Large Ice Streams Research Articles

Assemblages of submarine landforms produced by tidewater glaciers in Svalbard

High‐resolution swath bathymetry from the marine margins of several Svalbard tidewater glaciers shows an assemblage of submarine landforms that is probably linked to glacier surging. These landforms are essentially unmodified since their initial deposition over the past hundred years or so because they have not been subjected to subaerial erosion or periglacial activity. Swath images comprise an assemblage of superimposed landforms, allowing reconstruction of relative age of deposition: (1) large transverse ridges, interpreted as recessional moraines overridden by a subsequent ice advance; (2) a series of curvilinear streamlined bedforms orientated parallel to former ice flow, interpreted as lineations formed subglacially during rapid advance; (3) large terminal ridges, marking the farthest extent of ice at the last advance, with flow lobes immediately beyond interpreted as submarine debris flows; (4) a series of interconnected rhombohedral ridges, interpreted as a product of soft sediment squeezing into crevasses formed at the glacier bed, probably formed during immediate post‐surge stagnation; and (5) a series of fairly evenly spaced small transverse ridges, interpreted as push moraines produced annually at tidewater glacier termini during retreat. A simple descriptive landsystem model for tidewater glaciers of probable surge type is derived from these observations. We also show that megascale glacial lineations can form not only beneath large ice streams, but are also produced beneath surging tidewater glaciers lying on deforming sedimentary beds.

A modelling reconstruction of the last glacial maximum ice sheet and its deglaciation in the vicinity of the northern patagonian icefield, south america

ABSTRACT. A time‐dependent model is used to investigate the interaction between climate, extent and fluctuations of Patagonian ice sheet between 45° and 48°S during the last glacial maximum (LGM) and its subsequent deglaciation. The model is applied at 2 km resolution and enables ice thickness, lithospheric response and ice deformation and sliding to interact freely and is perturbed from present day by relative changes in sea level and equilibrium line altitude (ELA). Experiments implemented to identify an LGM configuration compatible with the available empirical record, indicate that a stepped ELA lowering of 750 to 950 m is required over 15000 years to bracket the Fenix I‐V suite of moraines at Lago Buenos Aires. However, 900 m of ELA lowering yields an ice sheet which best matches the Fenix V moraine (c. 23000 a BP) and Caldenius’ reconstructed LGM limit for the entire modelled area. This optimum LGM experiment yields a highly dynamic, low aspect ice sheet, with a mean ice thickness of c. 1130 m drained by numerous large ice streams to the western, seaward margin and two large, fast‐flowing outlet lobes to the east. Forcing this scenario into deglaciation using a re‐scaled Vostok ice core record results in an ice sheet that slowly shrinks by 25% to c. 14500 a bp, after which it experiences a rapid collapse, loosing some 85% of its volume in c. 800 years. Its margins stabilize during the Antarctic Cold Reversal after which it shrinks to near present‐day limits by 11 000 a bp.

Streaming flow in an ice sheet through a glacial cycle

AbstractGeological evidence indicates that the flow of the last European ice sheet was dominated by numerous large ice streams. Although some were ephemeral, most were sustained along well-defined axes at least during the period of retreat after the Last Glacial Maximum. A thermomechanically coupled three-dimensional numerical ice-sheet model has been used to simulate the ice sheet through the whole of the last glacial cycle, but with a spatial resolution that is high enough to capture streaming behaviour. An experiment with a smoothed bed is used to explore the self-organizing behaviour of streams when they are not forced by bed topography. On such a bed, streams typically have a width of 1–10 km, much narrower than the inferred European ice streams. An experiment using a realistic topography suggests that widths of ice streams are strongly influenced by topography, and tend to be of order 100 km. Moreover, even where the topography is muted, it stabilizes the locations of ice streams which, once formed, tend to be sustained along pre-existing axes. The model creates patterns of streaming that are similar to inferred patterns, suggesting strong topographic forcing. In a simulation using a realistic bed in which the ice was very cold and basal melting rarely occurred, streams were again very narrow. Widespread streaming under low driving stresses tends to reduce ice-sheet thicknesses compared with weak streaming or models that do not produce streaming. Consequently, ice thicknesses are smaller and tend to be consistent with the results of sea-level inversions based on geophysical Earth models.

High geothermal heat flow, Basal melt, and the origin of rapid ice flow in central Greenland.

