Deglaciation chronology inferred from marine sediments in a proglacial lake basin, western Spitsbergen, Svalbard

Sub-bottom sediment profiles and sediment cores show that the lacustrine sediments in lake Linnevatnet are underlain by marine sediments and a basal till that mantles the bedrock. The till was probably deposited by the glacier that during the Late Weichselian glacial maximum removed all pre-existing sediments from the basin. The cores were collected in closed basins, where continuous deposition is expected. The marine sediment in the studied cores is up to 8 m thick and consists of bioturbated clay and silt. Radiocarbon dates on shells from the base of the marine sequence suggest that glacial retreat from the lake basin occurred around 12,500BP. This is more than a thousand years older than basal shell dates from raised marine sediments on the slopes above the lake. Typical ice proximal litbofacies were not identified in the cores. stratigraphic record indicates both a rapid glacial retreat and that no younger glacial re-advances occurred. During the Younger Dryas local glaciers on western Svalbard were smaller than during the Little Ice Age. This is in sharp contrast to western Europe, where Younger Dryas glaciers were much larger than those the Little Ice Age.     

Geological and archaeological chronology of a late Holocene coastal enclosure: The Sanguinet lake (SW France)

Sanguinet lake is separated from the Atlantic Ocean by a wide Holocene coastal dunes system in SW France. The present day lake level is 21 m above mean sea level (msl). It formed when aeolian sand closed the mouth of the small La Gourgue river which gradually became a lagoon and then a lake. Dated sub‐lacustrine archaeological remains (human settlements, canoes, and wooden architectural structures), as well as paleoenvironmental evidence (drowned tree stumps and lagoonal deposits exposed on the beach) are used to interpret the formation and chronology of lake level rise during the past 4000 years. Around 2000–1650 B.C., the river flowed into a lagoon or an estuary which connected with the ocean west of the present Sanguinet Lake. Its level was affected by the tide, which ranged between 2 m below and 3 m above msl. The accumulation of aeolian sand before 1500–1000 B.C. began to close the connection with the sea. At this time, the elevation of the surface of the lake water was approximately 5 m above msl, but it still remained connected to the ocean. Around 1000 B.C., the lake level rose quickly by 1 to 2 m during a period of renewed mobility of the coastal aeolian sand, and continued to rise slowly until about 100 A.D. when there was a gradual closure of the lake outlet. This rise forced people who were living on the lake shore and along the rivers to move to higher land along the valley. The nearby Gallo‐Roman site of Losa was settled at the end of the 1st century B.C.; then the final blocking of the outlet occurred because of spit growth as a result of north‐south littoral drift accompanied by the deposit of aeolian sand. This led to the lake level rising rapidly. Consequently, Losa was abandoned in the 3rd century A.D. and ruins of its temple (at 17 m above msl) were submerged in the 6th century. Further oscillations of the lake level probably correspond to water table fluctuations before it became stable at around 1000 A.D. The highest lake level (23.35 m) was reached during the 18th century as a consequence of modern dune formation, and thus was artificially reduced to 21 m in 1840 by construction of an overflow channel. © 2008 Wiley Periodicals, Inc.     

A pollen record from the Agassiz Ice Cap, northern Ellesmere Island, Canada

A continuous pollen record from the last glacial period to 100 B.P. was obtained from an ice core drilled in 1977 near the top of the Agassiz Ice Cap. Pollen concentrations are low ( c . 15–173 grains/100 I) through-out the core and exotic pollen grains (from distant sources) dominate over regional pollen grains (from Ellesmere Island). The very low concentrations during the Wisconsinan glacial period and the early Holocene are attributed to the increased distance of potential sources as most of northern North America was ice-covered or supported tundra vegetation. An increase of exotic grains (mainly birch and alder) at c . 7,600 B.P. corresponds to the period of alder migration into the Low Arctic regions. The subsequent fluctuations of exotic pollen, especially the increase at c . 3,100 B.P., are unexplained at present. Regional pollen concentrations start to increase at c . 6,100 B.P. and a maximum concentration is reached at c . 3,100 B.P. The pollen record suggests that plant migration into northern Ellesmere was limited until 6,100 B.P., then increased gradually and continued to do so for about 1,000 years after the climate had started to deteriorate at about 4,000 B.P.     

