Direction Of Ice Movement Research Articles

The Pleistocene ice-sheet dynamics in north-central Poland based on magnetic fabrics of tills and landform analysis

Abstract This research was carried out in north-central Poland, which was occupied by the ice sheet of the Saalian (Marine Isotope Stage [MIS] 6) and Upper Stadial of the Weichselian (MIS 2) glaciation. The application of the anisotropy of magnetic susceptibility (AMS) method supported by a digital elevation model (DEM) analysis of the orientation of glacial landforms allowed for the reconstruction of ice-sheet extent and ice movement directions of these two Pleistocene glaciations. The research used an innovative method of collecting AMS till samples from the glacial plateau areas. Based on the research, it was found that during the general recession of the ice sheet of the Saalian glaciation, a previously undescribed glacial transgression occurred, characterized by a different direction of ice-sheet movement. On the basis of detailed geomorphological studies of the area of terminal moraines, previously described in fragments, the maximum extent of the ice sheet during the Weichselian glaciation was clarified. The recession of the ice sheet of the Weichselian glaciation from the area of north-central Poland took place in four regressive or transgressive–regressive stages with variable directions of ice-sheet movement. The results obtained indicate the great potential of the AMS method in paleoenvironmental studies of glaciated areas.

Reappraisal of exceptionally preserved s-forms, striae, and fractures from late Paleozoic subglacial surfaces in paleofjords, NW Namibia

Determining the grounded ice dynamics of deep-time glaciations is limited by the scarcity of well-preserved subglacial erosional features and their irregular distribution. In particular, small-scale erosional features known as s-forms that are subglacially sculpted in bedrock by water and/or ice are rarely preserved from the pre-Cenozoic record. A detailed re-examination of two late Paleozoic (late Carboniferous–early Permian) glacially-polished, surfaces at the base of the Dwyka Gp. within paleofjords located in the Kaokoveld region of northwest Namibia reveals a range of erosional features including: complex, multi-directional striae that crosscut each other, crescentic markings, chattermark trails, sinuous furrows, linear furrows, transverse troughs, comma forms, sichelwannen, muschelbrüche, cavettos, a pothole, and rock drumlins. The first study location in the Sanitatis paleovalley is previously undescribed and consists of striae and fractures on a polished granite bedrock surface located on the paleovalley floor. Striae, crescentic markings, and chattermark trails indicate ice movement to the west/northwest (striae mean azimuth of 276°). The second location in the Hoarusib paleovalley was previously described and is located on a multi-level, resistant, quartzite bedrock ridge close to or on the valley wall. This location contains numerous s-forms, striae, and fractures, as well as onlapping glaciogenic sediments, including diamictite plastered within a pothole. Some of these features are superimposed on rock drumlins. These erosional features were likely formed by a combination of pressurized subglacial meltwater and glacial abrasion underneath a glacier as it flowed over and around a resistant bedrock outcrop. Orientations of striae and chattermark trails at the second location indicate a primary direction of ice movement toward the west/northwest (striae modal azimuth of 275°), a minor secondary movement to the southwest (255°), and abundant third-order striae indicating ice flow around bedrock obstacles. However, cross-cutting relations suggest the primary and secondary striae orientations are not related to two distinct glacial advances as previously thought. The complex relationships between striae, fractures, and s-forms suggest that a combination of pressure melting, abundant subglacial meltwater, debris-rich basal ice, and variable ice flow paths around resistant obstacles was required to form these features. We conclude that the study locations were overridden by relatively thick (>210 m) warm-based or polythermal glaciers that were confined to a network of fjords as ice receded and stagnated. The glaciers flowed west into present-day Brazil during the late Paleozoic and likely overtopped the paleovalley walls during times of ice maxima.

