Valley Heads Research Articles

History of the Antarctic Ice Sheet in the McMurdo Sound region at and since the Last Glacial Maximum: Evidence from Miers Valley

Reconstructions of past Antarctic Ice Sheet (AIS) behavior can afford key information about likely ice-sheet response to future climate change. Here, we examine the history of a marine-based ice mass grounded in McMurdo Sound of the western Ross Embayment during the Last Glacial Maximum and termination. Geomorphological mapping and interpretation of stratigraphic sections, as well as 67 radiocarbon dates of algal mats and 16 234U/230Th ages of carbonate and gypsum in glaciolacustrine sediments, indicate that ice from the broader Ross Embayment entered McMurdo Sound by approximately 24,000 yr BP and blocked the seaward mouth of Miers Valley. Maximum thickness of the McMurdo Sound ice was maintained from ∼19,000 yr BP, with fluctuations, until ∼12,600 yr BP. A lake-ice conveyor operated on a large ice-dammed lake that accompanied this advance until dessication at 10,000–7900 yr BP concurrent with ice recession. In contrast to the expansion of grounded marine-based ice in McMurdo Sound, alpine glaciers at the valley head did not advance at the Last Glacial Maximum. This out-of-phase behavior of the marine-based ice sheet vs. independent terrestrial glaciers is consistent with previous studies suggesting that local glaciers respond primarily to accumulation, which rises during warm periods. The long and late maximum of the McMurdo Sound ice (corresponding largely to Termination I) and delayed recession until after the Antarctic Cold Reversal (∼14,700–12,900 yr BP) lends support to the hypothesis that the AIS in the Ross Sea sector is controlled by the competing effects of marine (sea level, ocean temperatures) and accumulation forcing.

The hidden glacial landscape of the Monti della Laga (central Apennines, Italy)

AbstractThe geological and geomorphological record of the Last Glaciation in the Apennines is extensively documented, except for the Monti della Laga sector, where limited indicators of glacier extension have been reported. This study presents field evidence of glacial deposits and landforms from two specific sites within the Monti della Laga Range: Monte Pelone and Tordino Valley head. Although limited, this evidence suggests glaciation within a structural and lithological setting that facilitated rapid slope evolution. The post-glaciation paraglacial processes in the area erased most glacial traces, but our geomorphological and stratigraphical investigations identified specific landforms of glacial origin. Notably, at Monte Pelone, a semi-circular depositional landform exhibiting a significant presence of coarse clasts and distinctive rotational features has been identified as a moraine. Additionally, a smaller moraine ridge with evidence of diamicton deposition was observed in the Tordino Valley head. These observations align with an Equilibrium Line Altitude estimated at 1750 m a.s.l. in the central Apennines in the Last Glacial Maximum and, considering the favourable topographic conditions of the area, significantly contribute to the understanding of glaciations within the central Apennines.

A reassessment of the Erie Interstade from field work in the Cayuga basin, central New York

The Erie Interstade is generally accepted to have been a relatively warm period associated with retreat of the Laurentian ice sheet into the Lake Ontario basin, followed by a readvance to the Valley Heads (Port Bruce Stade) ice front in central New York. Recent investigations in the Cayuga basin indicate that this ice front retreated only to the Ithaca area during the Erie Interstade. In the Cayuga trough, Erie glaciolacustrine strata underlie Valley Heads deposits. Plant assemblages in these strata indicate a sub-Arctic environment. Erie deposits also occur in the Fall Creek Valley, where a varved sequence below Valley Heads till and above Nissouri till indicate deposition close to a retreating ice front. Remanent magnetic declination in the varves is 357°, best correlated with lower Newport beds of the Middleville fm in the Mohawk Valley and suggests an age of 17.7 ± 0.1 ka cal for the peak of the Erie retreat. Retreat only to the Ithaca area is based on the lack of Erie deposits north of Ithaca, an oscillation of the ice front around the south end of Cayuga Lake, and the lack of a fluvial interval in the Cayuga trough that would have marked retreat into the Ontario basin. Ice core data from Greenland and sea level studies demonstrate that the Erie Interstade was neither a warm period nor a time of significant sea level rise. The Erie Interstade retreat/Valley Heads readvance appears instead to represent a minor reversal in the general, slow ice front retreat from the Last Glacial Maximum to the warm up at Melt Water Pulse 1.

