Glacial Processes Research Articles

A comparison of the lichenometric and Schmidt hammer dating techniques based on data from the proglacial areas of some Icelandic glaciers

Measurements of Rhizocarpon section and Schmidt hammer R-values are reported from the proglacial geomorphic features on the forelands of the Icelandic glaciers of Kvı́árjökull, Hólárjökull and Heinabergsjökull (Öræfi and south Vatnajökull), Sandfellsjökull and Öldufellsjökull (east Mýrdalsjökull), and Brúárjökull, Eyjabakkajökull and west Snæfell (north Vatnajökull). These data are used in reconstructions of patterns of glacier recession since the Little Ice Age maximum, and the geomorphic signals of climatic versus non-climatic events are discussed. Age control was obtained from various dated substrates by utilizing historical accounts, aerial photographs and grave stones. Three lichen growth rates are calculated: (a) 0.51 mm a -1 (corrected to 0.50 mm a -1) with a colonization lag time of <16 yr for the arid forelands of north Vatnajökull; (b) 0.56 mm a -1 with a colonization lag time of 5 yr for the Icelandic southeast coast; and (c) 0.80 mm a -1 with a colonization lag time of 6.5 yr for the south Vatnajökull and east Mýrdalsjökull forelands. These compare favourably with a previously published growth rate of 0.44 mm a -1 for the arid north of Iceland. This regional coverage of data allows a comparison between annual precipitation totals and lichen growth rates and the construction of a growth rate prediction curve for Iceland. The success of the Schmidt hammer in differentiating moraines based upon age varied according to the geomorphological setting. Reasonable R-value/lichen size correlations were obtained on the east Mýrdalsjökull and Heinabergsjökull forelands where unrestricted glacier advance into lowlands allows for a higher degree of debris surface freshening by direct glacial processes. Weak correlations were obtained at Kvı́árjökull, where the glacier was restricted by a precursor latero-frontal moraine loop and therefore the debris comprising the Little Ice Age recessional moraines was diluted with material of various ages being reworked by mass movement from the precursor moraine loop. Similar problems arise in areas affected by surging glaciers, such as Brúárjökull and Eyjabakkajökull. It appears that an accurate R-value age-prediction curve can not be constructed for a timescale of <100 yr in Iceland.

Cenozoic erosion and the preglacial uplift of the Svalbard–Barents Sea region

The creation of the huge fans observed in the western Barents Sea margin can only be explained by assuming extremely high glacial erosion rates in the Barents Sea area. Glacial processes capable of producing such high erosion rates have been proposed, but require the largest part of the preglacial Barents Sea to be subaerial. To investigate the validity of these proposals we have attempted to reconstruct the western preglacial Barents Sea. Our approach was to combine erosion maps based on prepublished data into a single mean valued erosion map covering the whole western Barents Sea and consequently use it together with a simple Airy isostatic model to obtain a first rough estimate of the preglacial topography and bathymetry of the western Barents Sea margin. The mean valued erosion map presented herein is in good volumetric agreement with the sediments deposited in the western Barents Sea margin areas, and as a direct consequence of the averaging procedures employed in its construction we can safely assume that it is the most reliable erosion map based on the available information. By comparing the preglacial sequences with the glacial sequences in the fans we have concluded that 1/2 to 2/3 of the total Cenozoic erosion was glacial in origin and therefore a rough reconstruction of the preglacial relief of the western Barents Sea could be obtained. The results show a subaerial preglacial Barents Sea. Thus, during interglacials and interstadials the area may have been partly glaciated and intensively eroded up to 1 mm/y, while during relatively brief periods of peak glaciation with grounded ice extending to the shelf edge, sediments have been evacuated and deposited at the margins at high rates. The interplay between erosion and uplift represents a typical chicken and egg problem; initial uplift is followed by intensive glacial erosion, compensated by isostatic uplift, which in turn leads to the maintenance of an elevated, and glaciated, terrain. The information we have on the initial tectonic uplift suggests that the most likely mechanism to cause an uplift of the dimensions and magnitude of the one observed in the Barents Sea is a thermal mechanism.

