Structure and Fabric on the Burroughs Glacier, South-East Alaska

Abstract

ABSTRACT. The Burroughs Glaci er, so uth-east A laska, is a slow-m ov in g remnant ( 14 X 3 km. ) of a much more ex tensive glacier. It is n ow entirely b elow th e firn line; ablat ion has revealed ice structures and fabri c once 300 m. or more below the g lacier surfacc .A t the present glacie r surface three kinds of ice are iden tified -foliated ice, coarse-grained border ice and very coarse-grained basal ice.Two systems of fine-grained foliation a re present. Differential m ovem ent in the g lacier has caused recrystallization a long closely spaced p lanes. At the g lac ier surface thi s produces a steeply d ipp ing longitud in a l foliation. A gently dippin g foliation , hav ing a reg iona l trough-like structure, may be associated with form er stratification p lanes or with former spoon-shaped sh ear su rfaces.T he optic ori entation of cr ystal s in the coarser layers of the foliated ice shows three weak maxima, and in th e fin er layers a singl e weak maximum , correspond ing to one of the coarse layer maxima, a nd normal to the gently d ipp in g foliation p lane. Th e other maxima in th e coarse layers a re orientated close to the poles of principal fracture planes .In the coarse ice the fabric shows a pattern w ith three maxima similar to that obta in ed in torsion shear exper iments. I n th e glacier the pattern may be formed by shear near the glacier bottom o r a long gent ly dipping foliation planes. Grain -size increases towards the glacier te rminus, especially in th e stagnant ice zone .Structura l ev idence suggcsts that in th e earl y stages of the Little Ice Age the ice Row was from west to eas t. Later it was to cast and west from an ice crest in the upper Burroughs G lacier. Structures produced by present movement have b een supe rimposed on o lder structures. R ESUME. L e glacier Burroughs, dans le Sud-Est de l'Alaska, es t un r este ( 14 X 3 km ) it mouvement lent d ' un glacier b eaucoup p lus important. 11 est maintenant entierement en -dessous de la li gne de nevc ; I'ablation a mis en evid ence des structures et des tex tures d e g lace, qui on e ex istees it 300 m au moin s e n-dessous d e la surface d u g lacier.Actue ll ement on peut id entifi er 3 sortes de g lace it la surface d u g lac ier: g lace feuill etce, g lace bordiere it gros gra ins, g lace d e fond it tres g ros grains.O n constate la presence d e 2 systemes d e foliation a gra in s fins. D es mouvements differentiels dans le glacier ont cause des recristallisation le long de p lans rapproches. Ceci se traduit a la surface du glac ier par une foliation it fort pendage longitud in a l. Vne foli at ion a faibl e pendage ayant une structure locale en form e d e c reux p eut etre assoc iee avec d 'a nci ens plans d e stratification ou avec d ' a n c iennes surfaces d e c isa illem en t en forme de cui ll er.L'ori entat ion optique d es cr istau x dans les couc h es it gros gra ins d e la g lace feuill etee , pn'sente 3 faibl es maxima et un se ul faibl e maximum dans les cr istaux a gra ins fins; ce max imum est perpend ic...

Nature and Rate of Basal Till Deposition in a Stagnating Ice Mass, Burroughs Glacier, Alaska

“…A topographic map of the area was constructed between 1890 and 1892 by A Reid (1896) which shows ice over the Bruce Hills (Figure 1) at an elevation between 610 and 685 m. The Wachusett Inlet area has been completely deglaciated and now lies at sea level (Figure 1). The elevation of the crest of Burroughs Glacier was between 500 and 525 m in 1960 (Taylor, 1963). Retreat of the calving ice margin in Wachusett Inlet was rapid and 8 FIGURE 1.…”

The Origin of Foliation in Glaciers: Evidence from Some Norwegian Examples

“…Longitudinal foliation forms between flow units and at the margins of a glacier, and transverse foliation forms below ice-falls and in zones of marked decrease in the logitudinal velocity component. The structures of Charles Rabots Bre in many respects are similar to those of other glaciers, but apparently the only previous references to the possibility that the development of foliation may be controlled by sedimentary stratification, and then only in a restricted sense, are in papers by Meier (1960), Taylor (1962) and Ragan (1969). However, it is clear that foliation can either be highly deformed stratification or a completely new structure.…”

“…As in earlier studies (Untersteiner, 1955; Meier, 1960; Allen and others, 1960; Taylor, 1962), the term foliation is here used to describe intercalated layers of ice of varying crystal size or bubble content and translucency, and stratification refers to the original depositional layering preserved in the ice. In foliation there are two main types of ice, coarse bubbly and coarse clear, which form intercalated layers, each about 5-100 mm thick and up to about 4 m in length.…”

“…By definition, foliation must be the result of deformation. Recent studies of ether glaciers (Schwarzacher and Untersteiner, 1953; Untersteiner, 1955; Meier, 1960; Allen and others, 1960; Taylor, 1962; Gunn, 1964; Rutter, 1965; Hashimoto and others, 1966; Ragan, 1969; Marangunic, 1972; Hooke, 1973) have shown that foliation develops as a result of shearing or compression. Longitudinal foliation forms between flow units and at the margins of a glacier, and transverse foliation forms below ice-falls and in zones of marked decrease in the logitudinal velocity component.…”

The Formation of Medial Moraines on Alpine Glaciers

“…There are thus important differences between the Sørbre and Arolla bands. It is well known that in zones of strong ice compression (as at the base of ice falls), there is a tendency towards up-shearing, as evidenced by the occurrence of steeply dipping transverse foliation (Taylor, 1963; Rutter, 1965). However, on Glacier de Tsidjiore Nouve, shearing in this position would hardly produce planes extending laterally for only a few metres, nor could it account for the strong localization of englacial debris, unless the latter had been raised along shears initiated at the glacier bed at a depth of 200 m—an unlikely mechanism.…”