Onset of the late Paleozoic glaciation in the Lhasa terrane, Southern Tibet

The Late Paleozoic Ice Age is characterized by diverse stratigraphic records with extensive spatial-temporal distribution in the Gondwana. The Lhasa terrane located in northeastern Gondwana during the late Paleozoic developed successive glacial sedimentary records, but the depositional evidence, as well as age constraint for the onset of glaciation, is unknown. This paper provides the first full description of the basal glacial deposits in the Xainza area of Lhasa terrane, southern Tibet. Conodonts present in the carbonate overlying the glacial bed (1 m) dominantly consist of Declinognathodus intermedius, indicating that the initiation of the Late Paleozoic Ice Age in the Lhasa terrane took place in Bashkirian. Three facies associations were recognized: (i) tidal flat association (subtidal channel, bar, intertidal mixed flat), (ii) subglacial association (megaclasts and melt-out diamictite), and (iii) lonestone-bearing heterolithic association (grounding-line fan and iceberg). These facies associations are vertically arranged into a complete transgressive-retrogressive glacial succession. Ice movement direction reconstructed from the imbricated megaclasts, the concordant folds and the lateral distribution of subglacial deposits and glacial marine deposits is consistently from NNE to SSW. These characteristics suggest that the Bashkirian glaciation in the Lhasa terrane is irrelevant to western Australia, implying that these two blocks were separated during the Late Carboniferous.

Origin of glacial curvilineations by subglacial meltwater erosion: Evidence from the Stargard drumlin field, Poland

AbstractDeciphering the origin of subglacial landforms is crucial for understanding processes operating under ice sheets. Due to the inaccessibility of the ice/bed interface limiting real‐time observations, the origin of these landforms is typically interpreted from the morphological and sedimentological record exposed after ice retreat, which makes the conclusions conjectural. Here, we focus on a recently discovered new type of subglacial landform called glacial curvilineations (GCLs), whose formation generates some controversy. We use LiDAR imagery and geological data to constrain the origin of GCLs found in a tunnel valley in the Stargard drumlin field, Poland. The tunnel valley is about 3800 m long, 520 m wide, and it occupies about 2 km2. A total of 61 individual GCLs have been mapped there; they are up to 550 m long and typically 0.5–1.5 m high, but some reach a height of up to 5 m. The GCLs are composed of mostly massive till represented by either a single or several diamicton units occasionally separated by thin layers of meltwater sand. Till macrofabrics in the GCLs and in the nearby till plain are distinctly consistent, and have high and uniform clustering strengths. At all locations, the dominant till fabric direction broadly corresponds with the ice movement direction given by the drumlin orientations, but it is oblique to the orientation of the tunnel valley and independent of the orientations of the individual GCLs. The lack of correspondence between the orientation of the GCLs and the till fabrics in them suggests that the GCLs consist of an antecedent till deposited during the drumlinizing ice advance and subsequently carved by subglacial meltwater erosion in the tunnel valley. These results substantiate the hypothesis that GCLs are erosional features postdating the material constituting them, and emphasize the role of subglacial meltwater flows as a landforming agent.

Combined pattern matching and feature tracking for Bohai Sea ice drift detection using Gaofen-4 imagery

ABSTRACT Sea ice is the main cause of winter marine disasters in the Bohai Sea, which is an important sea area in China. The sea ice in the Bohai Sea is easily deformed by strong water currents and rapid temperature changes. Consequently, low spatial and temporal resolution data cannot meet the needs of high-precision monitoring of the sea ice in the Bohai Sea. The geostationary satellite Gaofen-4 (GF-4) has the unique advantages of a high temporal resolution (20 s) and a high spatial resolution (50 m) and can be used to monitor sea ice drift in the Bohai Sea. We evaluate a new algorithm that combines the scale-invariant feature transform (SIFT) technique and the normalized cross-correlation (NCC) method to obtain accurate sea ice drift information. The interclass separability method is used to determine the optimal band combination (OBC), and the results show that the OBC for distinguishing sea ice from sea water is B2×(B1× B5)−1. The proposed method improves noise immunity and calculation efficiency and considers the characteristics of ice blocks. The proposed algorithm can also obtain more correctly matched feature points than the NCC method and approximately 10 times more correct points than the SIFT technique. The mean absolute error (MAE) of the velocity obtained with the proposed algorithm is 0.1615 m s−1 lower than those obtained with the SIFT and NCC methods, and the root mean square error of the velocity determination of the three algorithms is very small. The MAE of the ice movement direction obtained with the proposed algorithm is 0.605° lower than those obtained with the SIFT and NCC methods, and the MAE of a second data set is 12.378° better than that of the SIFT method. The motion of sea ice obtained by the proposed algorithm is close to the actual movement of the sea ice in the Bohai Sea.

