Rock Glaciers and Periglacial Rock-Ice
Features in the Eastern Sierra Nevada, CA
Mapping, Classification, Origins, & Significance?
Connie Millar
Sierra Nevada Research Center
PSW Research Station
USDA Forest Service, Albany, CA
“Rock glaciers are among the most enigmatic
forms produced in cold environments.”
--Benn & Evans 1998
“Significant controversy
surrounds their mode(s) of
origin, climatic significance,
and internal workings.”
-- Clark et al., 1998
“Disparities in nomenclature
have produced considerable
confusion and misleading views.”
-- Hamilton & Whalley 1995
Rock Glacier
• “A glacier whose motion and
behavior is characterized by a large
amount of embedded or overlying
rock material”;
• “A mass of poorly sorted angular
boulders and finer material, with
interstitial ice or an ice core.”
“Rock glaciers [sic] belong to the
most spectacular and widespread
periglacial [sic] phenomenon on
– K. Krainer, 1998
earth.”
Pro-Talus Rampart
“Unsorted, non-stratified, angular rock
debris forming arcuate ridges and
lobes; associated with former
persistent snowbanks in shaded sites.”
“Multiple pathways give rise to similar forms in diverse locations” -cim, 2005
Glacial Esoterica or More?
-Rock glaciers increase in abundance during paraglacial times;
-Rock mantling insulates ice and inhibits ablation;
-Importance in hydrological cycle unknown; unincorporated
into streamflow & run-off models.
Preliminary Mapping & Classification of Rock Glaciers
& Periglacial Rock-Ice Features in the E Sierra Nevada
N = 321
Mean Aspect:
Latitude Range: 38°11’N – 36°55’N
% Dead: 39
Elevation Range (low): 2365-3780 m
Elevation Range (high): 2895-3931 m
Size Range: 1-388 ha
Mean Size: 4 ha
Shape:
30% longer than wide
42% about equal length:width
27% wider than long
Longitude Range: 119°24’W - 118°21’W
Classification for Mapping
I. CONDITION Classes (Glacial/Periglacial) 2
1) Morphology States (Form) 4
a. Location Types (Situation) 18
I.GLACIAL FEATURES
1) Rock Glaciers
5 types
“By typing each feature in
the field I made an explicit
hypothesis of condition,
subject to revision…”
II. PERIGLACIAL FEATURES
1) Rock PeriGlaciers
6 types
2) Boulder Streams
6 types
3) Solifluction Fields
1 type
In addition, I mapped
(hypothesized) each feature as:
-- Active or Dead
-- Pleistocene (including
Recess Peak) or Holocene
P
Recess?
I. GLACIAL FEATURES
1) Rock Glaciers (RG)
a. Cirque RG
b. Valley RG
c. Slope RG
d. Morainal RG
e. Pocket RG
GLACIAL
- Glacial attributes: ice, move, erode
- Ice-cored or rock/ice matix
Rock Glaciers
- Sorted rock debris (often reverse sorting)
- Lobate, wedge, or tongue shape
– Oversteepened front
N = 93
Aspect:
% Dead: 41
% Pleistocene: 41
Mean Elev Low: 3231 m
Mean Elev Hi: 3450 m
Mean Shape: 44% long, 43% equal, 13% wide
Mean Size: 2.85 ha
a. Cirque RG
b. Valley RGIce-cored
b. Valley RGRock:Ice Matrix
c. Slope RG
d. Morainal RG
II. PERIGLACIAL FEATURES
1) Rock PeriGlaciers (RPG)
a. Cirque RPG
PERIGLACIAL
b. Avalanche Shoot RPG Periglacial attributes: ice seasonal or if
permanent, ice unaltered; moves as mass
c. Talus Cone RPG
wasting
d. Slope RPG
e. Cliff RPG
Rock PeriGlaciers
f. Valley RPG
- Sorted rock debris (often reverse sorting)
- Lobate, wedge, or tongue shape
– Oversteepened front
N = 134
Aspect:
% Dead: 14
