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Sherzer (1907), Gerlib (1961) and McCarthy (2003) have attempted to use tree-ring dating to estimate the timing of glacial advances and moraine formation at the Illecillewaet Glacier. Sherzer (1907: 96) produced a crude estimate of tree age by measuring diameters and using “the average breadth of the annual rings of growth measured in the Illecillewaet and adjoining Asulkan valleys.” He estimated the ages of 243, 447 and 550 years for three trees on the “Bear-den moraines” - a soil-covered medial moraine formed by the Asulkan and Illecillewaet Glaciers (Sherzer, 1907: 96-97).
Gerlib (1961) spent one day at the site in 1959 and obtained 12 increment cores from an unspecified number of trees at the trimline. He found three periods of reduced growth: 1685 - 1735 (two trees), 1800 - 1840 (three trees), and 1860 - 1884 (three trees). He reasoned that the growth suppression was caused by glacial advances at the “mean dates” of 1710, 1818 and 1872 (Gerlib, 1961: 105-107). Given his limited sample base and the lack of a standardized tree-ring chronology against which to compare the ring-width data, Gerlib’s ages must at best be viewed as crude approximations. Indeed, Gerlib (1961: 107) recognized this and concluded that his 1872 event is the 1887 advance that was witnessed by the Vaux family (Vaux and Vaux, 1908). No corroborative data were provided for the two older events.
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McCarthy (2003) mapped and correlated the morainic ridges, sampled and ring-counted over 300 trees on and between the moraines, estimated the lag time between ice retreat and the establishment of trees (ecesis), used direct and indirect measurement to estimate lichen growth rate and closely estimated the timing of moraine stabilization at the Illecillewaet. Figure 11 shows the nine morpho-stratigraphically correlated moraine segments located at and upvalley from the historically dated icefronts and Table 2 summarizes the botanical evidence used to assign ages to the moraines.
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In assessing these ages it must be recognized that counts of growth rings from the base of trees provide only minimum ages for deglaciation. Closer estimates of the timing of ice retreat are obtained by adding years to the earliest growth ring to compensate for tree ecesis and correct for sampling height and years lost due to missed piths. Unfortunately, the timing of moraine formation at the Illecillewaet Glacier has been difficult to determine. For example, the age of Moraine 2 West was obtained by cross-matching the ring-with record of a rooted snag with a tree ring-width chronology that first had to be developed from trees on the Bear-den moraine. Then, detailed sampling found that there was great variability in tree age, ecesis and lichen sizes on both sides of the forefield. This made it difficult to closely estimate the true age of the moraines. Nonetheless, the evidence showed that nine moraines formed between 1693 and 1889. The lichenometric ages and the ages estimated by the oldest trees on each unit (Table 2) showed that Moraines 1, 4, 5 and 6 were older than their lichens indicated, while Moraines 1 through 5 were older than their oldest living trees. The age differences arise from the fact that slight differences in microclimate or disturbances on the moraines affected the lichen and tree populations differently. For example, microenvironmental conditions that may have been tolerated by lichens may have prevented trees from becoming established. Analysis of the lichen and tree-ring information shows that Moraines 1 and 2 had formed by 1693, Moraines 3 to 7 were deposited by 1760, Moraine 8 formed by 1845 and Moraine 9 formed by 1889. |
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Very little is known about moraines that were formed by the Illecillewaet Glacier prior 1888 and the formation of Moraine 1. However Mazama tephra (Bacon, 1983) can be found in soil ca. 30m downvalley from Moraine 1 at the top of switchbacks along the Great Glacier trail. This indicates that the glacier has stayed within its outermost terminal-lateral moraine since at least 6700 yr BP. In addition, I found tephra in the thin soil that can be found atop the central portion of the Bearden Moraine. Microscopic inspection of this tephra under polarized light (Figure 12) reveals that the tephra has morphological characteristics consistent with those described by Matthewes and Westgate (1980) for Bridge River tephra. If this is indeed Bridge River tephra then it indicates that the Bear Den Moraine had formed by at least 2350 yr BP. Presumably the Bear Den moraine correlates with morainic ridge fragments that can be found at the base of the eastern valley side - where the Perley Lake trail first emerges from the forest. Unfortunately there are no known maps or detailed studies of these units and their true age is unknown. |
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Appendix I:
A short list of historical photographs that can
be used to identify icefront positions.
Appendix II:
Description of Dominion Water and Power
Bureau Survey and Photographic Reference Points.
Appendix III:
Dominion Water and Power Bureau: Annual Photographs Required at the Illecillewaet Glacier. |
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Figure 11: Download High Resolution Version
Figure 12: Sketch maps of the Illecillewaet Glacier forefield. Dated icefront positions and historical details are based on maps and photographs by the Vaux family (Vaux and Vaux, 1907 and photo NG4-894 - Archives of the Canadian Rockies, Banff). Dashed lines show ridges that are hidden from view by dense shrub cover or have been removed by erosion since the late 1880s. Morainic ridge segments have been correlated based on their size and position (A). The germination dates of the oldest trees on each ridge segment are also shown (B).
Figure 13: The striped shards in this image are Bridge River tephra taken from a shallow pit on the Bear Den moraine. The small piece on the bottom right is Mazama tephra sampled along the Great Glacier trail.
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