines that formed in glacial lakes differ from those that arose on land in that lacustrine
deposits are inter-stratified with drift. Glacial lake deltas are usually composed of sands and
gravels that are typically cross-bedded. By contrast, those sediments that accumulated on
the floors of glacial lakes are fine-grained, consisting of silts and clays. These fine-grained
sediments are sometimes composed of alternating laminae of finer and coarser grain size.
Each couplet has been termed a varve. Large boulders that occur on the floors of glacial
lakes were transported on rafts of ice and were deposited when the ice melted. Usually, the
larger the glacial lake, the larger were the beach deposits that developed about it. If changes
in lake level took place, then these may be represented by a terraced series of beach deposits.
Deposition that takes place at the contact of a body of ice is frequently sporadic and irregular.
Locally, the sediments possess a wide range of grain size, shape and sorting. Most are
granular, and variations in their engineering properties reflect differences in particle size distribution
and shape. Deposits often display abrupt changes in lithology and, consequently, in
relative density. They are deformed since they sag, slump or collapse as the ice supporting
them melts.
Kame terraces are deposited by melt water streams that flow along the contact between the
ice and the valley side (Fig. 3.29). The drift is derived principally from the glacier, although some
is supplied by tributary streams. They occur in pairs, one on each side of the valley. If a series
of kame terraces occur on the valley slopes, then each pair represents a pause in the process
of glacier thinning. The surfaces of these terraces are often pitted with kettle holes (depressions
where large blocks of ice remained unmelted while material accumulated around them).
Narrow kame terraces are usually discontinuous, spurs having impeded deposition.
Kames are mounds of stratified drift that originate as small deltas or fans built against the snout
of a glacier where a tunnel in the ice, along which melt water travels, emerges (Fig. 3.30).
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