]ou""Jl of Glacioloo , Vo1. 14, No. 70 , 19 75
DROP STONES RESULTING FROM SNOW-AVALANCHE
DEPOSITION ON LAKE ICE
By B. H . L UCKMAN
(Department of G e og raphy, University of Western Ontario, London 72, Ontario, Canada)
ABSTRACT. Dirty snow ava lanches have been observed to ca rry considerable amounts of rock debris
on to lake ice a t the foo t of scree slopes. As ice breaks up in the spring thaw, this material is carried back
and forth on ice floes and is graduall y deposited in the lake. In so me a reas this produces typical drop stones
of rock d ebris in predomin a ntly fin e-grained d eposits. Mos t avala nche debris is very angular which enables
avala nche drop stones to be diffe rentiated from those of glacial or other drift-ice o rigins. However, where
avalanches incorpora te gl acial debris, such d eposits may be indistinguishable from those formed by floating
glacier ice.
RESUME. SauP Judrage de s!diments rtsult ~Tl t des app )r!s d'avala"c"es de neige sur la glace de lac. On observe que
les ava la nches de neige sa le transportent v ne qua ntit e consid era ble de debris rocheux jusque sur la glace d e
la c au pied d es p ~ n t es d' eboulis. L:lrsque la glace se brise au m -:>m ent d e la fusion printa niere, le materic! est
tra nsport e ~a et la pa r les glaces f!o tt a nt ~s e t es t depose graduellem ent dans le lac. Dans certaines zones ceci
produit un saupoud rage d e s·.'diment ca racte ristique fait de d ebris rocheux dans d es depots OU dominent les
ma tt' ria ux fi ns. Bcaucoup d'a pports d 'avala nches son t t. e3 a:1guleux ce qui peut p erm ~ llre de differencier
les s edim~ n tl saupoudre ; issus d 'avala nche d e ceux dontl'origin e es t un gl aci er ou une congere construite par
le vent. er p ~ ndant , lc n que les avalanches o nt illcorpore d es d ebris glaciaires, leurs depots sont impossibles a
distingucr de c ~ ux forme ; pa r des fragments fl o llanB de gl ace d e gl a cier.
Z USAM.\fENFASSUNG. Tropfsteille als E rgehllis von Schneelawinellahlagerullgetl auI Seeeis. Es wurde beobachtet,
d ass schmutzige Schneela winen betrachtliche M engen von Felsschull a uf das Seeeis am Fusse von Gerollhalden bringen. \-Venn d as Eis ill der FrUhja hrsschmelze a ufbricht, wird dieses Material auf Eisschollen
hin und her getrieben und a llma hl ich in d en See abgelagert. In manchen Gebie ten cntstehen dadurch
typische Tropfsteine a us Felsschull in vorwiegend feinkornigen A blage rungen. D er meiste Lawinenschutt
is t sehr eckig ; dadurch lasse n sich Lawinen-Tropfsteine von solchen unterscheiden, die glazialen Ursprungs
sind odeI' andcrweitig von T reibeis herriihren. Wo jedoch La winen glazialen Schutt enthalten, mogen
solche Ablagerungen vo n j encn, die durch treibendes Gletsc here is entstchen, nicht unterscheidbar sein.
geomorphi c signific ance o f snow a val a nches in Arc tic and alpine areas has b een underestimated.
Unde r fav ourable conditio ns, especially wh e n t ra velling o ve r surfaces of loose , unconsolidated rock
fragm e nts, avala nch es may incorpora te large a mounts of roc k debris (R a pp, 1960 ; Gardnel·, 1970;
Luc kman , 19 72). Continued a vala nche acti vity in such loca t ions leads to the production of welld evelope d la ndforms known as a va la nch e b o ulde r tongu es (Rapp, 1959 ; Luc kman, unpublished) .
Usually th e a vala nch e d e bris is d epos ited w ithin the terminal zone of the trac k but where avalanches
extend on to ice-cove red w a te r bodi es there may be conside r a ble redistribution of these d eposits.