Age-depth relations from internal layering reveal a large region of rapid basal melting in Greenland. Melt is localized at the onset of rapid ice flow in the large ice stream that drains north off the summit dome and other areas in the northeast quadrant of the ice sheet. Locally, high melt rates indicate geothermal fluxes 15 to 30 times continental background. The southern limit of melt coincides with magnetic anomalies and topography that suggest a volcanic origin.

Ice‐stream‐related patterns of ice flow in the interior of northeast Greenland

Ice flow in the interior of the NE quadrant of the Greenland ice sheet is focused on the large ice stream draining the north side of the summit dome. The rapid ice flow in the stream is apparent in the surface features in the stream and at the margins, in the broad scale topography that drives ice flow, and in satellite‐derived ice motion information. The patterns of ice flow in the upper half of the stream are remarkable for their level of organization, simple geometry, and effects on local surface topography. The stream begins less than 100 km from the ice divide as a current 15 km wide and then broadens symmetrically downstream by the addition of ice from the sides to a width of more than 60 km. Elevation data and visible‐band imagery show that the stream has marginal troughs tens of meters deep in its upper reach which are coincident with regions of high shear strain rate. The topography of the margins and undulating surface of the stream is generated by the ice flow; the surface undulations in the stream are fixed in location and shape over the 5 year period from 1994 to 1999. The enhanced flow presents a challenge for researchers trying to understand the history of ice discharge from a significant area in the interior of the ice sheet.

Evidence for rapid retreat and mass loss of Thwaites Glacier, West Antarctica

AbstractThwaites Glacier, the second largest ice stream in West Antarctica, drains an area of 166 500 ± 2000 km2 which accumulates 55 ± 5 Gt a−1 (or 60 ± 6 km3 ice a−1) into the Amundsen Sea, unrestrained by an ice shelf. Using interferometric synthetic-aperture radar (InSAR) data collected by the European Remote-sensing Satellites (ERS-1 and -2) in 1996, an output flux of 71 ±7 Gt a−1 (or 77 ± 8 km3 ice a−1) is estimated at the grounding line, where ice thickness is deduced from hydrostatic equilibrium. A similar flux, 70 ± 7 Gt a−1 (or 76 ± 8 km3 ice a−1), is obtained at a gate located 20 km upstream, where ice thickness was measured in 1978 by ice-sounding radar. Total accumulation in between the two gates is 1.6 Gt a−1, or 1.8 km3 ice a−1. Ice discharge therefore exceeds mass accumulation by 30 ± 15%, and Thwaites Glacier must be thinning and retreating at present. The InSAR data show that the glacier floating ice tongue exerts no back pressure on the inland ice, calves into tabular icebergs along a significant fraction of its grounding line, and has a grounding-line thickness which exceeds a prior-calculated limit for stability. Glacier thinning is confirmed at the coast by the detection of a 1.4 ± 0.2 km retreat of its grounding line between 1992 and 1996 with InSAR, which implies 3.2 ± 0.6 m ice a−1 thinning at the glacier center and less near the sides. These results complement the decimeter-scale annual surface lowering observed with satellite radar altimetry several hundred km inland of the grounding line. The magnitude of ice thinning estimated at the coast, however, rules out temporal changes in accumulation as the explanation for surface lowering. Ice thinning must be due to changes in ice flow.

Ice flow in the northeast Greenland ice stream

AbstractWe have measured ice flow and detailed topography in northeastern Greenland using satellite-based synthetic-aperture radar (SAR) interferometry. The subject of this study is the large ice stream that drains this quadrant of the ice sheet. A single SAR interferogram allows the measurement of one component of motion over a several-day-long interval. We have used a set of such measurements from multiple look directions to produce a mosaic of ice-flow velocity. The resulting flow field is tied to an estimated balance-velocity distribution in slow-moving areas and assumes flow to be locally surface-parallel. The velocity field is the most detailed, consistent data-set available over a flow feature of this size. It compares with global positioning system surveyed velocity measurements at the 5 m a–1 level In the process of mapping ice-flow velocity an enhanced elevation model of the ice stream was produced. The elevation model is based on a blend of interferometrically measured short-wavelength topography and radar-altimetry-determined longer-wavelength topography. This enhanced model has improved information on local surface slope, which is useful for estimating the horizontal components of the velocity field.

