The glacier bed, where basal sliding occurs, was reached by cable-tool drilling and sand-pump bailing in seven bore holes in Blue Glacier, Olympic National Park, Washington. Basal sliding velocities measured by bore-hole photography and confirmed by inclinometry are unexpectedly low, ranging from 0.3 to 3.0 cm/day and averaging 1.0 cm/day. This is much less than about half the surface velocity of 15 cm/day, which was the sliding-rate expected from earlier deformation measurements in bore holes made by thermal drilling alone

Engelhardt, H. F.

Harrison, W. D.

Kamb, B.

Caltech Authors - Main

Edge/ 
arete 
Ttrm 
Chatter(ing) fractures/ 
fractures de broutage 
Conca ve chattermarks/ 
broulurts concaves 
Convex chaltermark/ 
brouture co1wexe 
I\ typ e of quarried wall / 
debiture 
Arcade/ 
arcade 
Arcaturc/ 
arcature 
A type of quarried plane/ 
tro11cat11re 
JOURNAL OF GLACIOLOGY 
Table I-continued 
Deji11itio11 
I ntersection line of two surfaces of a 
same glaciated bedrock form, 
parallel to the ice movement and 
produced at two different times. 
Small curvilinear and nested fractures, 
with their concavity facing down the 
ice movement. · 
Hollow and arched forms, generally 
occuring in series, with a concave 
wall facing up-stream and a floor 
gently sloping d own the ice move-
ment. 
H ollow and arched form, generally 
occurring a lone, with a convex wall 
facing up-stream and a floor gently 
sloping down the ice movement. 
Sloping Ice side of any outcrop plucked 
by glacial action . 
A type of quarried'wall with a concave 
surface, developed along the plane 
of weakness of a chatter fracture. 
Quarried wall ch a racterized by juxta-
posed arcades. 
A quarrymark slightly sloping down-
stream, or a bevelled plane 
developed on a weak joint ( ?) bear-
ing unorganized striations mostly at 
its base. 
Sit.t 
Length: decimetric to decametric . 
Chord and sine: a few centimetres to a 
few decimetres. 
Chord and sine: a few centimetres to a 
few decimetres. 
Chord and sine: a few centimetres to a 
few decimetres. 
Height: a few centimetres to a fow 
metres; wid th : longer still. 
Height and width: a few centimetres 
to a few d ecimetres. 
(See arcade.) 
Arca: a few squa re metres. 
TH E I CE-ROC K I NT ERFA CE AN D BASAL SLI D I NG P R OCESS 
AS R EVEA L ED BY D IR ECT O BSERVAT ION I N BORE HOLES 
AND TUNNELS 
By B. K AMB, H. F. ENGELHARDT, 
(Division of Geological and Planetary Sciences, California I nstitute of Technology, Pasadena, 
California 9 1125, U.S .A. ) 
and \V. 0 . H ARRISON 
(Geophysical I nsti tu te, University of Alaska, Fairbanks, Alaska 99701 , U.S.A.) 
ABSTRACT. The g la cier bed, where basal sliding occurs, was reached by cable-tool drilling 
a nd sand-pump bailing in seven bore holes in Blue Glacier, Olympic National Park, 
Washington. Basa l sliding velocities measured by bore-hole photography and confirmed by 
inclinometry are unexpectedly low, ranging from 0.3 to 3.0 cm/day and averaging 1.0 cm /day. 
T his is much less tha n about half the surface velocity of 15 cm/day, which was the sliding-rate 
expected from earlier deformation m easurements in bore holes made by thermal drilling a lone. 
The glacier bed consists of bedrock overlain by a layer of active subsole drift about 1 o cm 
thick, which intervenes between bedrock and ice sole, is partially to completely ice-free, and 
ABSTRACTS OF PAPERS PRESENTED 
is mechanically and visibly distinct from the overlying debris-laden basal ice. The subsole 
drift participa tes actively in the sliding process by moving along partially with the ice and by 
the rolling of its larger clasts when caught between bedrock and moving ice . The underlying 
bedrock is not definitely seen in bore-hole photographs, but is inferred from drilling perfor-
mance and penetrometer tests. 
A gap, of width up to a few centimeters, oflen inLervenes between ice sole and subsole 
drift. It may result from ice-bed separation caused by pressurization of the bed by water 
standing initially high in the bore holes, generating pressures c. 5 % greater than the ice 
ove_rburden pressure at the bed. The highest sliding velocities occur where a basal gap is 
present. 
The water pressure in subglacial conduits, as indicated by the behavior of bore-hole water 
levels, was essentially atmospheric in an area of the glacier where the ice was thin (c . 120 m ) . 
Wild fluctuations in water level that occur after an initial, precipitous drop reflect variations 
in water input and in flow capacity of narrow hydraulic connections from bore holes to 
subglacial conduits. In an area of thicker ice (c. 265 m ), the minimum basal water pressure 
was 1 0 bars above atmospheric, and a transient rise of about 6 bars occurred by input of 
turbid water from the bed after a day of heavy rain. A subglacial stream was encountered by 
one bore hole and followed by the sounding float for 65 m down-stream. 
The subsole drift is initially so impermeable to inflow of pressurized bore-hole water that 