Seismic interpretation and structural restoration of the Heligoland glaciotectonic thrust-fault complex: Implications for multiple deformation during (pre-)Elsterian to Warthian ice advances into the southern North Sea Basin

Despite a long history of research, the locations of former ice-margins in the North Sea Basin are still uncertain. In this study, we present new palaeogeographic reconstructions of (pre-) Elsterian and Warthian ice-margins in the southeastern North Sea Basin, which were previously unknown. The reconstructions are based on the integration of palaeo-ice flow data derived from glaciotectonic thrusts, tunnel valleys and mega-scale glacial lineations. We focus on a huge glaciotectonic thrust complex located about 10 km north of Heligoland and 50 km west of the North Frisian coast of Schleswig-Holstein (Northern Germany). Multi-channel high-resolution 2D seismic reflection data show a thrust-fault complex in the upper 300 ms TWT (ca. 240 m) of seismic data. This thrust-fault complex consists of mainly Neogene delta sediments, covers an area of 350 km2, and forms part of a large belt of glaciotectonic complexes that stretches from offshore Denmark via northern Germany to Poland. The deformation front of the Heligoland glaciotectonic complex trends approximately NNE-SSW. The total length of the glaciotectonic thrust complex is approximately 15 km. The thrust faults share a common detachment surface, located at a depth of 250–300 ms (TWT) (200–240 m) below sea level. The detachment surface most probably formed at a pronounced rheological boundary between Upper Miocene fine-grained pro-delta deposits and coarser-grained delta-front deposits, although we cannot rule out that deep permafrost in the glacier foreland played a role for the location of this detachment surface. Restored cross-sections reveal the shortening of the complex along the detachment to have been on average 23% (ranging from ca. 16%–50%). The determined ice movement direction from east-southeast to southeast suggests deformation by an ice advance from the Baltic region. The chronospatial relationship of the thrust-fault complex and adjacent northwest-southeast to northeast-southwest trending Elsterian tunnel valleys implies a pre-Elsterian (MIS 16?) age of the glaciotectonic complex. However, the age of these Elsterian tunnel valleys is poorly constrained and the glaciotectonic complex of Heligoland may also have been formed during an early Elsterian ice advance into the southeastern North Sea Basin. The glaciotectonic complex underwent further shortening and the Elsterian tunnel-valley fills that were incised into the glaciotectonic complex were partly deformed during the Saalian Drenthe and Warthe (1) ice advances.

Ice-flow directions of the last Scandinavian Ice Sheet in central Poland

The article presents the results of studies on the ice-flow directions of the last Scandinavian Ice Sheet (SIS) in central Poland, at the southern periphery of the SIS, and particularly within the Płock lobe. The ice-flow pattern was derived mainly from direct indices, such as: till fabric; vergence of glaciotectonic compression structures; orientation of small-scale subglacial deformations formed beneath the flowing ice; orientation of pavement stone longitudinal axes; stone lee ends; and clast surface striae. The study is supplemented with a reconstruction of ice-movement directions based on orientation analysis of both concave (tunnel valley) and convex (eskers) glacial morpholineaments (linear glacial landforms). Till age was also determined, using luminescence (OSL) datings from sand deposits. These were collected directly at numerous sites, from under and above the last SIS till unit. Combined with previously published data, the results of the present investigation of the last ice-sheet advance during the Late Weichselian (ca 23–18 ka) indicate that there were two different ice-flow phases and that the maximum extent was asynchronous. The western part of the said area experienced two advances during the Leszno (Brandenburg) and Poznań (Frankfurt) phases, while during the earlier one the ice masses came from the NW, presumably along the eastern limb of the Odranian (B2 after Punkari 1997) ice stream, and then from the N, along the Vistulian (B3) stream. The last SIS occupied the eastern sector of the area only once, during the younger Poznań phase, building the Płock lobe. The lobe-shaped ice margin with a fan-like flow pattern within the lobe may be indicative of a palaeo-ice stream terminal zone.