Quantifying Rates of Landscape Unzipping

AbstractMeasuring rates of valley head migration and determining the timing of canyon‐opening are insightful for the evolution of planetary surfaces. Spatial gradients of in situ‐produced cosmogenic nuclide concentrations along horizontal transects provide a framework for assessing the migration of valley networks and similar topographic features. We developed a new derivation for valley head retreat rates from the concentrations of in situ‐produced cosmogenic radionuclides in valley walls. The retreat rate is inversely proportional to the magnitude of the spatial concentration gradient and proportional to local nuclide production rates. By solving for a spatial gradient in concentration along a valley parallel transect, we created an expression for the explicit determination of valley head retreat, which we refer to herein as unzipping. We applied this expression to a seepage‐derived drainage network developing along the Apalachicola River, Florida, USA. Sample concentrations along a valley margin transect varied systematically from 2.9 × 105 to 3.5 × 105 atoms/g resulting in a gradient of 160 atoms/g/m, and from this value a valley head retreat rate of 0.025 m/y was found. The discrepancy between overall network age and current rates of valley head migration suggests intermittent network growth which is consistent with glacial‐interglacial precipitation variations during the Pleistocene. This method can be applied to a wide range of Earth‐surface environments. For the 10Be system, this method should be sensitive to unzipping rates bounded between 10−6 and 100 m/y.

Alabama Historical Association Markers, 2021

Alabama Historical Association Markers, 2021 Scotty E. Kirkland (bio) The texts and locations of the fourteen markers sponsored by the Association in 2021 are copied below. More information on the program, including guidelines for new marker requests, can be found at https://www.alabamahistory.net/historical-markers. JACKSON COUNTY PHILLIP HAMMAN, SOLDIER & EARLY ALABAMA SETTLER Of German ancestry, Phillip Hamman was a Colonial-era scout and a Revolutionary War soldier hailed as the “Savior of Green-brier” for his 1778 efforts to defend Ft. Donnally in present-day West Virginia. In 1780, he married Christina Cook (1763–1842). After nearly 40 years in Kentucky, they became early settlers of Jackson County in the 1820s, farming a large tract of land one mile north of this site. In 1827, Phillip Hamman helped organize the Friendship Church at Fackler and was ordained as its pastor. In 1972, the remains of Phillip and Christina Hamman were removed from family land and reinterred at the Valley Head Cemetery. [To be located along County Road 42, near Fackler] [End Page 163] JEFFERSON COUNTY HIGHLANDS SCHOOL Evalina Brown Spencer founded Highlands Day School in 1958, expanding a daycare she operated in her family’s backyard. In 1962, Highlands moved from rented space in St. Luke’s Episcopal Church to this site, on land donated by Joseph and Virginia Simpson. The early campus consisted of four buildings. Expansions in 1991, 1997, and 2019 added an auditorium, library, offices, a gymnasium, cafeteria, and classrooms. Students range from infants to eighth grade. Alumni remember school traditions fondly, including Blue Devils athletics, public speaking, gatherings on the quad, and time spent on the “Big Rock” located on campus. [4901 Old Leeds Road, Birmingham] THE ATTEMPTED BOMBING OF CONGREGATION BETH-EL Congregation Beth-El was founded in 1907 on Birmingham’s north side. Its leadership came from Knesseth Israel, the city’s Orthodox Jewish congregation. Beth-El was established as a modern, yet traditional congregation. Construction on the synagogue at this site began in 1926, with the addition of an education wing in 1952. By the 1950s, Beth-El was Alabama’s largest Conservative Jewish congregation. On Monday, April 28, 1958, a satchel containing 54 sticks of dynamite was placed along the eastern exterior of the synagogue’s lower floor. The bomb failed before it could detonate. Had the bomb exploded, the blast would have demolished the building. Part of a series of unsolved antisemitic attacks against Jewish institutions in the South in the 1950s, the attempted bombing of Beth-El came amidst a plague of racial terror during Birmingham’s modern Civil Rights era. Although Congregation Beth-El received sympathy and support from across Birmingham and beyond, the crime remained unsolved. [End Page 164] In the bombing’s wake, reaction within the local Jewish community ranged from fear to a determination to confront bigotry and intolerance amidst the ongoing struggle for justice and equality. [To be located at 2179 Highland Avenue, Birmingham] LEE COUNTY AUBURN UNIVERSITY AND THE ALABAMA FARMERS FEDERATION On January 31, 1921, farmers gathered on the campus of Alabama Polytechnic Institute (API), now Auburn University, to form the Alabama Farm Bureau, now known as the Alabama Farmers Federation. Former Alabama Extension Service Director Luther Duncan, who later became API president, encouraged farmers to organize as a unified force in public policy and to support agricultural education, research, and outreach. Extension agents promoted Farm Bureau membership throughout Alabama. By 1925, Farm Bureau groups were established in all 67 counties. Farm Bureau played a pivotal role in development of farm programs, cooperative marketing, rural electrification, and strengthening the land-grant university system. In 1946, Alabama Farm Bureau formed what is known today as Alfa Insurance® to provide coverage for rural residents. The organization changed its name in 1987. The Federation has emerged as the preeminent voice for farmers and rural Alabama in advocacy, communication, public policy, leadership development, commodity promotion, agricultural education, and research. In 2021, the Alabama Farmers Federation celebrated its centennial and the 75th anniversary of Alfa Insurance with a renewed commitment to its founding principles. [Ag Heritage Park Pavilion, S. Donahue Drive, Auburn University] [End Page 165] MADISON COUNTY ALICE BOARMAN BALDRIDGE, WOMEN’S RIGHTS...