A comparison of balance velocities, measured velocities and thermomechanically modelled velocities for the Greenland ice sheet

AbstractBalance velocities for the Greenland ice sheet have been calculated from a new digital-elevation model, accumulation-rates compilation and an existing ice-thickness grid, using a two-dimensional finite-difference scheme. The pattern of velocities over the ice sheet is presented and compared with velocities derived from synthetic-aperture-radar interferometry for part of northern Greenland and a limited number of global positioning system data. This comparison indicated that the balance-velocity scheme and boundary conditions used here provide a remarkably good representation of the dynamics of the ice sheet inland from the margins. It is suggested, therefore, that these balance-velocity data could provide a valuable method of constraining a numerical ice-sheet model. The balance velocities were compared with the diagnostic velocity field calculated from several different configurations of a numerical ice-sheet model. The general pattern of flow agrees well. The detail, however, is quite different. For example, the large (>300km) ice stream in the northeast is not generated by the numerical model and much of the detailed flow pattern is completely lost due to the limited model resolution and limitations in the model physics.

Geostatistical methods for mapping Antarctic ice surfaces at continental and regional scales

AbstractThe Antarctic ice sheet plays a major role in the global system and the large ice streams discharging into the circumpolar sea represent its gateways to the world’s oceans. Satellite radar-altimeter data provide an opportunity for mapping surface elevation at kilometer resolution with meter accuracy. Geostatistical methods have been developed to accomplish this. We distinguish two goals in mapping the Antarctic ice surface: (a) construction of a continent-wide atlas of maps and digital terrain models, and (b) calculation of maps and models suitable for the study of individual glaciers, ice streams and ice shelves. The atlases consist of accurate maps of ice-surface elevation compiled from Seasat, Geosat and ERS-1 altimeter data, covering all of Antarctica surveyed by Geosat (to 72.1° S) and by ERS-1 (to 81.5° S). With a 3 km grid they are the highest-resolution maps available today with continent-wide coverage. The resolution permits geophysical study and facilitates monitoring of changes in ice-surface elevation and changes in flux across the ice-ocean boundary, which is essential for monitoring sea-level changes.

Late Weichselian Glaciation of the Russian High Arctic

A numerical ice-sheet model was used to reconstruct the Late Weichselian glaciation of the Eurasian High Arctic, between Franz Josef Land and Severnaya Zemlya. An ice sheet was developed over the entire Eurasian High Arctic so that ice flow from the central Barents and Kara seas toward the northern Russian Arctic could be accounted for. An inverse approach to modeling was utilized, where ice-sheet results were forced to be compatible with geological information indicating ice-free conditions over the Taymyr Peninsula during the Late Weichselian. The model indicates complete glaciation of the Barents and Kara seas and predicts a “maximum-sized” ice sheet for the Late Weichselian Russian High Arctic. In this scenario, full-glacial conditions are characterized by a 1500-m-thick ice mass over the Barents Sea, from which ice flowed to the north and west within several bathymetric troughs as large ice streams. In contrast to this reconstruction, a “minimum” model of glaciation involves restricted glaciation in the Kara Sea, where the ice thickness is only 300 m in the south and which is free of ice in the north across Severnaya Zemlya. Our maximum reconstruction is compatible with geological information that indicates complete glaciation of the Barents Sea. However, geological data from Severnaya Zemlya suggest our minimum model is more relevant further east. This, in turn, implies a strong paleoclimatic gradient to colder and drier conditions eastward across the Eurasian Arctic during the Late Weichselian.

Thermomechanical modelling of the Scandinavian ice sheet: implications for ice-stream formation

AbstractThis work attempts to explain the fan-like landform assemblages observed in satellite images of the area covered by the former Scandinavian ice sheet (SIS). These assemblages have been interpreted as evidence of large ice streams within the SIS. If this interpretation is correct, then it calls into doubt current theories on the formation of ice streams. These theories regard soft sediment and topographic troughs as being the key determinants of ice-stream location. Neither can be used to explain the existence of ice streams on the flat, hard-rock area of the Baltic Shield. Initial results from a three-dimensional, thermomechanical ice-sheet model indicate that interactions between ice flow, form and temperature can create patterns similar to those mentioned above. The model uses a realistic, 20 km resolution gridded topography and a simple parameterization of accumulation and ablation. It produces patterns of maximum ice-sheet extent, which are similar to those reconstructed from the area’s glacial geomorphology. Flow in the maximum, equilibrium ice sheet is dominated by wedges of warm, low-viscosity, fast-flowing ice. These are separated by areas of cold, slow-flowing ice. This patterning appears to develop spontaneously as the modelled ice sheet grows.

