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The end
of the Little Ice Age, Mendenhall Glacier, Juneau, Alaska
Margo Burton, Class of 2002
Advisor: Robert J. Carson
Dept. of Geology, Whitman College, Walla Walla, WA 99362.
Mendenhall
Glacier, Mendenhall Lake, and a moraine complex are located northwest
of Juneau, Alaska. Lawrence (1950, The Geographic Review, v. 40, p.
191-223) used dendrochronology to date the most recent glacial retreat
and moraine deposition from the 18th century to the mid 20th century.
Miller (1975, U.S.G.S. Map I-885) mapped the Quaternary geology in the
vicinity of Mendenhall Lake and obtained many radiocarbon dates constraining
the glacial history.
The glacial trough and Mendenhall
Lake basin have likely been excavated throughout the Pleistocene. Modern
Mendenhall Lake, which has existed for only about a century, formed
when the ice retreated from the terminal moraine complex. Mendenhall
Lake has grown larger during this last retreat, and the increased wave
action has cut low bluffs and deposited beaches and spits. In addition
to the terminal moraine complex, there are a variety of depositional
and erosional landforms in the recently deglaciated area. Depositional
landforms include lateral moraines, kames, and possible kame terraces
and eskers. Erosional landforms include striations (some cross-cutting),
grooves, stoss-and-lee forms, and spectacular p-forms.
Although most of the margin
of the Mendenhall Glacier has active ice, there is local stagnation.
Historical records, topographic maps, aerial photographs, and GPS data
were used to establish rates of retreat. The retreat rate averaged 29.8
m/yr from 1910 to 2000. During the retreat of the late 20th century,
the glacier has exposed a cross-valley ridge of dark-colored metamorphic
rock at its terminus. Due to the rock's high heat absorption, the normal
ablation processes (melting, evaporation, and calving) are accelerated.
The ridge also deflects marginal drainage, which thermally undermines
the terminus.
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