Introduction

Fabric analyses, or the study of preferred orientations of stones in glacial and other superficial deposits has attracted increasing attention in recent years, following the pioneer work of K. RichterReference Richter ^{4 ,} Reference Richter ⁵ and HolmesReference Holmes ¹ . Fabric analysis has been frequently used in the study of Pleistocene glacial deposits, but studies in and around present day glaciers are rare.

The studies discussed here were carried out as part of the work of an expedition from Edinburgh University to Lyngenhalvöya in north Norway on moraines in Övre Lyngsdalen a 10 km.-long valley that descends eastward from the Jiek’kevarri massif to the fjord, Lyngen. (Fig. 1, p. 731.) Here lie two valley glaciers, Lyngsdalsbreen and Litle Jiek’kevarribreen, both largely maintained by avalanches from ice caps on the surrounding plateaux, and both terminating at an altitude of about 400 m. The former has a fairly smooth gradient, the latter ends in an ice fall. Both have retreated several hundred metres from prominent moraines that mark the limit of a considerable re-advance and which in places overlie an older set of moraines. (Fig. 2, p. 731.) In both cases, the portion of the morainic arc lying on the right hand side of the snout and outwash streams is larger (5–25 m. high), with a more mixed constitution than the block moraine (c. 2 m. high) resting on the opposite side of the valley. The inner face of the Litle Jiek’kevarribre moraine is fresh and so steep that all loose debris falls clear and does not mask the structure.

Fig. 1. Lyngsdalen and its glaciers. The “North Cwm” is the tributary glacier situated to the north of the “South Cwm”

Fig. 2. Terminal moraine of Litle Jiek’kevarribreen; preferred orientation indicated by arrows

Method

When making a fabric analysis, the bearing of the long axes of at least 50 stones about 10 cm. long was measured by compass. Only stones with a clearly defined long axis dipping at less than 45 degrees were chosen, and each sample of stones was gathered from as small an area as possible to minimize the risk of errors caused by unconscious selection of stones lying in a certain direction. It is obvious that the sample taken should be truly typical of the deposit being investigated. The readings were assembled into ten degree groups: 0–10, 11–20, 21–30, etc. and plotted on a rose diagram. (Fig. 3, p. 731.) In most cases a clearly defined maximum appeared, representing the preferred orientation of the stones. This maximum has been shown to be generally parallel to the direction of movement of the glacier but in some cases may be transverse to itReference Holmes ^{1 ,} Reference Richter ^{4 ,} Reference Richter ⁵ .

Fig. 3. Rose diagrams of fabric analyses

In Fig. 2 the preferred orientation at a particular point on the moraine has been plotted as an arrow pointing in the appropriate direction, its length being proportional to the percentage of stones in the largest ten degree group. Where two fabric analyses were made at the same point in the section but at different depths, the lower analysis is indicated by a double stroke across the tail of the arrow. Care was taken to ensure that the analyses were made at points where sliding and rolling of the stones since deposition by the ice had not occurred.

The orientation of englacial material was studied on Lyngsdalsbreen while the moraines studied were those of Litle Jiek’kevarribreen.

Orientation of Englacial Material

Wherever ablation revealed englacial moraines, whether lateral or medial, it was obvious that the stones had been generally lying in the ice with their long axes parallel to the movement of the glacier. This orientation is certainly due to dynamic forces related to the movement of the ice and contrasts with the orientation of stones in a stream bed which tend to lie at right angles to the current. In some cases the orientation may correspond to the original attitude in which the stone became incorporated in the glacier: at the head of the “South Cwm” branch of the glacier it was noted how blocks detached from the rock walls above adopted a sliding position, aligned down the slope, when they came to rest on the firn surface. In this attitude, parallel to the direction of movement of the firn towards the exit of the cwm, they would be eventually carried down to the main glacier.

Only one dirt band suitable for making fabric analyses was found.

It was a lens of stony material reaching a maximum thickness of 50 cm. of which the outcrop appeared for about 50 m. on a sloping ice surface in the “North Cwm” tributary glacier. Fabric analysis showed a majority of stones in the band lying parallel to the direction of ice movement at this point. (Fig. 3a.)

Structure of Moraines

Fabric analyses made on the steep inner face of the lateral part of the Litle Jiek’kevarribre morainic arc at two levels (c. 3 and 10 m. below the crest) showed clear preferred orientations transverse to the axis of the ridge, usually with a forward-pointing component. (Figs. 2 and 3b, 3c.) This transverse orientation was apparent to even a casual inspection and may be observed in Fig. 4 (p. 763). Since stones are generally transported by the ice in an attitude parallel to its direction of motion, this transverse orientation implies that the ice was moving radially in the tongue when the moraine was built.

Fig. 4. Summit of morainic are of Litle Jiek’kevarribreen (see R. W. Galloway, p. 730)

Where the moraine swung from the lateral to the terminal position, the preferred orientation changed and became parallel to the axis of the ridge. This suggests that the terminal portion of the moraine was built up by a process of thrusting and rolling in contrast to the lodgement that built the lateral part. Similar preferred orientations were noted in the block moraine on the other side of the outwash stream. In the lateral part, the blocks lay transversely to the ridge, with a slight forward-pointing component (Fig. 3d), while in the terminal part the stones lay approximately parallel to the ridge.

Nowhere did the upper and lower preferred orientations coincide exactly. This suggests that either 50 measurements were insufficient to give an accurate picture of the orientation, or that the structure of the moraine varied with depth. At close quarters no stratification could be discerned on the steep face, but when seen from a distance of about half a kilometre sub-horizontal, subparallel layers could be detected, some layers being distinctly richer in boulders than others. A detailed study of such a moraine with analyses made at close intervals vertically and horizontally might yield interesting information on variations in the moraine-building forces. The existence of important variations in orientation with depth means that great caution must be used in interpreting the results of fabric analyses from shallow sections in morainic ridges, even if the stones investigated are truly in situ.

Conclusions

• a. Stones in the englacial, lateral and terminal moraines studied show distinct preferred orientations ascertainable by fabric analyses.

• b. Englacial stones tend to lie parallel to the movement of the ice in the glaciers discussed here.

• c. In the lateral part of the morainic arc, the stones lie transverse to the ridge, suggesting that radial movements were strong when the moraine was formed.

• d. In the terminal part of the moraine, stones lie parallel to the ridge, suggesting accumulation by rolling and thrusting.

• e. The preferred orientation varies with depth, so that great caution is needed in interpreting results from shallow sections.

Acknowledgements

I would like to thank the following for invaluable advice and suggestions in connection with the preparation of the work discussed in this paper: Professors A. Cailleux, R. Miller, H. Poser, J. Tricart and Dr. R. Common.