Postglacial morpholineaments as an indicator of ice sheet dynamics during the Saale Glaciation in the Białystok Plateau and Sokółka Hills (NE Poland)

The palaeogeographical reconstruction of the deglaciation process in NE Poland during the Odranian Glaciation Wartanian Stadial (Saale MIS 6) is based mainly on the analysis of glacial morpholineaments, performed using the GIS programs that provide us with better understanding of the glacial landscape evolution, spatial reconstruction of glacial processes, and postglacial landscape development. Analysis of postglacial morpholineaments was carried out based on the Digital Elevation Model (DEM) as well as geological, geomorphological and topographic maps. It resulted in the delimitation of visible linear postglacial landforms, their identification and classification, and the determination of their genetic and chronological relations. Directions of the lineaments were determined through quantitative analysis. Qualitative analysis was applied to determine the spatial and temporal sequence of events, directions of ice movement and its range. Two glacial lobes, Biebrza and Neman, existed in the north-western part of the Bialystok Plateau and Sokolka Hills during the last glacial advance. The lobed nature of the ice sheet can be confirmed by field data collected from, e.g., sites in Knyszewicze, and by topographic analysis of Quaternary sediments, combined with their thickness and genesis. The obtained reconstruction of the last glacial advance and retreat in the study area can be used in further regional discussions on the character and dynamics of the last glaciation in this region.

The Lithological Characteristics of the Glacial Deposits of County Longford

A preliminary examination of the glacial geology of Co. Longford is presented. Four lithologically distinct facies of diamicton and three associated glaciofluvial members are examined in terms of their petrographic composition. The units are then interpreted with reference to direction of ice movement, nature of deglaciation and bedrock geology.

Application of till geochemical and indicator mineral data to the interpretation of the thick till sequence at Muhos, northern Finland

Recent drilling at Muhos, northern Finland has revealed an exceptionally thick Quaternary sediment cover, overlying the unmetamorphosed Neoproterozoic Muhos Siltstone Formation in the Oulujoki River Valley. Here we report preliminary petrographical, geochemical and heavy mineral results from four till units from the Muhos drill-core. The drilling site is located at 27 m asl, c. 1 km south of Oulujoki River and 30 km SE from the city of Oulu and the present shore of the Bothnian Bay of the Baltic Sea. The drilling at this site was performed with a GM 200 GTT drilling platform using 1 -m long sample tubes (HDPV, diameter 5 cm) inside the stainless steel cover tubes. The coring penetrated down to the siltstone bedrock 54 m below the ground surface and the sediment core recovery was over 75%. The sedimentary sequence from the ground surface down to the local siltstone bedrock consists of four till units interbedded with sand/silt/clay beds. The upper two till beds, Mu-1 and Mu-2, are interpreted as being deposited during the Weichselian Glaciation between 115 000–10 000 years ago, the third till bed Mu-3 during the Saalian and the lowest till bed, Mu-4, during the Saalian or pre-Saalian glaciations, i.e. till beds Mu-3 and Mu-4 prior to c. 130 000 years ago. Petrography of the coarse fraction, geochemistry of the fine fraction and heavy minerals of the sand fraction were studied from all four till units and the Muhos siltstone samples. Coarse and fine fractions were separated by dry-sieving. The fine fraction was analysed by XRF and AAS. The mineralogical composition of the heavy fraction was studied by FESEM+EDS and SEM+EDS. The lowest till bed Mu-4 can be divided into three subunits. On the basis of clast content, geochemistry of till fines the ice movement direction was from the NW or NNW during the deposition of the lowermost brown and red till units. The composition of the upper grey unit of till bed Mu-4 shows that the ice flow direction was from the WNW. Petrographical composition of till beds Mu-3 and Mu-2 indicates that the ice flow again was from the NW whereas the petrographical composition of the uppermost till bed Mu-1 suggests that the ice flow during this deposition phase was from the WNW. Younger till beds studied have in part also some reworked and re-deposited material from the older till beds. Supplementary material: The percentages of the rock types in pebble and gravel fraction, XRF and AAS analysis of the samples and identified mineral species of the heavy sand fraction are available at: http://www.geolsoc.org.uk/SUP18659

