FOSSIL ICE WEDGESAND GROUND WEDGES IN SEDIMENTS BELOW TILLAT VOSS, WESTERN NORWAY JAN MANGERUD & SVEIN ARNE SKREDEN Mangerud, J. & Skreden, S. A.: Fossil ice wedges and ground wedges in sediments below till at Voss, western Norway. Norsk Geologisk Tidsskrift, Vol. 52, pp. 73-96. Oslo 1972. In a 3 to 4 m high section, the following four distinct sediment units exist: At the base a well-sorted sand of uncertain origin; above this a bed of till (believed to be lodgement till); above this bedded sand, silt and clay, presum­ ably deposited in an ice-dammed lake; and on top, a younger lodgement till. All the sediments below the upper till are cut through by two different types of wedge-like structures. The wedges of the first type are filled with unsorted sediments and are interpreted as fossil ice wedges. The second type consists of vertically laminated clay and silt, presumably a kind of ground wedge. The sediments and the wedge structures are believed to be of either Allerod/ Younger Dryas age, or from older Weichselian interstadials. J. Mangerud, Geologisk institutt, avd. B, Universitetet i Bergen, 5014 Bergen­ Universitetet, Norway. S. A. Skreden, Geologisk institutt, avd. B, Universitetet i Bergen, 5014 Bergen­ Universitetet, Norway. Introduction Voss is situated in a broad and deep west Norwegian valley. The valley floor is 45-75 m a.s.l., while the surrounding mountains rise to between 1300 and 1400 m a.s.l. (Figs. l and 2). Bedrock is exposed in most of the mountain area, while considerable deposits of till and glacio-fluvial sediments occur in the valleys. Examination of the glacial striae (Skreden 1967) shows that the oldest ice movement was approximately towards the west (Figs. 1 and 2). Roughly speaking, this movement was independent of the topography, and is inter­ preted as representing a period of complete glaciation of the area. During this period the ice spread radially from an ice shed over the mountains further east. Later the ice movement changed direction towards SW and S (Fig. 2). During this later period the ice spread radially from an ice shed in the moun­ tain areas between Voss and Sognefjorden, northwest of Voss. As the ice melted it became increasingly influenced by the topography, and finally be­ came valley glaciers (Fig. 2, youngest) with outlets through the valleys of Voss-Granvin and Voss-Evanger. The final deglaciation of the Voss area seems to have taken place in Pre-Boreal time (Mæland 1963, Skreden 1967). 74 J. MANGERUD & S. A. SKREDEN LOCATION MAP TRONDHEI�(. ' :62• ... � ''\ .­ ' ' ' ' O OSLO ,--6'0"' ' Voungcst �� GLACIAL STRIAE Tlll FABRIC IN THE EXCAVATION u::uppcr till 1::: lowcr till 2km Contour intcrvat 90m Fig. l. Map of the Voss area. Glacial striae according to Skreden (1967). Numbers give altitude in m a.s.l. The described section at Lundarvatn is situated where the till fabric is plotted. lnset is a key map of southern Norway. The section at Lundarvatn During field work in 1965, an interesting section was discovered in a building excavation near Lundarvatn (Fig. 1). The field work was carried out by Svein Ame Skreden under Jan Mangerud's guidance, and although this article has been written by Mangerud most of the field descriptions used are from Skreden's thesis (Skreden 1967). Thick deposits forming an inclined ledge of 5-600 m in length along the western side of the valley are found here (Fig. 4). The building site is situated 130 m a.s.l. on a projecting ridge (remnant ridge) between two ravines cut­ ting through the ledge. The ravines fall from between 150 to 80 m a.s.l. Profiles along the ridge and ravines (Fig. 3) show that the sediments are at least 20 m thick, probably considerably more. FOSSIL ICE WEDGES AND GROUND WEDGES AT VOSS 75 ., Youngcr /t_," / \;: 10 km t;,:----,J Contour intcrval 500m tdcst Youngc1t <6J Are as above 1000 m Fig. 2. Schematic map showing the different ice movements in the Voss area mainly based on an interpretation of the glacial striae. m.a.s.l. 275 w 250 -- Profilel 225 ----Profi le TI 200 '.-:>. 175 150 125 Lundarv atn\ 100 �\ 75 o 500 900 Fig. 3. Profiles along the valley side at Voss. Profile I along the remnant ridge where the building site was located. Profile Il in the ravine just north of the building site. 76 J. MANGERUD & S. A. SKREDEN Fig. 4. On the valley floor Lundarvatn can be seen to the right in the fore·ground. The building site is indicated by an arrow. (Photo towards SW.) Fig. 5. Profile along the excavated walls. A sketch map of the building site is inset in the upper right corner. The corners in the building site are denoted by letters also shown in the profile. The excavated wall BC thus lies at right angles to walls AB and CD. r----Fi g. 6------, ",..