Groundwater seepage is a key driver of theater-headed valley formation in limestone

Abstract Groundwater seepage leads to the formation of theater-headed valleys (THVs) in unconsolidated sediments. In bedrock, the role of groundwater in THV development remains disputed. Here, we integrate field and remote-sensing observations from Gnejna Valley (Maltese Islands) with numerical modeling to demonstrate that groundwater seepage can be the main driver of THV formation in jointed limestone overlying clays. The inferred erosion mechanisms entail (1) widening of joints and fractures by fluid pressure and dissolution and (2) creeping of an underlying clay layer, which lead to slope failure at the valley head and its upslope retreat. The latter is slower than the removal of the talus by creep and sliding on the valley bed. The location and width of THVs are controlled by the location of the master fault and the extent of the damage zone, respectively. The variability of seepage across the fault zone determines the shape of the valley head, with an exponential decrease in seepage away from the fault giving rise to a theater-shaped head that best matches that of Gnejna Valley. Our model may explain the formation of THVs by groundwater in jointed, strong-over-weak chemical sedimentary lithologies, particularly in arid terrestrial settings.

Morphological differences of longitudinal profiles between glacial cirques and non-glacial valley heads, described by mathematical fitting

Glacial cirques have specific concave longitudinal profiles different from those of valley heads without glacial imprint. The degree of deepening of valley heads can be used to determine the extent of past mountain glaciation and serve as a tool in palaeoenvironmental reconstructions. The aim of the present paper is to compare longitudinal profiles of glacial cirques and non-glaciated valley heads. For this purpose, we analyzed an alpine area with classic well-developed glacial cirques in the Central Alps (Austria) and a mid-mountain area of the High Sudetes (Czech Republic/Poland) with both cirques and non-glaciated valley heads. We described each valley head by c-value function fitting using longitudinal profiles obtained from detailed laser-derived digital elevation models and tested them statistically. The results show, with high accuracy of fitting, significant differences between c-values of glacial cirques and those of non-glacial valley heads. This newly presented approach could be useful for distinguishing between valley heads of fluvial origin and glacial cirques, as well as for evaluating the degree of cirque development.