Quantitative geomorphological analysis of drumlins in the peterborough drumlin field, ontario, canada

Drumlins are enigmatic subglacial landforms that have been interpreted to form by a number of processes, including incremental accumulation of till, erosion of previously deposited sediment, catastrophic meltwater floods, and sediment deformation. However, relatively little is known about the controls on drumlin formation, such as spatially variable glacial processes or substrate characteristics, and how these controls may be identified from variations in drumlin morphology within a single drumlin field. This paper explores a computational method that allows identification of drumlins and extraction of their morphological characteristics from existing topographic digital data for a portion of the eterborough drumlin field in ntario, anada. Spatial and non‐spatial analysis of the form and distribution of drumlins across the study area identifies drumlin characteristics such as size, elongation ratio, symmetry and long axis orientation and shows that drumlins are not randomly distributed across the region and their form characteristics have distinct regional trends. Kernel density analysis is used to identify the regional trends in drumlin characteristics. Factors that appear to influence the form and distribution of drumlins in the study area include sediment thickness, length of time beneath the ice, ice velocity and direction of ice movement. The distribution of particularly well developed asymmetric and elongate drumlins coincides with the location of a broad bedrock low and is interpreted to identify the former location of a fast‐flowing ice stream.

Interlobate ice‐sheet dynamics during the Last Glacial Maximum at Whitburn Bay, County Durham, England

This research reconstructs ice‐sheet processes operating during the Late Devensian in northeast England. The article assesses the lithostratigraphy of the Devensian glacial tills of Whitburn Bay, eastern County Durham, and presents the first detailed analysis of petrological, geochemical and biostratigraphical data to reconstruct lithostratigraphy, provenance and iceflow pathways. Two Devensian tractions tills (the Blackhall and Horden tills) are separated by a boulder pavement, pointing to a switch in ice‐bed conditions and the production of a melt‐out lag prior to deposition of the upper traction till, the Horden Till. The Blackhall Till contains Magnesian Limestone, Carboniferous Limestone, Whin Sill dolerite and Old Red Sandstone, suggesting a northwesterly source, probably from the Midland Valley and the Southern Uplands. The Horden Till contains erratics and heavy minerals derived from crystalline bedrock sources in the Cheviot Hills and northeast Scotland. Within the Horden Till are numerous sand, clay and gravel‐filled canals incised downwards into the diamicton which are attributed to a low‐energy, distributed, subglacial canal drainage system. Coupled with hydro‐fractures and the boulder pavement, this suggests that a partially decoupled, fast‐flowing ice stream deposited the Horden Till. The uphill, landward direction of ice movement indicates that the ice stream was confined in the North Sea Basin, possibly by the presence of Scandinavian Ice.

Genesis of transverse kame trains in eastern Poland

Transverse kames, forming trains perpendicular to the direction of ice-sheet advance, are rare morphological elements in previously glaciated areas. The genesis of an example from the ice-contact zone of the Wartanian glaciation in eastern Poland is discussed. The transverse kames there form two main, distinctly separated, sub-parallel trains. Their sedimentary successions fill erosional troughs incised in the pre-Wartanian deposits on northern slopes. They consist of thick glaciofluvial sand and glaciofluvial/glaciolacustrine sandy/silty units that are covered with a thin, usually discontinuous, glacial till succession. The genesis of this kame type has been modelled. It is concluded that transverse kames developed in two phases: (1) erosion of the substratum in subglacial channels during initial deglaciation, and (2) glaciofluvial deposition in crevasses during advanced deglaciation (in the form of low-energy fans periodically submerged under stagnant water), followed locally by a cover of flowtills. Both the ablation of the ice and the accumulation of the kame deposits were controlled by the co-occurrence of ice zones either enriched or impoverished with sediment. Zonal enrichment of ice with debris was determined by the development of shear zones over substratum elevations that were inclined up-ice. The formation and subsequent infilling of crevasses both took place in zones of relatively clean ice, so that the resulting kames form a train perpendicular to the direction of ice movement.