-.----- -- Excavated Lundarvatn Sand fossile ice-wedge laminated wedges and veins Not excavatec o c FOSSIL ICE WEDGES AND GROUND WEDGES AT VOSS 77 The main characteristics of the section are shown in Fig. 5 with four well defined units: On the top a bed of till; upper till. Then follows stratified sand, silt and clay, which we call Voss Clay and Silt. A bed of till; lower till. In the bottom there is sand which we eaU Lundarvatn Sand. The three lower units are cut through by younger wedge structures which we will discuss in detail. LUNDARVATNSAND The Lundarvatn Sand is exposed in excavation walls AB and CD, as well as in a ditch from corner D (Fig. 5). Except for some clay which was found under the sand in this ditch, the material below the sand is unknown. The Lundarvatn Sand is mainly fine and is well sorted. Fig. 12 shows the grain size distribution of three samples from different levels in wall CD. The sand is very homogeneous with few visible structures. Weak stratification was seen in only a few places. In wall CD (Fig. 5) the sand was more consolidated in the upper part. Analysis of the porosity, both in a natura!, undisturbed condition as well as in the laboratory, by artificially decreasing the porosity by stamping, shows that the higher consolidation is partly due to tight packing. The origin of the sand is uncertain, but is most probably glacio-fluvial. A ._l ---=-o - r N 13.5m 1 7m 6 5 4 3 2 ------------------------------------ 0 ----------��------ � B A o 5 10m 78 J. MANGERUD & S. A. SKREDEN Fig. 6. Photo of wall CD (Fig. 5). The Jength of the spade is 70 cm. The natural surface is indicated by the unbroken line. The upper and lower boundaries of Voss Clay and Silt are indicated by stippled lines. LOWER TILL The lower till is exposed in all the excavated walls (Fig. 5). The thickness varies from 0.5 m to at least 1.5 m. The boundary between the till and the sand below is very well defined, and only small amounts of sand seem to be incorporated in the lower till. The upper boundary of the till is very uneven (Fig. 6); this is partly because of later deformation. The till is nonsorted (see sample l taken in corner A, Fig. 11). In wall CD the boulder content is normal for lodgement tills, while it is poor in wall AB. We do not attach any importance to this difference which is believed to be accidental. In wall CD a fabric analysis of the longest axes for 100 pebbles was car­ ried out (Fig. 7). This revealed a maximum in the sector W-NW or E-SE. We have interpreted this till as a lodgement till; the fabric indicates the direc­ tion of the ice movement. The till contains approximately 5% anorthositic rocks in the pebble fraction. Anorthosite occurs only E of V oss, therefore the most probable ice movement seems to have been towards N-NW. This movement was diagonally across the valley, and parallel with the oldest glacial striae in the area (Figs. 1 and 2). Accordingly we assume that the till was deposited simultaneously with the ice movement indicated by these striae. VOSS CLAY AND SILT The thickness of Voss Clay and Silt varies from 0.5 to 2 m. In places some beds are eroded and many deformations of the primary beds can be seen. FOSSIL ICE WEDGES AND GROUND WEDGES AT VOSS 79 uppertill N lower til l N l l w -- -- E w - - E ..._.. l s 5 pebbles s Fig. 7. Orientation of longest axes of 100 stones in each of the two beds of till. About the middle of the sequence, however, there is a bed of dark clay, 5-6 cm thick, which differs much from the rest of the sequence. We call this the 'clay marker bed' (Fig. 5), and the continuity of the bed shows an ab­ sence of faulting or displacement to any measurable degree. The sequence is complete in wall AB (to the right in Fig. 10) and will be described below. The base consists of about 20 lamina of well-sorted clay and silt, 20 cm thick. Isolated pebbles in the lamina must have been dropped from drifting ice simultaneously with the deposition of clay and silt. This is particularly evident at the top, where a clay lamina with many pebbles oc­ curs. Above the latter lamina follow sand and silt (Fig. 12, sample 8) de­ creasing in grain size upwards to the mentioned 'elay marker bed'. This consists of fine-grained bluish, black clay, with c.