% Pleistocene: 41
Mean Elev Low: 3227 m
Mean Elev Hi: 3352 m
Mean Shape: 13% long, 34% equal, 53% wide
Mean Size: 3 ha
a. Cirque RPG
b. Avalanche
Shoot RPG
c. Talus Cone
RPG
d. Slope RPG
e. Cliff RPG
f. Valley RPG
A Feedback Path for Rock Periglacier Development
1. Persistent Snowfield plus Rockfall,
Niveation, & Slumping
2. Slump catches rockfall and drainage;
toe slope supports new snowfield
3. New toe-snowfields become retaining & supporting walls Æ
catch fine sediments, furthering cementing front wall Æ
oversteepened front develops
4. Rockfall from above further collects on evolving bench;
interstitial ice further insulated by greater rock mantle;
movement begins and mass elongates; cycle continues
II. PERIGLACIAL FEATURES
1) Rock PeriGlaciers (RPG)
6 types
2) Boulder Streams (BS)
a. Cirque BS
b. Avalanche Shoot BS
c. Talus Cone BS
d. Slope BS
e. Cliff BS
f. Low-Gradient/Valley
Floor BS
PERIGLACIAL
Periglacial attributes: ice seasonal or if
permanent, ice unaltered; movement is
mass wasting
Boulder Streams
- Sorted rock debris (often reverse sorting)
- Flush with ground surface
– Abrupt edge of rock mass with turf
- Turf “rides” over rock mass
N = 55
Aspect:
% Dead: 2
Mean Elev Low: 3228 m
Mean Elev Hi: 3348 m
Mean Shape: 47% long, 51% equal, 2% wide
Mean Size: 2.8 ha
c. Talus Cone BS
d. Slope BS picture
f. Low-Gradient/Valley Floor BS
I. GLACIAL FEATURES
1) Rock Glaciers (RG)
5 types
II. PERIGLACIAL FEATURES
1) Rock PeriGlaciers (RPG)
6 types
2) Boulder Streams (BS)
6 types
3) Solifluction Fields (SF)
a. Solifluction Field
PERIGLACIAL
Periglacial attributes: ice seasonal or if
permanent, ice unaltered; movement is
mass wasting
Solifluction Fields
- Sorted rock debris
- Rock mass slumps & hummocky
– Regular concavities & hillocks
N = 29
Aspect:
% Dead: 8
Mean Elev Low: 3446 m
Mean Elev Hi: 3632 m
Mean Shape: 13% long, 79% equal, 8% wide
Mean Size: 12 ha
a. Solifluction
Fields
Clean Glaciers vs Rock Glaciers vs No Glaciers
Lo R
Hi T
NO
GLACIER
Precipitation
Hi R
Rock
PERIGLACIAL
R:I FEATURE
ROCK
GLACIER
CLEAN
GLACIER
Lo P
Temperature
Hi P
Lo T
Rock Glaciers & RIPGF vs Clean Glaciers:
Location: RGs increase eastward from the crest
Rock Glaciers & RIPGF vs Clean Glaciers:
Rock Type
Rock Glaciers & RIPGF vs Clean Glaciers:
Conversion
“An Unnamed Lake at the Foot
of Johnson Peak, July 21, 1927”
--Chiura Obata
“A Wonderful Day to be Alive,
July 21, 2004”
-- Connie Millar
Rock Glaciers & RIPGF vs Clean Glaciers:
Property Differences
1.
Erosion; Positive vs Negative
Form; “Till” Sorting; Moraines
Rock Glaciers & RIPGF vs Clean Glaciers:
Property Differences
2. Requirements for Flow Initiation
CG 30-70m; less for RG (weight of rock burden)?
Rock Glaciers & RIPGF vs Clean Glaciers:
Property Differences
3. Relation to Climate (esp Warming): Lag
RG insulated, ablation slower
Rock Glaciers & RIPGF vs Clean Glaciers:
Property Differences
3. Relation to Climate: ELA (Equilibrium Line Altitude)
Lower than CG because of insulation? Could move downhill
despite warming due to insulation, lag, and paraglacial conversion?
Finale: My 100th birthday Quest:
Ho: Ancient till from Tertiary & early Pleistocene glaciations, stranded on
highest surfaces, serves as a source of abundant, fractured rock for rock
glacier formation below