Experiments beneath Breidamerkurjökull in Iceland have led to development of flow laws for the subglacial till, relating strain rate to shear stress and effective pressure and assuming either Bingham fluid or nonlinearly viscous fluid behavior. Water pressures in the till are less than ice pressures and it is suggested that this may lead to infiltration of ice into the sediment, which inhibits sliding at the ice/sediment interface. Where water pressures are equal or near to ice pressures, infiltration does not occur and sliding may result. A one‐dimensional theory of subglacial deformation is developed in which the empirical flow law is coupled with a model of subglacial hydrology and consolidation. This predicts stable states in which subglacial sediment either does not deform or a dilatant deforming horizon forms with positive effective pressures at the ice/bed interface or unstable states where zero or negative effective pressures are predicted. Time dependent analyses show that response times following perturbations of the system may be of the order of 103 years and thus that unsteady behavior may be normal on glaciers flowing over unlithified sediment beds. It is suggested that the natural variability of material properties in subglacial sediment beds leads to the development of drumlins on the glacier bed. It is suggested that unstable deformation at zero or negative effective stress leads to “piping” in subglacial sediments at the glacier terminus and the growth of sediment‐floored, subglacial tunnels. Their frequency is that which is sufficient to draw down subglacial water pressures so as to prevent unstable deformation. Where they discharge large water volumes, subglacial sediments flow laterally toward them producing “tunnel valleys.” This sediment is then removed by water flowing along the axial tunnel. Tunnel valleys can be regarded as the equivalent in soft sediment areas of eskers in bedrock areas.

Debris transported by glacier is derived either supraglacially from nunataks and valley sides or from erosion of the subglacial bed. Debris produced above the glacier by fracturing of rock walls has a dominant coarse fraction with angular boulders. Subsequent englacial or supraglacial transport is relatively passive and little comminution occurs. Debris eroded from subglacial bedrock is initially transported in a basal zone of traction, where particles frequently come into contact with the glacier bed and are retarded by it so that large forces may be generated between particles and the bed and at interparticle contacts. The material introduced into this tractional zone may be subglacial bedrock which has undergone a crushing‐plucking event and which has a dominant coarse fraction, or supraglacially derived material which finds its way to the glacier bed. These parent debris assemblages are further comminuted by failure in response to locally concentrated compressive stresses, and attrition at shearing interfaces. Boulders transported through the tractional zone will tend to be rounded and bear several directions of striation. Large boulders embedded in lodgement till will tend to be streamlined with striae parallel to glacier flow and with an abruptly truncated distal extremity, rather like a roche moutonnée. Textural and boulder shape characteristics can be used to help distinguish different types of till.

The local climatic regime and the mass balance state are important determinants of the dynamics of terrestrial and marine glacier fronts, which in turn determine the sediments and landforms produced at the glacier front. Many modern glaciers undergoing overall retreat in areas of‘maritime’climate produce winter push moraines during a late winter readvance, followed by a summer retreat, whilst in more‘continental’regions no significant winter readvance occurs and annual push‐moraines are absent. The frontal dynamics which lead to these changes are analysed and the form, structure, sequence and field relations of both terrestrial and marine push‐moraines are described from Iceland, Spitsbergen and Baffin Island. Long‐term changes in mass balance leading to major glacier advances or readvances also generate large push‐moraines. In terrestrial environments push‐moraine formation is accompanied by uplift, rejuvenation and down‐cutting of outwash systems whose sediments become closely associated with glaciotectonic structures, which permit pre‐, syn‐ and post‐tectonic sequences to be identified. The development of ice marginal fan/moraine complexes is modelled as a function of the relative magnitude of two parameters: the velocity of ice movement and the calving rate. A high ice velocity just exceeded by the calving rate gives closely spaced push‐moraines and confluent ice marginal fans. A high velocity far exceeded by the calving rate produces closely spaced moraines but separate ice marginal fans. A low ice velocity in combination with a high calving rate results in well separated and feebly developed push‐moraines, while a low ice velocity and a low calving rate produces feeble push‐moraines and coalescent fans.

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Although theories of glacier movement ge n erally assume that glaciers flow over rigid rock beds, there are many places where glacie rs r est on beds of d eforma ble sedim e nt, a nd the great Pleistocene ice sheets which extend ed from time to time over much of Northern Europe a nd N orth America were largely underlain by such b eds. Observations sh ow that a large proportion of the forward movement ofa glacier lying on such a bed may be contributed by d eformation of the bed ra ther than the gl acier. A theory is d eveloped in which the glacier surface profile is rela ted to the hydraulic and strength prope rti es of potentially deform-able bed materials. If these have a high hydraulic transmiss ibility, melt wa te r is readily discharged sub-glacially, the bed is stable, and the profile is a normal p a ra boli c one, governed by the rheologica l properties of ice. If bed transmissibility is low, wa ter pressures build up, the bed begins to d eform, and a lower equilibrium profile will develop, so tha t in a n extreme case the gl acier a pproxim a tes to a thin flat sheet, similar to an ice shelf. It is suggested tha t su ch behaviour may h ave occurred a t the m a rgins of large Pleistocene ice sheets over North America and Europ e, a nd evidence in support of this is dra wn from the reconstructed shapes of these ice margins, a nomalo usly sm all a mounts of isostatic rebound, a n o ma lously high retreat-rates, and the presence of glaciotectonic structures. R easons a re suggested to expl a in why this behaviour should have been important for Pleistocene glaciers which p en etra ted into currently te mperate latitudes but does not appear to be important in la rge m od ern glaciers. REsuME. Stabiliti de calottes de glace tempi ree reposant sur des lits de sidiments diformables. Bien que les theories sur le mouvem e nt d es glaciers supposent en general que les glaciers s'ecoule nt sur d es lits rocheux rigides, il arrive souvent que les glaciers reposent e n fait sur des sed iments deforma bles et la grande calotte glaciaire pleistocene qui s' es t pa rfois etendue sur la plus grande pa rti e de l'Europe du N o rd et de l'Amerique du Nord etait en gran de partie supportee p a r d e tels lits. Des o bservati ons montre nt qu'une fort e proportion des avancees de glacie rs r eposant sur un tellit peut et re le res ultat de la deform atio n du lit plus que d e glacier. On a developpe un e theorie selon laquell e le profil de la surface du gl acier es t mis en relation avec les pro-prietes hydrauliques et plastiques d es m a te ria ux d'un lit d e fo rma ble. S' ils o nt un e forte permeabilite, I'eau d e fonte est rapid ement tra nsmise so us le glacier, le lit est sta ble et le profil est normalement parabolique, conformement aux lois rheologiques d e la glace. Si la pe rm eabili te a I'eau est fa ible, la pression hydrosta tique croit, le lit commence a se deforme r et un profil d'equilibre plus bas en a ltitude se developpe d e sorte que, e n d es cas extre m es, le glacier ressembl e a un e feuille min ce pl...