Comments on Sookhan et al. “Lidar-based quantitative assessment of drumlin to mega-scale glacial lineation continuums and flow of the paleo Seneca-Cayuga paleo-ice stream” [Quat. Sci. Rev. 263 (2021): 107003

Sookhan et al.‘s paper consists of two parts: an analysis of glacial bedforms in the Seneca-Cayuga Finger Lake area and a paleoglacial history following the Valley Heads readvance. This comment concerns only the paleoglacial history. The biggest problem with their history is that a radiocarbon age was used for the Valley Heads readvance whereas a calendar age was used for the Bølling Allerød interstadial, leading to their erroneous conclusion that the entire Quaternary fill of the Cayuga trough was deposited in a single pulse lasting only a few hundred years. The duration between the Valley Heads readvance and the end of proglacial deposition there was actually about 4000 years. There are also serious questions concerning their descriptions of the nature of the deposits during and after the Valley Heads readvance. Sookhan et al.‘s claim of subaqueous and “chaotically-bedded outwash” is without any observational support. Because the prerequisites for their model of southward ice acceleration do not exist, that model is suspect and an alternative is suggested.

How a New Cenozoic Geology and Glacial History Paradigm Explains Topographic Map Drainage System and Erosional Landform Evidence: Elbert and Lincoln Counties, Colorado, USA

Detailed topographic map drainage system and erosional landform evidence such as drainage route orientations, drainage divides, divide crossings (low points on drainage divides), erosional escarpments, and similar features in the east central Colorado Elbert and Lincoln County region are considered as pieces of a complex but solvable drainage history puzzle. A satisfactory solution to date has eluded investigators who have worked from the accepted Cenozoic geology and glacial history paradigm (accepted paradigm) perspective in which climatic and tectonic factors operating over long time periods lead to what might be considered to be a randomly determined regional drainage history. A new and fundamentally different Cenozoic geology and glacial history paradigm (new paradigm) in which immense and prolonged south-oriented continental ice sheet meltwater floods flowed across the Elbert and Lincoln County area which at that time was near the rising rim surrounding a thick continental icesheet created and occupied deep “hole”. Map evidence documents how northeast-oriented Republican River headwaters valleys eroded headward across must have been large southeast-oriented floods probably moving toward what at that time would have been an actively eroding and deep east-oriented Arkansas River valley head and how those massive southeast-oriented floods subsequently lowered the Colorado Piedmont surface before being beheaded and reversed when the deep northeast- and east-oriented South Platte River valley eroded headward to create in an identifiable sequence (from east to west) what are now long north-oriented South Platte River tributaries. New paradigm predicted massive and prolonged south-oriented meltwater floods flowing across what must have been a rising region explains much, if not all of the Elbert and Lincoln County detailed topographic map drainage system and erosional landform evidence.

Climatically Driven Holocene Glacier Advances in the Russian Altai Based on Radiocarbon and OSL Dating and Tree Ring Analysis

Analysis of new chronological data, including 55 radiocarbon, 1 OSL, and 8 dendrochronological dates, obtained in the upper reaches of trough valleys within the Katun, North Chuya, South Chuya, and Chikhachev ranges, together with the 55 previously published ones, specifies climatically driven glacier dynamic in the Russian Altai. Available data refute the traditional concept of the Russian Altai Holocene glaciations as a consecutive retreat of the Late Pleistocene glaciation. Considerable and prolonged warming in the Early Holocene started no later than 11.3–11.4 cal kBP. It caused significant shrinking or even complete degradation of alpine glaciers and regeneration of forest vegetation 300–400 m above the modern upper timber limit. Stadial advances occurred in the middle of the Holocene (4.9–4.2 cal kBP), during the Historical (2.3–1.7 cal kBP), and the Aktru (LIA thirteenth–nineteenth century) stages. New radiocarbon ages of fossil soils limited glaciers expansion in the Middle Holocene by the size of the Historical moraine. Lesser glacial activity between 5 and 4 cal kBP is also supported by rapid reforestation in the heads of trough valleys. Glaciers advance within the Russian Altai, accompanied by accumulation of the Akkem moraine, could have occurred at the end of the Late Pleistocene.