The evolution of Petrov lake and moraine dam rupture risk (Tien-Shan,Kyrgyzstan)

An interdisciplinary study of glacier-related hazards in the Petrov lake region (Ak-Shiirak Range, the Inner Tien-Shan, Kyrgyzstan) has been undertaken to identify potential dangers to the area. A cooperative effort from experts in the fields of hydrology, glaciology, geomorphology and geophysics has been employed in this study. For the hazard assessment, evolution of the Petrov glacier and lake was reconstructed using historical reports, aerial photographs and satellite images. Geomorphological mapping and geophysical soundings was applied to the lake territory and the moraine dam. This has identified potentially hazardous areas of the dam including subsurface drainage zones and cracks that could cause a sudden extremely high discharge. In the past three decades, the Petrov lake has doubled in size, while in recent years, its area has been increasing by more than 92,000 square metres per year. Although there is no evidence for an imminent outburst, the dramatic increase in the lake’s size emphasizes the importance of this study.     

Ice, moraine, and landslide dams in mountainous terrain

We review recent work on ice, moraine, and landslide dams in mountainous terrain, thus complementing several comprehensive summaries on glacier dams in intracontinental and Arctic areas of low relief. We discuss the roles of tectonic and climatic forcing on ice-, moraine-, and landslide-dam formation and sudden drainage, and focus on similarities and differences between their geomorphic impacts on confined valleys drained by steep bedrock and gravel-bed rivers.Despite numerous reported failures of natural dams in mountain belts throughout the world, their relevance to long-term dynamics of mountain rivers remains poorly quantified. All types of dams exert local base-level controls, thus trapping incoming sediment and inhibiting fluvial bedrock incision. Pervasive geomorphic and sedimentary evidence of outburst events is preserved even in areas of high erosion rates, suggesting that sudden dam failures are characterized by processes of catastrophic valley-floor aggradation, active-channel widening, and downstream dispersion of sediment, during which little bedrock erosion seems to be achieved.We find that, in the absence of direct evidence of former dams, a number of similarities among the geomorphic and sedimentologic characteristics of catastrophic outburst flows may give rise to ambiguous inferences on the dam-forming process. This is especially the case for tectonically active mountain belts where there is ample and comparable potential for the formation and failure of ice, moraine, landslide, and polygenetic dams concomitant with climatic oscillations or earthquake disturbance. Hence, the palaeoclimatic implications of erroneously inferring the cause of dam formation may be significant.We recommend that future research on natural dams in mountainous terrain addresses (a) climate- and earthquake-controlled systematics in the pattern of formation and failure; (b) quantification of response of mountain rivers to catastrophic outburst events and their concomitant process sequences; (c) elaboration of a comprehensive classification of natural dams in mountainous terrain with special attention to polygenetic dams; (d) physical-based modelling of dam formation, failure, and routing of water and sediment outbursts; and (e) quantitative controls on the contribution of natural dams to sediment budgets in mountainous terrain.     

Holocene moraine and paleosol stratigraphy, Bugaboo Glacier, British Columbia

Sequences of tills, buried paleosols, wood and tephra in lateral moraines provide a record of Holocene advances and retreats of the Bugaboo Glacier in British Columbia. The oldest paleosol is tentatively classified as a Spodosol (Cryorthod). It incorporates Mazama tephra (6,800 B.P.) and charcoal and humus dated at 3,390 and 4.400B.P., respectively, and records early and middle Holocene warming and/or drying. This paleosol overliesa latest Pleistocene or early Holocene till associated with a nearby end moraine and assigned to the regionally known 'Crowfoot Advance'. Less-developed paleosols (Cryumbrepts) are formed on Neoglacial tills deposited shortly before 3,000 B. P., between c. 2,500 to 1,900 B. P., and between c. 900 B.P. and the 19th century. The paleosols and surface soils form microcatenas with morphological variations due to differences in original topography and vegetation. The chronology derived from these paleosols and tills generally agrees with, but increases the resolution of, what is known of Holocene glacier histories in the Canadian Cordillera.     

