Geology Reference
In-Depth Information
The most extensive areas of such terrain occur in Eurasia, central and eastern Siberia,
and northwestern Arctic North America (Alaska and interior Yukon Territory). The Tibet
Plateau, thought by most Chinese scientists to have been unglaciated during the Quater-
nary, cannot be regarded as an equilibrium periglacial landscape because it is tectonically
young, the result of exceptionally rapid uplift during the Late Pleistocene (
10 mm/year
for the last 130 k/years). Thus, the permafrost and frost action which currently character-
ize the plateau refl ect the ever-increasing elevation of the plateau. Equally, most alpine
periglacial environments are, by defi nition, proglacial in nature. As for the boreal forest,
tundra, and much of the High Arctic, the majority have only recently emerged from
beneath continental ice sheets; these regions have moved temporally and spatially from
being proglacial to paraglacial. The degree to which they are in balance with current cold-
climate conditions varies and depends largely upon the susceptibility of local bedrock to
cryogenic weathering and landscape modifi cation.
A second perspective is to examine landscape where it is clear that no glacial legacy
exists. The Holocene and Recent deltaic environments of the large northern rivers, such
as the Ob, Lena, Yenisei, Yukon, and Mackenzie, or the emergent coastal relief of many
of the High Arctic islands, are obvious examples. But it is also clear that these are special
cases which do not easily permit historic insight of the time scale required. Accordingly,
our attention must focus fi rst upon the cold and never-glaciated regions of the world. There
are surprisingly few.
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2.3. UNGLACIATED PERIGLACIAL TERRAIN
In this section, we examine two unglaciated cold regions as being possible type-examples
of periglacial terrain. One is upland, the other is lowland. Both are located in the western
Canadian Arctic. The prevailing climatic conditions are summarized in Table 2.1. Figure
2.2 outlines the limits of Quaternary glaciation in this part of Arctic North America.
2.3.1. Beaufort Plain, Northwest Banks Island, Arctic Canada
The northwest corner of Banks Island (latitude 73°30
W) consists of
a gentle surface developed upon Late-Tertiary age gravel and sand of the Beaufort Forma-
tion. The area appears to fulfi ll the essential requirements for periglacial designation.
First, a near-complete Quaternary record is available from a sequence of sediments on
southwestern Banks Island that indicates the climate was cool in this part of the Arctic
throughout Quaternary times (Vincent, 1982, 1989). The oldest sediments, know as Worth
Point Formation, contain macrofossils of larch, shrubs, and herbaceous plants that suggest
an open sub-arctic forest-tundra existed in Middle Pleistocene times (
′
N, longitude 124°00
′
790 ka/years).
Glacial sediments veneer the central part of the island, a spatially-complex association of
glacigenic sediments and much-modifi ed glacial landforms occurs in the east-central area,
and fresh glacial landforms are found only in the east, south, and along the north coast.
No glacial sediments are found in the northwest corner of the island. It seems reasonable
to conclude that northwest Banks Island has experienced uninterrupted cold-climate
conditions throughout at least the last one million years.
Second, during the most recent glaciation (the Late Wisconsinan), a major ice lobe
extended west through McClure Strait, impinged on the north coast, and left a well-
developed lateral moraine system. Part of this northern coastal area is illustrated in
Figure 2.3. A series of sinuous, fl at-fl oored ice-marginal channels parallel the former ice
lobe (French, 1972b). Some appear to have been plugged by fl ow-till that mass-wasted
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