Monitoring changes of ice streams using time series of satellite-altimetry-based digital terrain models

The Antarctic Ice Sheet plays a major role in the global system, and the large ice streams discharging into the circumpolar sea represent its gateways to the world's oceans. Satellite radar altimeter data provide an opportunity for mapping surface elevation at kilometer-resolution with meter-accuracy. Geostatistical methods have been developed for the analysis of these data. Applications to Seasat data and data from the Geosat Exact Repeat Mission indicate that the grounding line of Lambert Glacier/Amery Ice Shelf, the largest ice stream in East Antarctica, has advanced 10–12 km between 1978 and 1987–89. The objectives of this paper are to explore possibilities and limitations of satellite-altimetry-based mapping to capture changes for shorter time windows and for smaller areas, and to investigate some methodological aspects of the data analysis. We establish that one season of radar altimeter data is sufficient for constructing a map. This allows study of interannual variation and is the key for a time-series analysis approach to study changes in ice streams. Maps of the lower Lambert Glacier and the entire Amery Ice Shelf are presented for austral winters 1978, 1987, 1988, and 1989. As a first step toward understanding the dynamics of the ice-stream/ice-shelf system, elevation changes are calculated for grounded ice, the grounding zone, and floating ice. In the absence of (sufficient) surface gravity control for the Lambert Glacier/Amery Ice Shelf area, altimetry-based maps may facilitate improvement of geoid models as they provide constraints on the terrain correction in the inverse gravimetric problem.

Complex ice stream flow revealed by sequential satellite imagery

The velocity field of the confluence area of two large ice stream tributaries forming Ice Stream D in West Antarctica is studied using sequential Landsat images. Sequential satellite image analysis allows for a very high density of velocity measurements, based on computer-measured displacements of features such as crevasses, crevasse scars, and ice mounds recognizable in both images. Automated displacement measurement of these features results in a detailed map of surface velocities from which surface-horizontal strain-rate fields can be calculated. Correlations between the surface morphology, the velocity field, and the strain-rate field of Ice Stream D reveal a number of important characteristics of ice stream flow: • the characteristic flowband appearance of streaming ice is present at velocities from below 100 m a−1 to above 350 m a−1; • in the upstream areas, there appears to be no sharp transition between “sheet” flow, typical of the surrounding ice sheet, and “streaming” flow; • the fastest moving portions of the ice stream are nearly devoid of surface topography undulations; • the confluence area is characterized by acceleration of the ice in the slower tributary as it impinges on faster-moving ice, and by highly convergent flow. Velocity in the faster-moving tributary changes little, and there is no persistent evidence of the shear margins of the joined tributaries downstream of the confluence. This study demonstrates that sequential satellite image analysis, coupled with computer-determined displacement measurements, can provide accurate velocity and strain-rate information on a regional scale rapidly and cost-effectively. Such data sets are required for modelling ice sheet evolution, and for monitoring any changes in ice flow within the ice streams.

Complex ice stream flow revealed by sequential satellite imagery

The velocity field of the confluence area of two large ice stream tributaries forming Ice Stream D in West Antarctica is studied using sequential Landsat images. Sequential satellite image analysis allows for a very high density of velocity measurements, based on computer-measured displacements of features such as crevasses, crevasse scars, and ice mounds recognizable in both images. Automated displacement measurement of these features results in a detailed map of surface velocities from which surface-horizontal strain-rate fields can be calculated. Correlations between the surface morphology, the velocity field, and the strain-rate field of Ice Stream D reveal a number of important characteristics of ice stream flow:• the characteristic flowband appearance of streaming ice is present at velocities from below 100 m a−1 to above 350 m a−1;• in the upstream areas, there appears to be no sharp transition between “sheet” flow, typical of the surrounding ice sheet, and “streaming” flow;• the fastest moving portions of the ice stream are nearly devoid of surface topography undulations;• the confluence area is characterized by acceleration of the ice in the slower tributary as it impinges on faster-moving ice, and by highly convergent flow. Velocity in the faster-moving tributary changes little, and there is no persistent evidence of the shear margins of the joined tributaries downstream of the confluence.This study demonstrates that sequential satellite image analysis, coupled with computer-determined displacement measurements, can provide accurate velocity and strain-rate information on a regional scale rapidly and cost-effectively. Such data sets are required for modelling ice sheet evolution, and for monitoring any changes in ice flow within the ice streams.

