Geology Reference
In-Depth Information
Figure 14.14 Glacial Erratics
a A glacial erratic
in the making. This
boulder on the surface
of the Mendenhall
Glacier in Alaska will
eventually be deposited
far from its source.
Landforms Composed of Till
Landforms composed of till include several types of moraines
and elongated hills know as drumlins .
End Moraines The terminus of any glacier may become
stabilized in one position for some period of time, perhaps a
few years or even decades. Stabilization of the ice front does
not mean that the glacier has ceased to fl ow, only that it has a
balanced budget (Figure 14.7). When an ice front is station-
ary, fl ow within the glacier continues, and any sediment trans-
ported within or upon the ice is dumped as a pile of rubble at
the glacier's terminus (Figure 14.7 and
Figure 14.15). These
deposits are end moraines, which continue to grow as long
as the ice front remains stationary. End moraines of valley
glaciers are crescent-shaped ridges of till spanning the valley
occupied by the glacier. Those of continental glaciers simi-
larly parallel the ice front but are much more extensive.
Following a period of stabilization, a glacier may advance
or retreat, depending on changes in its budget. If it advances,
the ice front overrides and modifi es its former moraine. If it
has a negative budget, though, the ice front retreats toward
the zone of accumulation. As the ice front recedes, till is de-
posited as it is liberated from the melting ice and forms a layer
of ground moraine . Ground moraine has an irregular, rolling
topography, whereas end moraine consists of long ridgelike
accumulations of sediment.
After a glacier has retreated for some time, its terminus
may once again stabilize, and it deposits another end moraine.
Because the ice front has receded, such moraines are called
recessional moraines (Figure 14.7b). During the Pleistocene
b This is the Airdrie erratic near Calgary, Alberta, Canada. It is about
14 m long and 7.6 m high. The world's largest erratic is about
one-third again as large as this one.
sorted by particle size or density, and they show no stratifi ca-
tion. The till of both valley and continental glaciers is simi-
lar, but that of continental glaciers is much more extensive
and usually has been transported much farther.
As opposed to till, stratifi ed drift is layered—that is,
stratifi ed—and it invariably exhibits some degree of sorting
by particle size. As a matter of fact, most stratifi ed drift is
actually layers of sand and gravel or mixtures thereof that
accumulated in braided stream channels. In Chapter 12,
we mentioned that streams issuing from melting glaciers
are commonly braided because they receive more sediment
than they can effectively transport.
 
Search WWH ::




Custom Search