Source and grain-size influences upon the clay mineral distribution in the Skagerrak and northern Kattegat

AbstractBottom sediments from 78 sites in the Skagerrak-Kattegat area have been used to determine the semi-quantitative distribution of clay minerals, and to interpret sediment provenance and the transport pathways. The sediment reflects incorporation of four sources, affected both by earlier glacial processes and on-going marine mixing. The southern North Sea, characterized by dioctaheadral illite, kaolinite and smectite, is the dominating source. The central North Sea provides a limited, but significant, source for chlorite. The sediment from the Swedish west coast has an unweathered character, containing trioctaheadral illite, vermiculite, illite-vermiculite mixed-layer minerals and chlorite. The contribution from the southern Kattegat (southern Sweden and Denmark) is dominated by kaolinite and smectite. Correlation between clay content and the contents of smectite, kaolinite and chlorite in the Skagerrak indicate that the clay mineral distribution is influenced by sorting. This grain-size dependency must be considered when interpreting sources.

Detrital transport of metals by glaciers, an example from the Pinchi Mine, central British Columbia

Abundant cinnabar (HgS) mineralization is associated with the Pinchi Fault in central British Columbia. Two formerly producing mercury mines have been developed on this fault: Pinchi and Bralorne Takla. The mercury content of till (a sediment type directly deposited by glaciers) in the area of this fault is primarily controlled by the occurrence of cinnabar mineralization in bedrock and the direction of ice flow. Cinnabar-bearing bedrock was eroded by glaciers, transported in the direction of ice flow, and deposited "down-ice" from its source. An example of such a dispersal train is documented for the Pinchi Mine area where mercury ore was transported over a distance of 12 km, as measured in the clay-sized fraction ( 3.3) of this same sediment. Antimony, chromium, and nickel dispersal trains were also detected in the region. These data indicate that natural glacial processes can result in the "mobilization" of metals in the surficial environment, a factor which has to be considered at mine sites in glaciated terrain, where mine reclamation and remediation measures are now required.

Supervised classification of types of glaciated landscapes using digital elevation data

Automated approaches for identifying different types of glaciated landscapes using digitally processed elevation data were evaluated. We tested the ability of geomorphic measures (e.g. elevation, relative relief, roughness, and slope gradient) derived from digital elevation models (DEMs) to differentiate glaciated landscapes using maximum likelihood classification and artificial neural networks (ANN). The automated methods were trained and validated using an existing Quaternary geology map and a manual interpretation of the contour data portrayed on topographic quadrangles. The need for such methods arises from efforts to classify types of landscapes (e.g. ecoregions) in Michigan. One fundamental control of the landscape structure in Michigan, including soil type and vegetation, is the underlying sedimentary and landform assemblages produced by an array of glacial processes during the waning phase of the Pleistocene. Traditional methods for identifying different landscapes (e.g. ice-contact landscapes, stagnation landscapes) have relied on printed topographic maps and have been very effective, but time consuming. The maps resulting from the four supervised classification trials had between 51% and 61% agreement with the original Quaternary geology map. The output from the maximum likelihood classification had slightly higher agreements than the output from the neural net, which is attributed to the generalization inherent in the Quaternary geology map compared with the nature of the classifier for the neural net. The neural net, however, identifies significant detail and non-linear relationships between classification inputs and output classes. Future work should incorporate a map of soils into the classification.

Sediment dating by luminescence: a review

In the present article we review applications of luminescence methods for the determination of the age of sediments for Quaternary Earth science. “Sediment” is taken to include any mineral particles transported by wind, water and/or ice and subsequently deposited. Methodology is not discussed per se but appears in context. The emphasis is on recent work and is illustrative rather than exhaustive. Applications in archaeology are mentioned only briefly. Successful dating requires that the luminescence signal has been zeroed by sunlight at the time the sediment was laid down. Dune sands and loess transported in full daylight usually satisfy this condition and have been dated by both traditional thermoluminescence and by optical methods. Optical dating is advantageous for less well-bleached sediments, such as are found in colder climates and those laid down by fluvial and glacial processes, although the prospects for dating the latter are shown to be poor. Among less common contexts are volcanic eruptions, earthquake faults and tsunami deposits. The dating of very young sediments, less than 2 ka, is demonstrated and the factors limiting the dating of very old sediments, 500 ka to 1 Ma, are discussed. Validation of any dating method by comparison with other methods is necessary; luminescence ages have been successfully compared with ages obtained by, for example, 1 4C, U–Th series and δ 1 8O isotope ratios. The most difficult problems encountered in dating quartz, feldspars and undifferentiated fine grain mixtures include: anomalous fading (which leads to underestimates of age), incomplete zeroing (which gives overestimates) and sample inhomogeneity. Methods devised to minimise these effects include preheating regimes, selection of observing wavelengths and of stimulating wavelengths for optical dating, the use of single aliquot and single grain measurements. The use of appropriate data analysis procedures is also important. Advice on current best practice on procedures for obtaining reliable ages is offered and, in all applications, there are suggestions as to where further research might be directed.

