Cordilleran Section - 97th Annual Meeting, and Pacific Section, American Association of Petroleum Geologists (April 9-11, 2001)

Paper No. 0
Presentation Time: 1:00 PM-5:00 PM


BURTON, Margo A., Dept. of Geology, Whitman College, Walla Walla, WA 99362, CARSON, Robert J., III, Whitman College, 345 Boyer Ave, Walla Walla, WA 99362-2067, ROTH, Monica J., 3 David Lapsley Rd, Bedford, NY 10506-1000, FLEISHER, P. Jay, Earth Sciences, SUNY - Oneonta, Oneonta, NY 13820-4015 and NEWTON, Robert M., Smith College, College Ln, Northampton, MA 01063-0001,

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, and aerial photographs (supplemented with limited dendrochronology and lichenometry) are used to establish rates of retreat. The retreat rate averaged 8.4 m/yr from 1962 to 1984 and 20.9 m/yr from 1984 to 1996. 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.