Reconstruction of Holocene lake level from diatoms, chrysophytes and organic matter in a drainage lake from the Experimental Lakes Area (northwestern Ontario, Canada)

A paleolimnological study of lake-level changes in Lake 239 (Rawson Lake), a headwater lake in the Experimental Lakes Area in northwestern Ontario, indicates large fluctuations have occurred over the Holocene. Analyses are based on diatoms, the proportion of chrysophyte scales to diatoms and organic matter content from near-shore sediment cores. Quantitative estimates of lake level are based on a diatom-inferred depth model that was developed from surficial sediments collected along several transects in Lake 239. Declines of ∼ 1-3 m occurred during the late Holocene, whereas declines of at least 8 m occurred during the more arid mid-Holocene. These results provide the first substantive evidence of large declines in lake level in northwestern Ontario during the mid-Holocene. Conditions during the mid-Holocene may provide a partial view of future conditions under increasing global temperatures.     

Recently exposed vegetation reveals Holocene changes in the extent of the Quelccaya Ice Cap, Peru

Radiocarbon dating of well-preserved, in-place vegetation exposed by the retreating Quelccaya Ice Cap of southeastern Peru constrains the last time the ice cap's extent was smaller than at present. Seventeen plant samples from two sites along the central western margin collectively date to 4700 and 5100 cal yr BP and strongly indicate that current ice cap retreat is unprecedented over the past ∼ 5 millennia. Seventeen vegetation samples interbedded in a nearby clastic sedimentary sequence suggest ice-free conditions at this site from ∼ 5200 to at least ∼ 7000 cal yr BP, and place minimum constraint on early- to mid-Holocene ice cap extent.     

Analysis of a core through the Meighen Ice Cap,Arctic Canada,and its paleoclimatic implications

Analyses of crystal size, bubble content, oxygen isotope ratio, specific electrolytic conductivity, and the distribution of firn and dirt layers in a core, 121.2 m long, from surface to bedrock near the highest point of the Meighen Ice Cap, leads to the following outline of the ice cap's history. The ice cap, which has always been stagnant, originated in the cold period that followed the postglacial Climatic Optimum. After initial growth came a period of negative mass balance in which the area and thickness of the ice cap diminished and the surface slope at the core site steepened. The end of this period, at least 600 y.a., is marked by a discontinuity at 54 m depth in the core; above this level, the values of most parameters differ significantly from their values below. There followed a period of growth by the end of which, some 80 y.a., the ice cap had attained its maximum thickness; this period included the coldest interval in the ice cap's history. Ablation has predominated since then and up to 13 m of ice have been lost at the core site. This history resembles that of the Ward Hunt Ice Shelf.     

End moraine construction by incremental till deposition below the Laurentide Ice Sheet: Southern Ontario,Canada

Eyles, N., Eyles, C., Menzies, J. & Boyce, J. 2010: End moraine construction by incremental till deposition below the Laurentide Ice Sheet: Southern Ontario, Canada. Boreas, 10.1111/j.1502‐3885.2010.00171.x. ISSN 0300‐9483. Just after 13 300 ¹⁴C a BP in central Canada, the retreating Ontario lobe of the Laurentide Ice Sheet briefly re‐advanced westwards through the Lake Ontario basin to build a large end moraine. The Trafalgar Moraine (27 km long, 4 km wide) is composed of a distinctly red‐coloured silt‐rich till (Wildfield Till, up to 16.5 m thick) formed by the reworking of proglacial lake deposits and soft shale bedrock. The moraine has a pronounced ramp‐like longitudinal form passing upglacier into fluted till resting on exposed shale. Analysis of water well stratigraphic data, drilled sediment cores, downhole gamma‐ray logs and exposures in deep test pits shows that within the moraine the Wildfield Till is built of superposed beds up to 7 m in thickness. These are inferred to result from the repeated incremental deposition of fine‐grained debris being moved towards the ice margin as a deforming bed such as identified at modern glaciers. A total till volume of 0.81 km³ was produced in a very brief time‐span along a transport path probably no greater than 10 km in length. Subglacial mixing of pre‐existing sediment and soft shale was clearly a very effective process for generating and moving large volumes of till to the ice margin. Similar till‐dominated end moraines occur widely around the margins of the Great Lake basins, where the markedly lobate margin of the retreating Laurentide Ice Sheet re‐advanced repeatedly into proglacial lakes and over fine‐grained sediment. This suggests the wider applicability of the till transport and incremental depositional model presented here.     