Paleoseismicity in France: Fault trench studies in a region of moderate seismicity

France is a country of moderate to low seismicity with a ten-century record of historical seismicity. Paleoseismicity appears to be the only available tool which is able to extend the record of seismic activity beyond this short time-window. However, moderate to low seismicity should be associated with low slip rates and, thus, geomorphic evidence for detecting active fault traces should be weak, impeding the development of paleoseismic fault studies. To test the paleoseismic method in France, we have developed a programme on paleoseismology during the past few years. First, we performed a critical reappraisal of existing data on French active tectonics, then we selected several suitable sites, and finally we trenched on some sites. Much of the revised evidence from recent and active tectonics was not selected because their origins were either dubious or non-tectonic. In particular, many reported sites from the Alps, seen in moraines, actually correspond to glacial processes (‘glacitectonics’). In addition, some clearly observed scarps could result from gravitational processes such as post-glacial rebound. Among the selected sites, three were studied in detail; they correspond to the Nimes, Moyenne Durance, and Argentera fault zones. The most reliable information was provided by trenching observations on the Moyenne Durance Fault. The trench site exhibited a N35 °E-striking knee fold, parallel to the Moyenne Durance Fault, covered unconformably by colluvial deposits. Radiocarbon dates indicate that folding occurred during the latest Pleistocene or earliest Holocene. The observed stratigraphical relations strongly suggest that folding resulted from a single coseismic slip event associated with a strong earthquake of magnitude Mw = 6.4–6.9. Our data also indicate long recurrence intervals for such earthquakes, at least of the order of 25 ka. Paleoseismic data collected on the other studied fault zones confirm that nearly Mw = 7 events may be generated by French active faults and that the recurrence intervals should be over 10 ka. Thus, the paleoseismic study of active faults is a robust tool to reconstruct seismic history where slip rates are low to moderate; however, specific exploration methods should be developed to localize fault traces more accurately and study hidden faults.

The sedimentology of a Late Pleistocene drumlin near Kingscourt, Ireland

An exposure in a Late Pleistocene drumlin near Kingscourt, Ireland, provides a good insight into some of the processes that give rise to such subglacial bedforms. The drumlin is located on a ridge, cored by red sandstone of Carboniferous (Namurian) age, which rises to 150 m above m.s.l. The drumlin itself is about 380 m long by 170 m wide and is 20 m in height. It is orientated VVNW-ESE. Ice flow direction in the area, as inferred from general drumlin orientation, striae, and erratic dispersal, was NW-SE. The drumlin is composed of diamicton (with four constituent facies), containing large-scale (up to 20 m long and 1.5 m high) slabs of the sandstone bedrock, which have been displaced tens of metres by ice dragging. The thin diamicton matrix is sand-rich with few clasts greater than pebble size. It contains green sandstone erratics from west and northwest of the study site along with clasts of weathered Namurian sandstone which have been sheared from local bedrock. The diamicton attains a maximum thickness of 5.4 m. The slabs are confined to the basal 2.5 m of the diamicton. Thus the drumlin is essentially rock cored. Parts of the matrix: are interpreted as injection sediments that have been squeezed into fractures and voids in the bedrock, and between rock slabs, under high porewater pressures, when the ice began to displace fractured substrate. Deformation structures, at a variety of scales are seen in the matrix. Deformation, erosion and deposition were all important in the formation of the drumlin. Much of the upper part of the diamicton has been sheared, subsequent to initial deposition, by ice action. The sheared units are uppermost in the stratigraphic sequence. The shearing is the last glacial process apparent in the drumlin sediment and may have been contemporaneous with drumlinisation. Both the squeezing process, which deposited the injections within the matrix, and the deflection in flow of ice were influenced by the obstructing bedrock ridge which, in this case, is also responsible for the anomalous orientation of the feature.

