Geology Reference
In-Depth Information
for ice-foot accretion. The tidal range and beach slope are other factors which infl uence
ice-foot width and thickness. The greater the tidal range, the greater is the zone of spray
and swash accretion, while the steeper the beach profi le, the smaller is the width and
thickness of the ice foot. Usually, the ice foot ablates during the early part of the open-
water season. Initially, a shore lead develops some 5-15 m off-shore to isolate the sea ice
proper from the ice foot. Then the ice foot is breached by water draining seawards from
melting snow in the backshore zone.
The effectiveness of storm events can be infl uenced by either the presence of unusually
long-lasting beach-fast ice, which would inhibit erosion, or its removal by a prior storm
event, which would accentuate erosion.
The more direct effects of ice on the beach occur when sea ice is forced, by strong
onshore winds, to impinge upon the shoreline. Typically, tabular ice bodies push and scour
beach material to form irregular ridges and scours in and above the high-tide zone
(Barnes, 1982; Dionne, 1975, 1989; Hume and Schalk, 1964; Owens and McCann, 1970;
Taylor and McCann, 1976). Where ice becomes buried by beach material, its eventual
melt produces pits, depressions, and irregular topography. In general, these beach forms
are short-lived because the shorelines are systematically reworked every year.
Boulder barricades are also typical of many cold-climate coastlines. These are elongate
rows of closely packed boulders that fl ank the coastline and which are usually separated
from the shore by a low-gradient nearshore zone (Rosen, 2005). They are the result of ice
movement and the pushing of boulders during late winter and spring (Lauriol and Gray,
1980). They are especially well developed along the coastline of eastern Canada (Dionne,
1994, 2002), where they consist of glacial erratic boulders that rest on rock-cut shore
platforms. Ice-push ridges also occur around the shores of inland-water bodies that are
large enough to permit wind-induced movement of the ice cover. Smaller ice-push ridges
can also form around large thaw lakes. Because of the absence of a tidal range and no
well-developed ice foot, all these lacustrine push-ridges form at the same elevation each
year and are relatively regular in plan and form.
10.4.4. The Infl uence of Permafrost and Ground Ice
In areas where the coastline is developed in permafrost terrain composed of unconsoli-
dated and ice-rich sediments, rapid bluff retreat may occur. Fine-grained sediments are
usually associated with high ice contents, and storm waves result in either the exposure
of ice bodies to rapid melt or the undercutting and formation of thermo-erosional niches
at high-tide level. The former induce retrograde thaw- slumping (see Chapter 8) while the
latter lead to dramatic block collapse, often controlled by ice wedges which act as lines of
weakness (see Figure 10.4B). In both cases, fi ne sediment is released, carried away, and
subsequently deposited elsewhere along the coast. The coastal processes of melt and
erosion are sometimes termed thermo-abrasion (Are, 1983, 1998). This is common along
the coasts of the permafrost lowlands of Siberian and arctic North America and along the
banks of major rivers (see earlier). It is especially effective wherever coastal bluffs are
developed within ice-rich and unconsolidated sediments (Dallimore et al., 1996a; Hume
et al., 1972; Mackay, 1963, 1986a; Shur et al., 2002).
In northern Alaska and the western Canadian Arctic, average rates of bluff retreat of
between 2 and 4 m/year are typical (Harry et al., 1983; Hume et al., 1972; Mackay, 1986a)
but where beach gravels are present, retreat is less. Along the Yukon coast and in the
Mackenzie District of Canada, a similar pattern of rapid coastal retreat can also be
observed (Mackay, 1963; McDonald and Lewis, 1973; Ruz et al., 1992, table 1). Measure-