High‐resolution proglacial lake records of pre‐Little Ice Age glacier advance,northeast Greenland

Understanding Arctic glacier sensitivity is key to predicting future response to air temperature rise. Previous studies have used proglacial lake sediment records to reconstruct Holocene glacier advance–retreat patterns in South and West Greenland, but high‐resolution glacier records from High Arctic Greenland are scarce, despite the sensitivity of this region to future climate change. Detailed geochemical analysis of proglacial lake sediments close to Zackenberg, northeast Greenland, provides the first high‐resolution record of Late Holocene High Arctic glacier behaviour. Three phases of glacier advance have occurred in the last 2000 years. The first two phases (c. 1320–800 cal. a BP) occurred prior to the Little Ice Age (LIA), and correspond to the Dark Ages Cold Period and the Medieval Climate Anomaly. The third phase (c. 700 cal. a BP), representing a smaller scale glacier oscillation, is associated with the onset of the LIA. Our results are consistent with recent evidence of pre‐LIA glacier advance in other parts of the Arctic, including South and West Greenland, Svalbard, and Canada. The sub‐millennial glacier fluctuations identified in the Madsen Lake succession are not preserved in the moraine record. Importantly, coupled XRF and XRD analysis has effectively identified a phase of ice advance that is not visible by sedimentology alone. This highlights the value of high‐resolution geochemical analysis of lake sediments to establish rapid glacier advance–retreat patterns in regions where chronological and morphostratigraphical control is limited.     

Neoglacial chronology and floristics in the Middle Teton area,central Teton Range,Western Wyoming

The relative ages of Neogfacial deposits above 2900m near Cloudveil Dome, Middle Teton, and Teepe glaciers, central Teton Range, western Wyoming, were determined using topographic position, weathering features, lichenometry, vegetation characteristics, and soils. A three-fold deposit sequence is identified and correlated with the Gannett Peak, Audubon and Indian Basin Neoglacial advances described elsewhere in the Teton and nearby Wind River ranges. While soil profile morphology proved very useful in distinguishing deposits of different age, other age indicators such as clay mineralogy and soil chemistry, proved of only marginal value in age discrimination. This chronosequence, emplaced over the last ～5000yr, provides an excellent setting upon which to test changes in plant species composition and its use as a relative dating method. Vascular plant species composition cannot be used to discriminate deposits of the different advances, possibly because of random colonisation and establishment combined with extensive mass wasting during the Neoglacial. However, certain vegetation features, such as total cover and species densities, appear useful as relative dating methods.     

Within-valley asymmetry and related problems of Neoglacial lateral moraine development at certain Jotunheimen glaciers, southern Norway

A difference in the size of Neoglacial lateral moraines on either side of a valley axis (within-valley asymmetry of lateral moraine development) is described. Analysis of clast roundness has revealed subangular material in latero-terminal and terminal moraines; lateral moraines, however, exhibit a compositional gradient of increasing angularity with distance from the former glacier snout. Comparisons with clasts of known origin suggest that this 'roundness gradient' may be explained with reference to either or both of two hypotheses: (1) a variable proportion of supraglacial (or englacial) to subglacial transported material; and (2) the variable composition of regolith incorporated by a push mechanism from the valley sides. Within-valley asymmetry is inferred to result where the supply of debris to lateral moraines from these sources is unequal either side of a valley axis. Both interpretations are also consistent with the relatively large size of latero-terminal sections of end moraines. In order to account for the discrepancy between moraine size and apparent debris supply rates, it is suggested that the largest lateral moraines may have been formed over a longer time scale than the 'Little Ice Age', and that reworking of deposits may have occurred. The supply of debris to the north-facing lateral moraine at Nordre Illåbreen has been so great that it has developed into a rock glacier; this suggests the possibility that subglacial material and valley-side regolith, as well as supraglacial material, contributes to the formation of ice-cored rock glaciers.     