Alluvial Heavy Minerals as Indicators of Late Pleistocene Ice Flow in the Irish Midlands

The alluvial heavy minerals of Slieve Bloom consist of an allochthonous group, glacially imported from the Carboniferous limestone piedmont, and an autochthonous group derived from the Lower Palaeozoic rocks of Slieve Bloom. The spatial distribution of each group is distinct from the other along the river valleys; the allochthonous minerals are found only where till is being fluvialiy eroded; the autochthonous dominate in valley reaches devoid of till. Given this spatial arrangement, it is proposed that the distribution of allochthonous minerals now hosted by alluvial sediments reflects the pattern of glacial depositional processes within the mountains, in particular a directionally specific distance decay in the carryover of minerals from the limestone piedmont into Slieve Bloom.

Volcanic impact on the southern Basin of Mexico during the Holocene

The interstratification of volcanic and glacial deposits in Central Mexico has been helpful in correlating the time sequence of these events. Two glacial advances and many eruptions have occurred during the last 10,000 years, although conditions have changed drastically since the last eruptions in the 1920s (Popocatepetl). The origin and evolution of several lakes (Basin of Mexico) are related to vulcanism and periods of warmer and colder climatic conditions. Fifty monogenetic volcanoes have erupted near Mexico City during the Holocene, probably during 20–22 different eruptive periods since some are superimposed vents similar to the Jorullo vents which erupted 200 years ago in Michoacan, Mexico. The youngest of these volcanoes are Chichinautzin and Xitle (2030±60 years BP) whose lavas covered approximately 80 km each. The Xitle eruption began with ash emission mainly towards the NW and was later dominated by lava flow fed by tubes. This lava covered the Cuicuilco prehispanic urban center and the agricultural areas near it. The lava dammed and caused silting of the Magdalena, Xicotitla and Chicuautitla Rivers and deflected the drainage pattern in general. The eruption of Chichinautzin and the other Holocene volcanoes have drastically changed the topography and caused fires as can be seen in the carbonized branches at the base of the flows. Chichinautzin Volcano was formed by successive thin lavas which flowed to the south and by thicker flows to the east, west and north. Older Holocene volcanoes are responsible for the formation of the Zempoala and Xochimico Lakes. Popocatepetl is a 5450-m high stratovolcano to the east of Mexico City which has been subject to glacial processes. Its eruptions have been documented since the 14th century, the last being in the 1920s. Most of these eruptions caused ash fall mainly to the east and northeast but lava emissions and flooding have also been reported. Larger eruptions in the earlier history produced pyroclastic flows and surges, lahars, Plinian pumice falls and two debris avalanches. A preliminary map of the type of volcanism which has affected the different areas and its implications for future hazards is presented.

Interpretation of lithofacies of the Ashtabula Till along the south shore of Lake Erie, northeastern Ohio

The final advance of the Erie lobe into Ohio during the Port Bruce Stade of the Late Wisconsinan deposited the Ashtabula Till. Wave erosion and mass wasting along the south shore of Lake Erie show that the Ashtabula Till consists of laterally traceable lithofacies, which are used to determine the depositional history of the Ashtabula Till. At each section, lithofacies sequences are divided into two sub-sequences, each consisting of massive, matrix-supported diamicton (Dmm) overlain by stratified, matrix-supported diamicton (Dms). Some Dmm are sheared (Dmm(s)) and are interpreted as lodgement till, whereas other Dmm and Dms were deposited as melt-out till. Some sections contain lenses of fines (Fm and Fl), current-reworked diamictons (Dmm(c) and Dms(c)), and resedimented diamictons (Dmm(r) and Dms(r)). The two sub-sequences represent two advances of Ashtabula ice that deposited the Euclid and Painesville moraines about a kilometre apart. During and after recession of the Ashtabula ice, waves and currents in Lake Maumee and its successors reworked outwash and diamictons to form the lake plain. The texture of Dmm(s) is significantly different from that of most other diamictons, and Dmm has the smallest carbonate content of all diamictons. Analysis of the variations in texture and composition among lithofacies provides additional evidence of the effectiveness of lithofacies logging in interpretation of glacial processes.