Thinning and Grounding-Line Retreat on Ross Ice Shelf, Antarctica

Detailed measurements of surface topography, ice motion, snow accumulation, and ice thickness were made in January 1974 and again in December 1984, along an 8 km stake network extending from the ice sheet, across the grounding line, and on to floating ice shelf in the mouth of slow-moving Ice Stream C, which flows into the eastern side of Ross Ice Shelf, Antarctica. During the 11 years between surveys, the grounding line retreated by approximately 300 m. This was caused by net thinning of the ice shelf, which we believe to be a response to the comparatively recent, major decrease in ice discharge from Ice Stream C. Farther inland, snow accumulation is not balanced by ice discharge, and the ice stream is growing progressively thicker.There is evidence that the adjacent Ice Stream B has slowed significantly over the last decade, and this may be an early indication that this fast-moving ice stream is about to enter a period of stagnation similar to that of Ice Stream C. Indeed, these large ice streams flowing from West Antarctica into Ross Ice Shelf may oscillate between periods of relative stagnation and major activity. During active periods, large areas of ice shelf thicken and run aground on seabed to form extensive “ice plains” in the mouth of the ice stream. Ultimately, these become too large to be pushed seaward by the ice stream, which then slows down and enters a period of stagnation. During this period, the grounding line of the ice plain retreats, as we observe today in the mouth of Ice Stream C, because nearby ice shelf, no longer compressed by ice-stream motion, progressively thins. At the same time, water within the deformable till beneath the ice starts to freeze on to the base of the ice stream, and snow accumulation progressively increases the ice thickness. A new phase of activity would be initiated when the increasing gravity potential of the ice stream exceeds the total resistance of the shrinking ice plain and the thinning layer of deformable till at the bed. This could occur rapidly if the effects of the shrinking ice plain outweigh those of the thinning (and therefore stiffening) till. Otherwise, the till layer would finally become completely frozen, and the ice stream would have to thicken sufficiently to initiate significant heating by internal deformation, followed by basal melting and finally saturation of an adequate thickness of till; this could take some thousands of years.

Thinning and Grounding-Line Retreat on Ross Ice Shelf, Antarctica

Detailed measurements of surface topography, ice motion, snow accumulation, and ice thickness were made in January 1974 and again in December 1984, along an 8 km stake network extending from the ice sheet, across the grounding line, and on to floating ice shelf in the mouth of slow-moving Ice Stream C, which flows into the eastern side of Ross Ice Shelf, Antarctica. During the 11 years between surveys, the grounding line retreated by approximately 300 m. This was caused by net thinning of the ice shelf, which we believe to be a response to the comparatively recent, major decrease in ice discharge from Ice Stream C. Farther inland, snow accumulation is not balanced by ice discharge, and the ice stream is growing progressively thicker. There is evidence that the adjacent Ice Stream B has slowed significantly over the last decade, and this may be an early indication that this fast-moving ice stream is about to enter a period of stagnation similar to that of Ice Stream C. Indeed, these large ice streams flowing from West Antarctica into Ross Ice Shelf may oscillate between periods of relative stagnation and major activity. During active periods, large areas of ice shelf thicken and run aground on seabed to form extensive “ice plains” in the mouth of the ice stream. Ultimately, these become too large to be pushed seaward by the ice stream, which then slows down and enters a period of stagnation. During this period, the grounding line of the ice plain retreats, as we observe today in the mouth of Ice Stream C, because nearby ice shelf, no longer compressed by ice-stream motion, progressively thins. At the same time, water within the deformable till beneath the ice starts to freeze on to the base of the ice stream, and snow accumulation progressively increases the ice thickness. A new phase of activity would be initiated when the increasing gravity potential of the ice stream exceeds the total resistance of the shrinking ice plain and the thinning layer of deformable till at the bed. This could occur rapidly if the effects of the shrinking ice plain outweigh those of the thinning (and therefore stiffening) till. Otherwise, the till layer would finally become completely frozen, and the ice stream would have to thicken sufficiently to initiate significant heating by internal deformation, followed by basal melting and finally saturation of an adequate thickness of till; this could take some thousands of years.