Revision of the early Holocene lake sediment based chronology and event stratigraphy on Hochstetter Forland, NE Greenland

The previously established and often debated lake sediment based chronology and event stratigraphy from Hochstetter Forland, NE Greenland, has been re-examined. These new studies show that the last deglaciation of the coastal area is several thousand years younger than previously described. The main reason behind the difference is the fact that the older chronology was based on ¹⁴C datings of bulk sediment samples, which are now shown to contain enough coal fragments to produce erroneous ages. The re-examination was performed on sediments from two lakes located within the Nanok moraine system: one is situated at or slightly above the marine limit around 65 m. the other at 21–22 m a.s.l. The combined stratigraphy from the two lakes shows that the area was deglaciated before 9000 BP, after which followed deposition of glaciomarine sediments, fining upwards. The first vegetation seems to have been dominated by grasses, Lycopodium and Polypodiaceae. At c . 8000 BP the limnic production increased significantly and a pioneer vegetation characterized the area. At this time the 'Artemisia' grains appear. A short but distinct climatic cooling occurs at c . 7500 BP causing a significant drop in lake productivity and possibly also producing coarser sediments in the (glacio)marine environment. About 200 years later the lake productivity again increased, very rapidly, and the marine sediments became finer and more rich in molluscs, as a response to the beginning of the climatic optimum. Because of the time-lag between climate and vegetation response it took another 300 years before Betulci nana immigrated, at the same time as the 'Artemisia' grains disappeared, and another several hundred years before a real dwarf-shrub vegetation developed in these parts of Hochstetter Forland. Before the lower lake was isolated from the sea at c . 6000 BP, coarse wave-washed sediments, followed by a typical isolation sequence, were deposited in it.     

Dimensions and deglacial chronology of the Outer Hebrides Ice Cap,northwest Scotland: implications of cosmic ray exposure dating

Cosmic ray exposure ages of frost-weathered bedrock from mountain summits in the Outer Hebrides exceed the age of Late Devensian glaciation. Exposure ages of most glacially-abraded bedrock surfaces at low and intermediate elevations are younger than the age of maximum Late Devensian glaciation. These results confirm that previously mapped periglacial trimlines in the Outer Hebrides define the upper limit of bedrock erosion by Late Devensian ice. They are consistent with the interpretation, based on geomorphological evidence, that the trimlines mark the approximate upper limit of a Late Devensian Outer Hebrides Ice Cap. A postglacial exposure age from the summit of Oreval (662 m) suggests that this mountain was overrun during the last glaciation, indicating thicker ice cover and a lower surface gradient west of the ice-cap divide than previously inferred. Although bedrock surfaces below the trimlines are strongly ice-moulded, some show evidence of prior cosmic ray exposure, which we attribute to limited erosion during Late Devensian glaciation. If this interpretation is correct, the youngest apparent ages from these surfaces give the most reliable dates for deglaciation, at ca. 14.5–14 ka. This implies that ice persisted at favourable sites through the warm opening phase of the Windermere Interstade. Comparison with radiocarbon-dated evidence from offshore cores suggests net ice margin retreat of ∼74 km eastwards across the adjacent shelf in > 2.3 ± 1.0 ka. The dating evidence is consistent with relatively rapid retreat of calving margins to the coast, then slower withdrawal of ice margins to high ground. Copyright © 2005 John Wiley & Sons, Ltd.     

A local clay-varve chronology and proglacial sedimentary environment in glacial Lake Peipsi, eastern Estonia

Ten cores consisting of varved clay from the northern part of Lake Peipsi in eastern Estonia have been correlated using varve thickness variations and specific marker varves into a 375-year floating varve chronology. Continuous sedimentation during gradual ice recession is concluded from a clear transition from proximal to distal varves. Cyclic variations in varve thickness are caused mainly by thickness changes of clayey winter layers. This is interpreted to indicate increased influx of finer material due to faster melting of the glacier. The cyclic pattern of thickness change is explained by alternating periods of increased and decreased melting of the ice. Simultaneous accumulation of varved clay in glacial Lake Peipsi and in the Luga and Neva basins of Russia is concluded from the good visual correlation between the mean varve thickness diagrams for the three chronologies. Because the varve chronologies from northwestern Russia have been tentatively correlated to the Swedish varve chronology, the timing of the clay accumulation in glacial Lake Peipsi is placed between c . 13 500 and 13 100 varve years BP.     