Glacial processes, sediments, landforms, and stratigraphy in the terminus region of Myrdalsjökull, Iceland: two interdisciplinary case studies

Journal of Quaternary ScienceVolume 11, Issue 5 p. 435-436 Book Review Glacial processes, sediments, landforms, and stratigraphy in the terminus region of Myrdalsjökull, Iceland: two interdisciplinary case studies DANNY McCARROLL, Corresponding Author DANNY McCARROLL Department of Geography, University of Wales, Swansea SA2 0RA, WalesDepartment of Geography, University of Wales, Swansea SA2 0RA, WalesSearch for more papers by this author DANNY McCARROLL, Corresponding Author DANNY McCARROLL Department of Geography, University of Wales, Swansea SA2 0RA, WalesDepartment of Geography, University of Wales, Swansea SA2 0RA, WalesSearch for more papers by this author First published: September/October 1996 https://doi.org/10.1002/(SICI)1099-1417(199609/10)11:5<435::AID-JQS272>3.0.CO;2-RAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume11, Issue5September/October 1996Pages 435-436 RelatedInformation

Glacial processes, sediments, landforms, and stratigraphy in the terminus region of Myrdalsjökull, Iceland: two interdisciplinary case studies

Glacial processes, sediments, landforms, and stratigraphy in the terminus region of Myrdalsjökull, Iceland: two interdisciplinary case studies

Pleistocene permafrost of West Siberia as a deformable glacier bed

Subglacial permafrost is usually referred to as a factor impeding basal glacial processes, and the deformation of soft substrata in particular. In West Siberian, widespread glacial disturbances present in permafrost of Pleistocene age suggest that frozen sediments, if clayey and/or icy, can readily deform, thus translating basal glacial stress into sliding of the entire glacier/sediment complex along subglacial shear zones. Ductile deformations such as folds and diapirs are also widespread. The mode of deformation of frozen subglacial sediments is dependent on their lithology, ice content and temperature conditions. Signatures of former subglacial permafrost in currently thawed sediments are deduced from contrasting deformation behaviours of lithologically different sedimentary formations.

Late Cenozoic evolution of the western Barents Sea-Svalbard continental margin

Seven regionally correlatable reflectors, named R7 (oldest) to R1, have been identified in the Upper Cenozoic sedimentary succession along the western continental margin of Svalbard and the Barents Sea. Regional seismic profiles have been used to correlate between submarine fans that comprise major depocentres in this region. Glacial sediment thicknesses reach up to 3 seconds two-way time, corresponding to 3.5–4 km. Despite limited chronostratigraphic control, ages have been assigned to the major sequence boundaries based on ties both to exploration wells and to shallow boreholes, and by paleoenvironmental interpretations and correlations with other regions. Lateral and vertical variations in seismic facies, between stratified and chaotic with slump structures, have major implications for the interpretation of the depositional regime along the margin. The main phases of erosion and deposition at different segments of the margin are discussed in the paper, which also provides a regional seismic stratigraphic framework for two complementary papers in the present volume. Reflector R7 marks the onset of extensive continental shelf glaciations, but whereas the outer Svalbard shelf has been heavily and frequently glaciated since R7 time, this did not occur, or occurred to a much less extent, until R5 time in the southern Barents Sea. The present study provides the background for a quantification of the late Cenozoic glacial erosion of Svalbard and the Barents Sea. The rates of erosion and deposition exhibit large temporal and spatial variations reflecting the importance of glacial processes in the Late Cenozoic development of this nearly 1000 km long margin.

Glaciation, erosion and the evolution of the Transantarctic Mountains, Antarctica

Modelling studies of the tectonic evolution of the Transantarctic Mountains in Antarctica have drawn differing conclusions as to the primary mechanisms involved. None has considered the role of the East Antarctic ice sheet in detail. We use a denudation—flexural model to examine the isostatic response of the continental margin to glacial erosion to determine whether glacial processes have played a role in forcing mountain uplift.The conclusion is that, although there are insufficient data formally to delimit the role of glacial erosion, available geophysical and geomorphological data are not inconsistent with the results of the differential denudation model, providing certain conditions are met. These results indicate that the current topography of the Transantarctic Mountains can be simulated, in part, from the isostatic response of the lithosphere to glacial erosion. The short wavelength and high amplitude of the Transantarctic Mountains do not require a low flexural rigidity in the unrated lithosphere, provided there is a fast escarpment retreat from the rift hinge, high escarpment denudation rates and a large differential in denudation between the coastal zone and the interior.

