| Abstract
The spatial extent of glaciation varied across the eastern range-front of the Lemhi
Range, east-central Idaho, indicating multiple episodes of glaciation during the last two
glacial cycles. This study utilized extensive field mapping, in conjunction with satellite
imagery and digital elevation models (DEMs) to document the glacial and alluvial
deposits that represent distinct ice advance and retreat events from marine isotope stage
(MIS) 6-2. Surficial mapping of glacial and alluvial features delineates four distinct
glacial advances evidenced by mapped Qm1-Qm4 end moraines and coeval Qf1-Qf4
outwash fans. The geomorphic characteristics of the mapped end moraines, including
moraine crest angularity, moraine relief, and distal moraine slope steepness, were
analyzed semi-quantitatively from transverse profiles. These comparisons concluded that
slope angle, crest angularity, and relief decrease as moraine age increases, and permit
broad correlation of the glacial advances with widespread climatic events. The glacial
advance sequences and estimated timings include Lemhi Advance 1 (MIS 6), Lemhi
Advance 2 (MIS 4-3), Lemhi Advance 3 (MIS 2) and Lemhi Advance 4 (late MIS 2).
Glacial recession in the Lemhi Range was completed by 14 ± 0.5 cal ka BP, as
determined from radiocarbon dating of organic sediment from a Meadow Lake sediment
core. Paleo-glacier ice surfaces were reconstructed for seven glaciated valleys across the
eastern flank of the range from inferred MIS 2 age, Qm3 end moraines and weakly
identifiable trimlines. Equilibrium-line altitudes (ELAs) were calculated using the
accumulation-area ratio (AAR) and the toe-to-headwall-altitude ratio (THAR). The two
methods of calculation are broadly consistent with one another, revealing average AAR-
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and THAR-derived ELAs of 2650 m and 2690 m, respectively, across the Lemhi Range,
with an overall ELA depression of ca. 600 m.
The calculated AAR ELAs in this study show similarities to MIS 2 and
LGM/Pinedale age AAR-derived ELAs in the Lemhi Range (2720 m), Sawtooth
Mountains, Idaho (2640 m) and Beaverhead Mountains, Montana (2650 m) (Meyer et al.,
2004; Locke, 1990). This ELA similarity likely results from decreases in precipitation
from rain shadowing by the Sawtooths, moisture diversion around these high ELA ranges
to the north and south into the Snake River Plain, and weakening of westerly flow from
glacial anticyclone easterly winds from the Cordilleran and Laurentide ice sheets. It is
likely that cold easterly winds migrated over low portions of the Beaverhead Mountains
(e.g., Railroad Canyon), directly east of the Lemhi Range, causing a cold and dry climate.
Thus MIS 2 climate in the central Lemhi Range may have been influenced by the
continental interior and its ice sheets, resulting in depressed ELAs. Similar ELA patterns
of the Lemhi Range, Beaverhead Mountains, and Sawtooth Mountains may result from
uniform cooling and consistent precipitation contrasts across the ranges, or greater
temperature depression in the Lemhi Range and Beaverhead Mountains from cold
easterly winds, coupled with reduced precipitation from decreased westerly winds in the
Sawtooth Mountains during the last glacial cycle.
The character and relative position of Lemhi Advance 2 end moraines, which lie a
short distance down-valley of or merge with Lemhi Advance 3 moraines, indicate
glaciation of similar magnitude during MIS 4/3 (Lemhi Advance 2) and MIS 2 (Lemhi
Advance 3). These relationships suggest that similar climatic conditions affected the
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Lemhi during MIS 4/3, or that wetter and milder conditions drove glaciers to similar
positions. |