there must exist permeability barriers between the drift and subglacial conduits, such as 
interconnecting areas where ice is in direct contact with bedrock. Because of these barriers, a 
high interstitial water pressure can be maintained in the subsole drift, which can prevent the 
basal ice from rapid ly invad ing the subsole drift by regelation. 
Anomalously low basal shear stress was present in an area of ice- bed separation, and 
anomalously high basal shear stress in an area oflowest sliding velocity, reflecting a local stress 
concentration due to high local resistance to sliding. These conclusions follow from detailed 
bore-hole tilt measurements near the bottom. 
Factors responsible for the smallness of the sliding velocities are ( 1) high bed roughness due 
to subsole drift, (2) partial support of basal shear stress by rock friction, and (3) minimal basal 
cavitation because of low water pressure in subglacia l conduits. 
Observed conditions at and near the bed do not closely correspond to the idealized state of 
affairs assumed in theories of sliding. The abundance of rock debris and the lack of a clean 
ice-rock interface are features quite different from what is assumed in the theories. 
A paper reporting much of this work has already been published in J ournal of Glaciology, 
Vol. 20, No. 84, 1978, p. 469- 508. 
DISCUSSION 
R. H. THOMAS: If the base of the ice is debris laden, could there be two sliding interfaces : 
one at the base and one at the top of the debris-laden layer? 
W. B. KAMB: Yes, there might, and there might also be distributed shear within the subsole 
drift layer. It is an interesting problem to consider at what level or levels the sliding should 
occur. When there is a basal separation gap, the sliding should certainly be at the level of the 
gap, but otherwise the choice of level is not so obvious. 
S. M. HooGE: If ice is continually invading the subsole drift and being melted within the drift, 
what happens to the water produced if this drift is sealed off from the active drainage system of 
the glacier? 
'!.7 
JOURNAL OF GLACIOLOGY 
KAMB: The water remains where produced , as pore water in the layer of subsole d rift that is 
growing. The pressure in this pore water inhibits ice invasion of the subsole d rift layer a nd 
allows this layer to grow by bottom melting. 
HODGE: Could you comment on why the slid ing velocity increased when the water level in 
the bore hole dropped ? 
K AMB: Possibly the increase in sliding speed resulted in increased ice- bed separation, which 
opened up a connection to a subglacial channel and thus a llowed the bore-hole water to drain 
away. There is certainly a problem of what is cause and what is effect in the correlation of the 
two events, if the correla tion is not simply accidental. 
L.A. LLIBOUTRY: Are you sure that there is not a continuous inflow of water into your bore hole 
along its walls, because glacier ice is not perfectly impervious? H ave you tried to pump off the 
water in the bore hole? 
K AMB: For bore holes in firn (and all but two of our holes were in firn), there is continuous 
inflow of water from the firn . With the bore-hole television we can see it enter a nd cascade 
down the hole. I doubt that there is any significant inflow through the ice walls. We have 
pumped water out of some bore holes with an air-lift pump, at a limited rate, and we have 
carried out several experiments in which water was pumped into the holes and the response 
was followed. Hermann Engelhardt has a paper on this which will appear in a forthcoming 
issue of Zeitschrift fur Gletcherkunde und Glazialgeologie. 
LLIBOUTRY: Can the value of the ice pressure not actually be higher tha n its mean value 
pgh cos O'. as a consequence of a horizontal variation of the shear stress? This would give 
perhaps N = pgh cos ry.-p zero instead of negative. 
KAMB : We do not think that the pressure of the base of the ice could generally be increased 
by this effect. More likely are local increases representing stress concentrations over the stoss 
surfaces of bedrock obstacles. Such an increased ice pressure may explain why the water in 
one bore hole (Y) never dropped from its high initial level. 
LLIBOUTRY: Have you any idea of the source of the basal drift you found? 
KAMB: The debris is derived from the local bedrock, which is relatively homogeneous, so that 
unique " tracer" lithologies whose source area can be pinpointed are not available. 
W. H. MATHEWS: In your model of shearing at the base of the debris layer, can you conceive 
of the shear surface from t ime to time or place to place shifting up to the top of the debris 
layer, at its contact with clear ice above? Shearing might then provide heat to maintain a 
· water layer here, and isolate a mass of frozen debris below. 
K AMB: This is conceivable. The subsole drift does appear locally to contain some interstitial 
ice, which might have originated in this way. Geothermal heat would of course tend to 
