keck geology consortium proceedings of the twenty-fourth annual ...

24th Annual Keck Symposium: 2011 Union College, Schenectady, NY
RECONSTRUCTING THE PINEDALE GLACIATION, GREEN
LAKES VALLEY, COLORADO
KEITH M. KANTACK, Williams College
Research Advisor: David P. Dethier
INTRODUCTION
During the Pinedale Glaciation (~32-15 kya) alpine
glaciers covered much Colorado’s Front Range
(Madole 1998). While these glaciers are small and
scarce today, their work still dominates the landscape.
As powerful erosive agents, glaciers remove massive
volumes of material from their beds and leave
equally impressive volumes of sediment in the form
of moraines and outwash plains. By using field and
modeling techniques based on glacial evidence, we
can determine glacier size and ice flow direction, as
well as how much sediment they deposited, which
ultimately helps to constrain how quickly glaciers cut
their cirques and valleys.
While the scoured bedrock surface of the deglaciated
alpine zone is not typical of the critical zone, glaciers
are sediment factories. Much of the sediment that
ends up covering the lower reaches of alpine and
subalpine basins and flanking channels downstream is
generated by glacial erosion of bedrock. In this way,
glaciers are not just icy blocks high in the mountains,
but significant players in the all important critical
zone.
In this paper, I use field and LIDAR evidence to model
the extent of the Pinedale glaciation in the GLV, as
well as its contributions to the critical zone.
SETTING
The Green Lakes Valley (GLV) of the Colorado Front
Range is about 13 kilometers northwest of the town of
Nederland in the headwater area of the North Branch
of Boulder Creek (Fig. 1). The valley floor runs
extends from 3300 to 3900 meters in elevation and
is walled by 3900 to 4100 meter peaks that form the
continental divide. The valley, which is part of the
City of Boulder watershed, is protected and access is
limited to researchers working on specific topics.
Figure 1. Colorado, Boulder County, and a panoramic
view of the GLV, looking west toward the continental divide
over the step between Green Lake 3 (left) and 4.
During the peak of the Pinedale glaciation, glaciers
flowed out of small cirques in the Front Range,
through valleys, and joined forces, reaching lengths
of 15 kilometers and elevations as low as 2500 meters
(Madole et al. 1998). The GLV bears many marks
of glaciation. In favorable sites, the bedrock holds
polish and striations, and moraines are preserved in
downvalley locations. On the valley floor, bedrock is
ubiquitously smoothed, and bears evidence of plucking.
In places, the valley walls are oversteepened,
reflecting undercutting by debris sheared along by the
moving ice. Post-glacial talus fields cover the base of
these cliffs and cover large tracts of the lower walls.
The valley is U-shaped, with six lakes strung along
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