Mountain Landforms and Geomorphic Processes - Mountain Geography: Physical and Human Dimensions - page 266

tinuum, a cycle describing the transition among glaciers, rock glaciers, and slope de-
posits (Corte 1987; Johnson 1987; Giardino and Vitek 1988). Followers of this viewpoint
accept that both glacial and periglacial geomorphic processes can operate on any rock
glacier, leading to questions about the usefulness of an active, inactive, and relict clas-
siication.
FIGURE 5.17 California rock glacier in the Sangre de Cristo Mountains, Colorado. (Photo by J. D.
Vitek.)
From their research in the Alaska Range, Wahrhaftig and Cox (1959) provided a
framework for form classification. Lobate rock glaciers have single or multiple lobes
which have a greater width than length, whereas tongue-shaped rock glaciers are
longer, extending downslope from a cirque. Spatulate rock glaciers resemble tongue-
shaped rock glaciers, but display an abrupt widening beyond lateral topographic con-
straints. Subsequent studies used classifications based upon topographical or geo-
graphical descriptors such as valley wall, valley floor, protalus, debris, talus, or glacial
(Outcalt and Benedict 1965; Linder and Marks 1985; Barsch 1988; Humlum 1998). A
single classification scheme has not gained widespread acceptance; however, a subset
of Wahrhaftig and Cox's (1959) classification most commonly appears in the literature:
(1) tongue-shaped and (2) lobate.
Rock glaciers that are moving exhibit pronounced ridge and furrow complexes, re-
sembling a viscous flow substance such as lava (Vitek and Giardino 1988). Transverse
ridges and furrows form perpendicular to the direction of movement; they originate
from overthrusting of internal shear planes, differential movement of distinct layers, or
changes in debris supply (Ives 1940; Wahrhaftig and Cox 1959; Potter 1972; Haeberli
1985; Barsch 1987; White 1987). Longitudinal ridges and furrows form parallel to the
principal direction of movement and result from extensional flow or resistance to flow,
or are remnants of lateral moraines (Barsch 1987; Calkin et al. 1987; Ackert 1998).
Rock glaciers with greater flow rates have a steep front slope near the angle of repose,
but near the head or rooting zone there is a gradual transition from the source of debris
input to the rock glacier (Barsch 1996). Because of thermokarst or ice ablation, the sur-