eliminate it. 
R . J. R OGERSON : Did you make any observations on the character of pebbles at the bottom of 
the holes? Particularly, is there any difference in roundness of stones beneath the firn and 
those beneath the holes further down the glacier. 
K AMB: We study each bailer haul (as well as the bottom photographs) with a n interest in the 
answer to your question. Some of the clasts, particularly the larger ones, show the effects o f 
contact with the bed (rounding, striated surfaces), but most are quite angular and thus appear 
locally derived. 
ABSTRACTS OF PAPERS PRESENTED 
J. \ VEERTMAN: You suggest that a boulder layer, such as the one you observed under Blue 
Glacier, may exist under most glaciers. But the very small sliding velocity that you measured 
for Blue Glacier is surely exceptional. The seasonal velocity fluctuations m easured on many 
glaciers g ive an order-of-magnitude estimate of sliding velocities and these velocities are large 
compared with the Illue Glacier sliding velocities. If the Blue Glacier sliding velocity is 
exceptional, then perhaps its basal boulder layer-whose existence is the presumed cause of 
the low sliding veloci ty-is also exceptional. 
KAMB: The sliding velocities found in Blue Glacier may well be similar to the wintertime sliding 
velocities of g laciers genera lly, which cannot be inferred from the seasonal fluctuations. I t 
seems likely that a basal debris layer is a ra ther general feature of glaciers, and that it may 
limit the wintertime sliding to velocities of a few cen time tres per day. The lubrication 
mecha nism that I described may account for a summertime increase in sliding velocity when 
it occurs. 
G. S. BouL TON: I am not convinced that you have found bedrock in your holes. There are 
many case studies in which cable-tool dri ll ing has proved incapable of penetrating a very hard 
till, which is quite often more difficult to penetrate than the bedrock from which the till is 
derived. I suggest tha t the soft layer could eq ually well be in terpreted as a deforming layer 
lying above a relatively rigid consolidated lower layer. The relative uniformity of the "active 
subsole drift" would support this idea. If this latter is correct it would explain the inconsistency 
in the sliding estimates produced from inclinomctry and bore-hole pho tog raphy, and that 
there is no fixed datum a t the bottom of the holes but that the subsole drift is deforming 
continuously. 
The idea of an open-work gravel beneath the glacier, which allows ice to penetrate it to a 
substa ntial depth does not seem compa tible with most subglacial observations of sediment, 
which show a matrix be tween large particles and no significant ice penetration. 
The pressure drop you observed is sim ilar to one we observed after several days in sub-
g lacial sediments in Iceland. There we believe it to be associated wi th consolidation of the 
sediment. Could that no t also be true in your case? 
K AMB: From our experience we do no t believe that the cable tool would have more difficulty 
penetrating till than bedrock under Blue Glacier. We believe that the abrupt cessation of drill 
penetration indicates tha t we encounter bedrock in our bore holes, but, as I said, this is not 
proven. \ Ve give the de tailed arguments for these conclusions in our paper in the .Journal <?f 
Glaciology. As also explained in detail in this paper, we think that our measurements of basal 
sliding by bore-hole photography are consistent with those from surveying and inclinometry 
to within the no t inconsidera ble errors involved in these methods, but in any case, the agree-
ment or disagreement between these measurements does not help dist inguish between bedrock 
or rigid till as the substrate for the active subsole d rift. 
Penetration of ice into basal rock debris (mixture of gravel, sand, and finer particles) is 
seen in some of our bore-ho le photographs and also in some subglacial tunnels, specifically 
Bondhusbreen, Glacier d 'Argentiere, Mer de Glace, and Blue Glacier. 
Consolidation of subglacial sediment would reduce its permeauili ty and therefore inhibit 
the loss of water from a bore hole reaching the bed. Application of water pressure would 
not cause permeable sediments to consolida te. You might think that the opening of a basal 
ga p between ice and subglacial debris could result from consolidation of the debris, but we 
think that more probably it results from local uplift of the ice mass under water pressures 
greater than the overburden pressure of ice. The bore-hole water can then escape through 
the ga p thus opened, and the bore-hole water level therefore drops. 


The Ice-Rock Interface and Basal Sliding Process as Revealed by Direct Observation in Bore Holes and Tunnels

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The Ice-Rock Interface and Basal Sliding Process as Revealed by Direct Observation in Bore Holes and Tunnels

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