Annual d ebris acc umulatio n from rock fa lls a nd snow avalanch es has been recorded on seve n scree
slopes in Surprise Vall e y, Jasp e r N atio na l Park, since 1968 (Luc kman , 1971, unpublished ). Two of the
sites studi ed flank lakes so tha t in seve ral cases avala nche d e p os itio n has extended o n to the lake ice.
When this ice breaks up, th e rock d e bris, a blated from the ava lanc he snow, is carried by the ice floes
over the lake basin a nd subsq uently d eposited as the ice melts (Fig . I ). This seconda ry d eposition may
be concentra ted in onc loca tion or dispe rsed over the lakes d e p e nding on the manne r of d ebris release
(tiltin g, overturning 01' g ra dual a bla tio n ) and the pa ttern of drift-ice movem e nt in response to the
winds or la ke currents.
The ava lanche d ep osits themselves m ay range from pure sno w to a dirty snow and rock admixture.
The c harac teristics of th e rock d ebris d ep e nd on its origin. The most common d e bris sources are sc ree
slopes or loose d ebris swe pt from the cliff zon e of th e ava lanc h e trac k. These mate rials are a charac teristically h e te rogeneous, poorly sorted mixture of a ngular, often fr eshly broken fl'agments ranging from
boulde l· to silt size (see Fig. I ). Since the roc k d ebris can b e d e rived from any un consolidated material
in the avalanche trac k , it could a lso include fluvial or glacially moulded d ebris.
The m ajor impli : a tions o f these obser vations are three-fold.
THE
(a ) Although these spe::ific observa tions only relate to two small a lpine lakes over a relatively short
time p e riod (6 years) , morpholog ical evid en ce suggests these processes have bee n active over a much
longe r p e riod (several thousand years ?) , and similar d eposits have been observed on larger water bodies
186
SHORT
NOTES
elsewhere in th e Canadian Rockies, in Scandinavi a (Rapp , 1960, fig. 48) a nd on sea ice in the Canadian
Arctic (Bones, unpublished , pI. 14) ' This sugges ts that, over long peri ods of time, snow avalanches
and other mass move m ents (rock falls , slush avalan ches, e tc. ) could locall y d eposit significant amounts
o f d ebris on la ke or n ea r-shore ice. Estimates from debris-accumulation m easurements on sc rees with
co nsiderabl e ava la nch e erosion indicate mean annu a l d ep os ition of th e o rd er of 0.5- 5 mm m - Z year- I
(see Luckman , unpublished , p. 273 ) .
Fig .
Partially subm Jrged ice fioe on Lake H elen, Surprise Valltry, J asper Na tional Park, 8 June 1970. The thick d !bris
cover is typical of avalanche-incorporated scree material. The large boulder is about 0.20 rn' ( 1. 2 m long ) .
1.
(b ) The di sp ersal and melting of this d ebris-covered ice m ay result in the formation of "drop stones"
in areas of otherwise predomin antly fine-grained sediments (lacustrine o r marine). Such d eposits are
commonly ascribed to the action of floating glacier ice or drift ice which derive their rock debris from
glaciers, fluvial deposition on near-shore ice (Dangeard and Vanney [1974] ) or ice-foot erosion of beach
material (Dionne, 1974) . Since the vast majority of avalanche drop stones are markedly angular, it
should be possible to differentiate them by their morphology (absm :e of rcu nd ed, polished or striated
debris and their locale (evidence of past or present avalanche activity near the shore). Howeve r, where
avalanches have incorporated till or fluvial material this distinction cannot be made. An important
corollary of this is that in some cases the presence of striated or polished drop stones is not conclusive
evidence for floating glacier ice.
(c) The dispersal of avalanche d eposition by lake ice may inhibit the development of typical
avalanche landforms in the lowest part of avalanche tracks which terminate in lakes.
ACKNOWLEDGEMENTS
The author wishes to thank McMaster University and the National Research Council of Canada
for support during field work.
MS. received 31 July 1971
188
JOURNAL
OF GLACIOLOGY
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