Recent glacial history and rapid ice stream retreat in the Amundsen Sea

Microfossil analyses of core top samples from 20 sites in the eastern and southern Amundsen Sea delimit three biotic provinces, which are related in part to the activity of a large ice stream and modern sea ice distribution. In Recent sediments on the Outer Shelf, planktonic and calcareous benthic foraminifera are abundant, but diatoms are rare. Here, low diatom abundances in sediments, despite high abundances in the overlying sea ice, are attributed to nondeposition caused by ocean currents. Pine Island Bay sediments are almost barren of microfossils, even though diatoms are abundant in overlying surface waters. We attribute this discrepancy to deposition of exclusively non‐organic sediments beneath a former extension of the floating terminus of Pine Island Glacier, which probably retreated from this area during the last century. The polynya which occupies this area during present‐day summers is probably so recent a feature that little or no biogenic sediment has accumulated. Recent sediments in the Eastern Margin Province, between Pine Island Bay and the Outer Shelf, contain abundant diatoms and arenaceous foraminifera. High diatom abundances here probably record the presence of the Amundsen Sea polynya during the last few millennia. The presence of a compact diamicton beneath Recent sediments on the Outer Shelf and in the Eastern Margin Province suggests that grounded ice formerly occupied the Amundsen Sea shelf with ice streams in deep, elongated troughs. These ice streams may have retreated up to 200 km during the Holocene.

Deformation of till beneath ice stream B, West Antarctica

The behaviour and possible instability of the West Antarctic ice sheet depend fundamentally on the dynamics of the large ice streams which drain it. Model calculations show that most ice-stream velocity arises at the bed1,2, and radar sounding has shown the bed to be wet3, but the basal boundary condition is not well understood. Seismic evidence from the Upstream B camp (UpB) on the Siple Coast of West Antarctica4 shows that the ice stream there rests on a layer of unconsolidated sediment averaging 5 or 6 m thick, in which the water pressure is only ∼50 kPa less than the overburden pressure. Because this thin layer occurs well inland beneath an active ice sheet and rests on a surface showing flutes4 characteristic of glacial erosion5, we presume that it is glacial till. We propose here that deformation within the till is the primary mechanism by which the ice stream moves, and we discuss implications of this hypothesis.

Glacial Geology of the Frying Pan River Drainage, Colorado

On the west flank of the Sawatch Range, Colorado, evidence is found for six distinct glacial advances One glaciation is pre-Wisconsin, four are Wisconsin, and one post-Wisconsin in age. In addition to end and lateral moraines of each advance, terrace remnants of six valley trains were identified and studied for a distance of 25 miles along Frying Pan River and its major tributaries. Elevations above stream level of these outwash terraces are 400-450, 90-120, 50-40, 20-30, 12-17, and 6-8 feet. Five of the tributary valleys contained large ice streams which did not join the trunk Frying Pan glacier during the Wisconsin stage. Extensive review and testing of the numerous criteria used to distinguish deposits of multiple glaciations show that nine of these criteria can conveniently be expressed in parameters indicative of relative age. Estimates based on these criteria, coupled with a recent radiocarbon dating of late Mankato till in the Midwest, yield the following approximate ages for deposits of the six glaciations in Frying Pan Valley: 230,000, 63,000, 46,000, 17,000, 11,500, and 5,750 years. The accuracy and reliability of the procedure used cannot be evaluated without further absolute Carbon 14 age determinations.

THE CORRELATION OF GLACIAL RETREAT STAGES ACROSS THE PENNINES

Introduction. The sequence of events during the Glacial Period in the North of England has long been a subject of study, and a wide literature has grown up around it in recent years. All attempts to give a complete picture have hinged on the correlation of the glacial deposits as developed on the east coast, and those, until recently less well known, inland, and on the west of the Pennines. It is perhaps not very surprising that many of the accounts give sequences that are very different, varying from a single glaciation, to one with five glacial episodes. As the field mapping of glacial deposits has slowly been completed during the last ten years, it has become more and more evident that a complete correlation would be most certainly attained, by close study of the three areas where ice streams from the west of England, crossed the Pennines and fed large ice streams on the eastern lowlands and on the coast —the Tyne valley, Stainmoor passes, and the Ribble-Aire Wharfe gap. These west to east ice streams are well known by their very distinctive trains of erratics, and their typical boulder clays can be followed through the lowlands and into relation with the coastal boulder clays, with certainty. Similarly it has been possible to trace many of the features associated with the shrinkage and retreat of these glaciers, and establish stages in their retreat, from complete contact with the eastern ice, back to the final stages of the western ice ...