Subglacial deformation of trees within overridden foreland strata, Bering Glacier, Alaska

The foreland stratigraphy overridden during recent Bering Glacier surges bears evidence of subglacial deformation. The pre-existing, fine textured substrate (till and diamicton) experienced diminished strength because of saturation, thus resulting in shallow mobilization and the formation of a new till of limited thickness. Glacial coupling with well drained sediment resulted in ploughing that generated a diamicton that retains vestiges of outwash sorting and stratification. The outwash sequence extending decimeters beneath the surface till contains four prominent sub-meter sand beds. Each sand bed holds multiple small, fossil trees still rooted in underlying layers of gravel. Virtually all trees in the upper two sand beds are deformed. Several are offset by centimeter to decimeter horizontal shears confined to thin, silt, and clay-rich zones at the base of each sand bed. Trees that escaped shearing are warped and kinked. Deformed trees are present at depths that range from 15.76 to 5.31 m. beneath potential ice/substrate interface surfaces. The most likely source of deforming stress in this foreland setting is related to glacial advance. The style and orientation of tree deformation are consistent with the direction of ice movement. Therefore, the occurrence of deformed trees is attributed to stress applied by overriding ice.

Large-scale glaciotectonic thrust structures in the eastern Danish North Sea

Abstract The distribution of shallow, detached thrust fault complexes in the eastern Danish North Sea has been mapped based on 6400 km of high-resolution multichannel seismic data. Individual thrust segments are 200–1000 m long and 100–250 m thick with up to 200 m of horizontal displacement. Thrusting mainly affected upper Middle Miocene to lower Pleistocene strata and predated an extensive system of now buried subglacial valleys. The thrust structures are located along a 200 km long NW-SE trend. The general thrust direction is towards the SW, parallel to inferred directions of ice movement during the Elsterian and Saalian glaciations. This direction is, however, also parallel to the dip of the underlying detachment surface. The detachment surface corresponds to the mid-Miocene unconformity in the SE and the base of the Quaternary to the northwest. Detachment is generally located at depths of 130–270 m below sea level. The structures are interpreted as glaciotectonic thrust structures of Elsterian and/or Saalian age. The driving mechanism is interpreted to be gravity spreading in front of glaciers advancing from the north, NE and east. Thrusting was facilitated by the presence of SW-dipping detachment surfaces.

Maximum altitude of Late Devensian glaciation on the Isle of Mull and Isle of Jura

Synopsis Evidence provided by striae, ice-moulded rock, erratics and perched boulders indicates that the last (Late Devensian) ice-sheet reached an altitude exceeding 760 m on Mull and 660 m on Jura. The highest summits on both islands support periglacial blockfields, suggesting that they remained as nunataks above the ice surface. This interpretation is supported by analyses of clay-fraction mineralogy, which shows that gibbsite (a pre-Late Devensian weathering product) is widespread in blockfield samples but rare in samples below the inferred limit of glaciation, implying removal by Late Devensian glacial erosion. Maximum ice-sheet altitudes of 760–840 m and 660–700 m are inferred for the Ben More massif on Mull and Paps of Jura respectively. Reconstruction of ice-sheet configuration in the Inner Hebrides area suggests that the 900 m ice-surface contour followed the west coast of the mainland, but the altitude evidence is insufficient to constrain the westwards extent of the ice sheet. Inferred ice-surface altitudes and directions of ice movement are incompatible with most theoretical models, and even the ‘best fit’ model of ice dimensions in this area underestimates maximum ice thickness by ≥ 60 m.