Glaciation, erosion and the evolution of the Transantarctic Mountains, Antarctica

Modelling studies of the tectonic evolution of the Transantarctic Mountains in Antarctica have drawn differing conclusions as to the primary mechanisms involved. None has considered the role of the East Antarctic ice sheet in detail. We use a denudation—flexural model to examine the isostatic response of the continental margin to glacial erosion to determine whether glacial processes have played a role in forcing mountain uplift. The conclusion is that, although there are insufficient data formally to delimit the role of glacial erosion, available geophysical and geomorphological data are not inconsistent with the results of the differential denudation model, providing certain conditions are met. These results indicate that the current topography of the Transantarctic Mountains can be simulated, in part, from the isostatic response of the lithosphere to glacial erosion. The short wavelength and high amplitude of the Transantarctic Mountains do not require a low flexural rigidity in the unrated lithosphere, provided there is a fast escarpment retreat from the rift hinge, high escarpment denudation rates and a large differential in denudation between the coastal zone and the interior.

A mini-symposium on glacial and periglacial processes in permafrost environments held at the Department of Geography, University of Oslo, on 4 and 5 February 1994

The Department of Geography at the University of Oslo is administrated by the Faculty of Mathematics and Natural Sciences. Geography is organized as part of the Geo-sciences, together with geophysics and geology. Physical geography offers four different main scientific topics: geomorphology, geometries (computing geographical information, GIS), hydrology, and geographical resources and landscape analyses. The department has many students in all of these groups.

The influence of englacial drainage on sediment-transport pathways and till texture of temperate valley glaciers

Field studies at Mueller and Tasman Glaciers in New Zealand and in Iceland have revealed extensive ice-marginal moraine sequences fed by a series of debris bands containing characteristic rounded debris. We interpret these debris bands as meltwater deposits from relict conduits. The process of sediment accumulation described requires that debris undergoing basal or high-level ice transport is entrained by running water, becoming comminuted and rounded before being returned to high-level ice transport and eventual deposition in marginal moraines. Tins sequence of events suggests a link between glacier hydrology and ice-marginal sedimentation, possible mechanisms for which are explored. Pronounced moraines with rounded debris can be expected if (a) large quantity of debris is carried within the englacial drainage network; (b) there is a tendency for this debris to be abandoned within the ice; and (c) ice-flow trajectories and ablation ensure that this debris congregates at the ice surface. Streams which flow at high level within the ice can intercept and entrain englacial debris derived from rock fall. However, debris sources at Gigjökull and Steinholtsjökull in Iceland are restricted to the glacier bed, suggesting that, in certain cases, the presence of a basal overdeepening acts as a key factor controlling the accumulation of these moraines. Water pressure rises as channels encounter an overdeepening, forcing debris-laden streams to leave the bed and take an englacial route. That debris may pass from ice to water and back into ice before deposition has gone largely unrecognised in accounts of glacial process systems, yet it provides an explanation of how temperate alpine glaciers can include water-worked debris in their marginal depositional facies.

The influence of englacial drainage on sediment-transport pathways and till texture of temperate valley glaciers

Field studies at Mueller and Tasman Glaciers in New Zealand and in Iceland have revealed extensive ice-marginal moraine sequences fed by a series of debris bands containing characteristic rounded debris. We interpret these debris bands as meltwater deposits from relict conduits. The process of sediment accumulation described requires that debris undergoing basal or high-level ice transport is entrained by running water, becoming comminuted and rounded before being returned to high-level ice transport and eventual deposition in marginal moraines. Tins sequence of events suggests a link between glacier hydrology and ice-marginal sedimentation, possible mechanisms for which are explored. Pronounced moraines with rounded debris can be expected if (a) large quantity of debris is carried within the englacial drainage network; (b) there is a tendency for this debris to be abandoned within the ice; and (c) ice-flow trajectories and ablation ensure that this debris congregates at the ice surface. Streams which flow at high level within the ice can intercept and entrain englacial debris derived from rock fall. However, debris sources at Gigjökull and Steinholtsjökull in Iceland are restricted to the glacier bed, suggesting that, in certain cases, the presence of a basal overdeepening acts as a key factor controlling the accumulation of these moraines. Water pressure rises as channels encounter an overdeepening, forcing debris-laden streams to leave the bed and take an englacial route. That debris may pass from ice to water and back into ice before deposition has gone largely unrecognised in accounts of glacial process systems, yet it provides an explanation of how temperate alpine glaciers can include water-worked debris in their marginal depositional facies.