Mountain Landforms and Geomorphic Processes - Mountain Geography: Physical and Human Dimensions - page 258

FIGURE 5.12 Slumping taking place on a disturbed slope in the Andes, near Santiago, Chile. Several
homes were destroyed beneath the slump. (Photo by J. R. Janke.)
Rockfall is most intense above the treeline in steep terrain where frost action oper-
ates to loosen rocks from surrounding bedrock. Under these conditions, the slightest
disturbance may dislodge rocks and send them plummeting. The triggering agent may
be blowing wind, running water, melting snow, disturbance from animals or people, or
the contraction and expansion of rock with diurnal or seasonal heating and cooling. The
sound of falling rock is very common in mountains. Tumbling rocks sound like thunder
as the dislodged rock strikes solid cliffs or bedrock. Sounds are often amplified, boun-
cing of nearby cliffs. Rockfalls are an eloquent testimony to the inherent instability of
the alpine environment.
Gardner (1970) investigated the importance of rockfall as a geomorphic agent by
simply listening to and recording rockfall events in a small area of the Rocky Mountains
of Alberta, Canada. Over three summers, he listened for 842 hours, recording 563 rock-
fall events (0.7 per hour). The greatest number occurred at mid-afternoon, when the
temperature was warmest, but a second period of high frequency was observed during
the initial daily warming and thawing of the surface. In general, rockfalls were most
frequent in the highest and steepest terrain on northeast- and east-facing slopes, where
frost processes were more active (Gardner 1970).
Rockfall rates can also be used to determine cliff retreat rates in small watersheds.
Caine (1986) examined the Green Lakes, Eldorado Lake basin, and Williams Fork in
the Colorado Rockies to understand movement of coarse debris. The cliffs of the Green
Lakes valley lose about 10 m 3 of rock each year, a mean cliff retreat rate of 0.02 mm
yr −1 . The rockfall record at Eldorado basin gave a rate of 0.012 mm yr −1 . At the Wil-
liams Fork site, the rate is about 0.3 mm yr −1 , an order of magnitude higher. The
coarse debris systems in the upper Rhine basin (Jäckli 1956) and in Karkevagge (Rapp
1960) show much higher levels of geomorphic work by rockfall, talus shift, debris flow,
and rock glacier flow, suggesting that the Colorado alpine environment is stable. In
the Swiss Pre-alps, dendrochronology was used to identify 301 rockfall events between