Rates of Deglaciation during the Last Glaciation and Holocene in the Cordillera Vilcanota-Quelccaya Ice Cap Region, Southeastern Perú

Moraine chronology is combined with digital topography to model deglacial rates of paleoglacier volumes in both the Huancané Valley on the west side of the Quelccaya Ice Cap and the Upismayo Valley on the northwest side of the Cordillera Vilcanota. The fastest rates of deglaciation (39×10⁻⁵ to 114×10⁻⁵ km³ yr⁻¹ and 112×10⁻⁵ to 247×10⁻⁵ km³ yr⁻¹ for each valley, respectively) were calculated for the most recent paleoglaciers, corresponding to the last few centuries. These results are consistent with observations in the Venezuelan Andes showing high rates of deglaciation since the Little Ice Age. These rates also fall within the range of 20th century rates of deglaciation measured on the Quelccaya Ice Cap (29×10⁻⁵ to 220×10⁻⁵ km³ yr⁻¹, Brecher and Thompson, 1993; Thompson, 2000). These results imply that rates of deglaciation may fluctuate significantly over time and that high rates of deglaciation may not be exclusive to the late 20th century. Equilibrium line altitude (ELA) depressions for the ice volumes of the last glaciation modeled here were computed as 230 m for the Quelccaya Ice Cap and 170 m for the Cordillera Vilcanota. Maximum ELA depressions are lower than previously published: <500 m for the Cordillera Vilcanota and <400 m for the Quelccaya Ice Cap. These lower values could imply a topographic control over paleoglacier extent.     

Stratigraphy and age of a Neoglacial sedimentary succession of proglacial outwash and an alluvial fan in Langedalen,Veitastrond, western Norway

This study presents the sedimentary succession of an outwash plain and an alluvial fan located along the valley Langedalen at the south-eastern side of the Jostedalsbreen ice cap in inner Sogn, western Norway. A newly exposed ~2.8-m-high section along the southern riverbank of Langedøla river shows alternating layers of minerogenic sediments and peat layers with tree logs, identified as Salix sp. The section is situated in the distal part of an alluvial fan built out from the southern slope of Langedalen. Six AMS radiocarbon dates of tree fragments indicate that the accumulation of the fine-grained sediments in the lower part of the section was initiated earlier than the basal radiocarbon date of 914–976 calibrated years CE (1σ age range). These basal, fine-grained sediments are interpreted as proglacial outwash deposited in a floodplain depression or abandoned river channel in a low-energy glaciofluvial environment. Periods of low glacier cover, low river discharge or low-water stands over the floodplain allowed peat formation and the growth of trees and shrubs in the valley. The radiocarbon dates further indicate relatively rapid sediment accretion (~2.7–3 cm a⁻¹) between 190 and 125 cm below the sediment surface, equivalent to approximately 1220 to 1250 cal. a CE (1σ age range). At ~60 cm depth below the surface, dated to approximately 1590 to 1620 cal. a CE (1σ age range), a transition to more coarse-grained, sandy and gravelly sediments indicates increased sediment supply and distal expansion of the alluvial fan. This occurred most likely as a consequence of increased sediment yield from expanding glaciers along the southern valley side of Langedalen as a response to the initial Little Ice Age glacier growth. Based on these results, the accretion and progradation of glacier-fed alluvial fans mainly occur during periods of glacier advance rather than during glacier recession.     

Holocene shore displacement and deglaciation chronology in Norrbotten, Sweden

The coastal zone of Norrbotten, northern Sweden, was gradually inundated by the Ancylus Lake following the retreating ice margin and forming a highest coastline approximately 210 m above the present sea level. The succeeding shore displacement is reconstructed based on lithological investigations and radiocarbon datings of identified isolation sequences from 12 cored lake basins. The highest lake basins, along with two basins above the highest shoreline, suggest ice-free conditions already at 10 500 cal. yr BP. This is at least 500 years earlier than previously thought and implies rapid ice-sheet break-up in the Gulf of Bothnia. The shore displacement (RSL) curve represents a forced regression of successively decreasing rate through the Holocene, from 9 m/100 yr to 0.8 m/100 yr. During the first 1000-1200 years, the isostatic uplift is exponentially declining, followed by a constant uplift rate from c. 9500 cal. yr BP to 5500-5000 cal. yr BP. The last 5000 years seem to be characterized by a low but constant rebound rate. The development of the Ancylus Lake stage of the Baltic may also be discerned in the Norrbotten RSL curve, suggesting that the chronology of the Ancylus Lake stages may have to be revised. The Littorina transgression is also reflected by the RSL curve shape. In addition, a series of early to mid-Holocene beach terraces were OSL-dated to allow for comparison with the ¹⁴C-dated shore displacement curve. Interpretations of these ages and their relation to former sea levels were clearly more problematic than the dating of the lake basin isolations.