Initial ice movement directions from the East and South South East during a late Weichselian readvance in NE Germany

Abstract. Im Hinterland der Pommerschen Endmoräne in Nordostdeutschland sind zwei Kiesgruben (Kavelpaß und Wusseken) untersucht worden. Beide weisen glazifluviale Sedimente im unteren, sowie Geschiebemergel im oberen Teil der Lagerfolge auf. Sedimentologische Studien und Studien zur Eisbewegungsrichtung sind durchgeführt worden. Es ist deutlich, daß das Eis zu Beginn des Vorstoßes von Ost nach West, bzw. von Südsüdost nach Nordnordwest geflossen ist, und somit die Eisfließrichtung als abweichend von der erwarteten regionalen nordost-südwestlichen Bewegungsrichtung betrachtet werden muß. Eine Erklärung dieses Phänomens unter Zuhilfenahme des Marginaldomkonzeptes wird in dieser Arbeit vorgeschlagen.

High‐resolution reconstruction of the last ice sheet in NW Scotland

Reconstructions of the last (late Devensian) British ice sheet have hitherto been based on assumptions regarding its extent and form. Here we employ observational evidence for the maximum altitude of glacial erosion (trimlines) on mountains that protruded through the ice (palaeonunataks) to reconstruct the form of the ice sheet over ≈ 10 000 km2 of NW Scotland. Contrasts in the clay mineralogy of soils and exposure ages of rock surfaces above and below these trimlines confirm that they represent the upper limit of late Devensian glacial erosion. The reconstruction yields realistic values of basal shear stress and is consistent with independent evidence of ice movement directions. The ice sheet reached ≈ 950 m altitude over the present N–S watershed, descended northwards and north‐westwards, was deflected around an ice dome on Skye and an independent Outer Hebrides ice cap, and probably extended across the adjacent shelf on a bed of deforming sediments.

Palaeonunataks and the altitude of the last ice sheet in the SW Lake District, England

The mountains of the SW Lake District support a high-level weathering limit that separates an upper zone of shattered bedrock, blockfields and tors from a lower zone of glacially-moulded bedrock. This weathering limit declines from c. 870 m in the east of the area to 800—830 m in the NW. Analyses of the depths of stress-release joints and clay fraction mineralogy indicate more advanced weathering of bedrock and soils above the weathering limit, and thus imply that it marks the upper limit of Late Devensian glacial erosion. The weathering limit is therefore interpreted as a periglacial trimline cut around palaeonunataks, and thus as representing the approximate altitude of the surface of the last ice sheet at its maximum thickness. High-level striae and roches moutonnees confirm that lower peaks and cols in the area were over-run by ice moving westwards or southwestwards. consistent with radial movement of ice away from an independent ice dome located over the central Lake District. The inferred ice altitude and direction of ice movement are reasonably consistent with ice-sheet models proposed by Lambeck (1993, Geophysics Journal International , 15 , 960–990; 1995, Journal of the Geological Society, London , 152 , 437–448), but not with earlier models.

Glacial dynamics during the deglaciation of the Bohnen area, SW Sweden

An area west of Lake Bolmen in southwestern Sweden was investigated by geomorphological, lithostratigraphical, and sedimento‐logical methods. Ice movement directions inferred from striae, end moraines, and drumlins indicate that the initial deglaciation of the western part of the area was towards the NE. This was probably followed by a readvance from the N and a final deglaciation towards the N. In the eastern part of the Bohnen area the deglaciation was continuous and the ice movement gradually changed from a direction from the NE to a direction from the N. Sedimentological investigations were made in two moraine ridges. These and other ridges are connected, together named the Unnaryd moraine and are interpreted to consist of ice slope colluvium and debris flows deposited proglacially. The Unnaryd moraine is the eastward continuation of a previously known end moraine called the Torup moraine. The combined Torup‐Unnaryd moraine marks the southern limit of the readvance from the N. It was also found that the Unnaryd‐Torup moraine postdates a geomorphological border‐line across southern Sweden with which it was previously correlated. Andersson, G., 1997: Glacial dynamics during the deglaciation of the Bolmen area, SW Sweden. GFF, Vol. 119 (Pt. 3, September), pp. 207–212. Stockholm. ISSN 1103–5897.