No abstract

Ice-walled melt ponds on the surfaces of active valley-floor rock glaciers and Matthes (Little Ice Age) moraines in the southern Sierra Nevada indicate that most of these landforms consist of glacier ice under thin (ca. 1 - 10 m) but continuous covers of rock-fall-generated debris. These debris blankets effectively insulate the underlying ice and greatly reduce rates of ablation relative to that of uncovered ice. Such insulation explains the observations that ice-cored rock glaciers in the Sierra, actually debris-covered glaciers, are apparently less sensitive to climatic warming and commonly advance to lower altitudes than do adjacent bare-ice glaciers. Accumulation-area ratios and toe-to-headwall-altitude ratios used to estimate equilibrium-line altitudes (ELAs) of former glaciers may therefore yield incorrect results for cirque glaciers subject to abundant rockfall. Inadvertent lumping of deposits from former debris-covered and bare-ice glaciers partially explains an apparently anomalous regional ELA gradient reported for the pre-Matthes Recess Peak Neoglacial advance. Distinguishing such deposits may be important to studies that rely on paleo-ELA estimates. Moreover, Matthes and Recess Peak ELA gradients along the crest evidently depend strongly on local orographic effects rather than latitudinal climatic trends, indicating that simple linear projections and regional climatic interpretations of ELA gradients of small glaciers may be unreliable.

No abstract

The valley of Bishop Creek, which drains part of the eastern fl ank of the Sierra Nevada, California, contains an unusually well-preserved set of middle to late Quaternary moraines. These deposits have been mapped by previous investigators, but they have not been quantitatively dated. We used the accumulation of cosmogenic 36 Cl to assign a chronology to the maximal glacial positions mapped in the valley. Our results indicate that the terminal moraines mapped by previous investigators as Tahoe were all deposited between ca. 165 and ca. 135 ka, during marine isotope stage (MIS) 6. Moraines mapped as Tioga were deposited between 28 and 14 ka, during MIS 2. These can be subdivided into Tioga 1 (28-24 ka), Tioga 3 (18.5-17.0 ka), and Tioga 4 (16.0-14.5 ka) advances (no moraines dated to Tioga 2 [21-19 ka] were found, presumably because the Tioga 3 advance either overrode or fl uvially eroded them). At 15.0-14.5 ka, the Tioga 4 glacier retreated abruptly to the crest of the range. This was followed by the brief and fairly minor Recess Peak advance at ca. 13.4 ka. No Holocene advances extended beyond the very restricted limits of ice during the Matthes (Little Ice Age) advance. All preserved terminal moraines at lower elevations were deposited during either the Tahoe or Tioga stades. The Tahoe terminal moraines are extensive and voluminous, whereas the Tioga moraines are relatively narrow and have small volumes. However, this notable difference may be more a result of idiosyncrasies in the local glacial history than the result of differences in the length or intensity of glaciation between the two glacial episodes. The history of glacial advances at Bishop Creek exhibits a strong correspondence to global climate cycles, and to paleoclimate events in the North Atlantic in particular.

No abstract

Rock glaciers, common in many alpine and polar regions, have poorly understood internal structure, dynamics, and origins. A renewal of interest in the climatic and geomorphic significance of these striking landforms has served to intensify a long-standing controversy surrounding the genesis of rock glaciers. The controversy, which began more than 30 years ago, has resolved into two primary viewpoints. One holds that rock glaciers form through a continuum of glacial to periglacial processes and encompass features that vary from debris-covered glaciers to slightly remobilized talus or till. The opposing view holds that all rock glaciers are exclusively features of creeping permafrost, genetically distinct from glaciers. Several factors have prolonged this debate: (1) sparse direct observations of internal composition and processes of ice formation;(2) few long-term measurements of rock glacier deformation; (3) difficulties in establishing geophysical, geochemical, or petrographic methods that unequivocally distinguish between ice of glacial and periglacial origins; (4) difficult access and remote locations of most rock glaciers; and (5) often arbitrary terminological distinctions between "glacial" and "periglacial" processes. Results from several recent studies, some presented in this volume, demonstrate conclusively that at least some rock glaciers are glacigenic, making untenable the view of rock glaciers as exclusively periglacial. This conclusion indicates that several previously held concepts of rock glacier dynamics and development should be re-evaluated. In addition, it highlights the need for researchers to move beyond taxonomic arguments, and to improve understanding of fundamental aspects of rock glaciers such as climatic sensitivity, geochemistry, hydrology, dynamics, structure, mass balance, and genetic and spatial variability.

Galena Creek rock glacier (GCRG), northwest Wyoming, exhibits most of the classic characteristics of rock glaciers. Clean ice with silty bands was found beneath a c. 1 m thick debris mantle by Potter. He inferred that the ice is glacigenic, originating in the small snowfield in the cirque at the head of GCRG. This view was challenged by Barsch, who asserted that the ice in GCRG is of "permafrost" origin. Since then GCRG has become a lightning rod for opponents and proponents of the glacigenic ice model for rock glaciers. We review evidence for that model here.Movement marks emplaced on GCRG in the 1960s were resurveyed in 1995 for a 30+ year record of movement. Maximum surface velocity is 45 cm/yr on gentle slopes and 80 cm/yr in a steep reach where GCRG spills out of the cirque. The less active, downvalley third of GCRG is moving at a maximum 14 cm/yr, and lobes formed between the more and less active parts have complex movement and are advancing down-valley over adjacent lobes at a maximum of 6.5 cm/yr. New refraction seismic profiles on GCRG were used to determine the thickness of the debris mantle over ice. On the up-valley, active part of GCRG, the debris mantle is a relatively uniform c. 1 m thick. On the down-valley, less active part, the thickness of the debris mantle is much more variable, but it is generally thicker. We cannot tell, on the basis of seismic data alone, whether the frozen material beneath the debris mantle is ice or a debris-ice mixture, but the results are not inconsistent with the glacigenic model for the origin of the ice. Two long-profiles in the cirque may identify bedrock at about 20-25 m depth.

No abstract