Origin of theater-headed valleys in the Valles Marineris region, Mars

This paper reports high-resolution observations of theater-headed valleys in the Valles Marineris region and proposes an integrated model of their formation and localization across the walls of the canyons of Valles Marineris. These theater-headed valleys have formed over a long period of time from Middle-Hesperian to Early-Amazonian. The valleys show several common features, namely, proximity to areas of high ejecta mobility on the Tharsis plateau; alignment along the major tectonic features such as wrinkle ridges, grabens, pit crater chains; association with fluvial channels and layered deposits on the plateau adjacent to the valley heads; and occurrences of mass wasting features at the valley mouths. Elevation profiles of the walls of Valles Marineris display ramp and cliff topography indicating mechanical stratification in the wall rocks. These observations reveal multiple controls over the formation and localization of theater-headed valleys in Valles Marineris. However, the major controls appear to be strong-over-weak stratigraphy in the canyon walls and the pre-existing zones of structural weakness in the Valles Marineris region. The source of water could likely be groundwater; however, snowmelt accumulated on the upper parts of the wallrock would additionally facilitate the process. Based on the above observations, we propose a sequence of events for the evolution of theater-headed valleys in the Valles Marineris region.

Reversible glacial-periglacial transition in response to climate changes and paraglacial dynamics: A case study from Héðinsdalsjökull (northern Iceland)

The objective of this work is to chronologically establish the origin of the different glacial and rock glacier complex landforms deposited by Héðinsdalsjökull glacier (65°39′ N, 18°55′ W), in the Héðinsdalur valley (Skagafjörður fjord, Tröllaskagi peninsula, central northern Iceland). Multiple methods were applied: geomorphological analysis and mapping, glacier reconstruction and equilibrium-line altitude calculation, Cosmic-Ray Exposure dating (in situ cosmogenic 36Cl), and lichenometric dating. The results reveal that a debris-free glacier receded around 6.6 ± 0.6 ka, during the Holocene Thermal Maximum. The retreat of the glacier exposed its headwall and accelerated paraglacial dynamics. As a result, the glacier terminus evolved into a debris-covered glacier and a rock glacier at a slightly higher elevation. The front of this rock glacier stabilized shortly after it formed, although nuclide inheritance is possible, but its sector close the valley head stabilized between 1.5 and 0.6 ka. The lowest part of the debris-covered glacier (between 600 and 820 m altitude) collapsed at ca. 2.4 ka. Since then, periods of glacial advance and retreat have alternated, particularly during the Little Ice Age. The maximum advance during this phase occurred in the 15th to 17th centuries with subsequent re-advances, namely at the beginning of the 19th and 20th centuries. After a significant retreat during the first decades of the 20th century, the glacier advanced in the 1960s to 1990s, and then retreated again, in accordance with the local climatic evolution. The internal ice of both the debris-covered and the rock glacier have survived until the present day, although enhanced subsidence provides evidence of their gradual degradation. A new rock glacier developed from an ice-cored moraine from around 1940–1950 CE. Thus, the Holocene coupling between paraglacial and climatic shifts has resulted in a complex evolution of Héðinsdalsjökull, which is conflicting with previously proposed models: a glacier, which had first evolved into a debris-covered and rock glacier, could later be transformed into a debris-free glacier, with a higher sensitivity to climatic variability.

Northward subglacial drainage during the Mackinaw Interstade in the Cayuga basin, central New York, USA

The history of deglaciation in the Finger Lakes region since the Valley Heads readvance is questioned by recent research in the Cayuga basin, which concludes that, instead of forming a series of proglacial lakes, drainage during the Mackinaw Interstade was into the Laurentide ice sheet. First suspected in the Dryden–Virgil Valley where there is an absence of a lake outlet or surficial lacustrine deposits, this conclusion was explicitly revealed in the Sixmile–Willseyville trough where ice margin channels funneled water into the ice front. Further support was found in the Cayuga Inlet Valley, where a kettle kame terrane sloped northward into the ice front. Northward drainage was preceded by southerly drainage, with reversal occurring about 16.3 kyr ago. Multi-channel seismic profiles at the south end of Lake Cayuga reveal a south-sourced subaqueous sedimentary fan at the base of the lacustrine sequence. This fan is correlated with a coarse and heterogeneous clastic sequence penetrated in water wells in the City of Ithaca and requires northward drainage into a subglacial lake, which precludes the existence of proglacial lakes Ithaca, Newberry, and Hall. The proposed subglacial flow path is through the Cayuga trough, exiting the ice front eastward in the Mohawk Valley. Subglacial drainage from the Cayuga trough probably was part of a regional subglacial drainage system during the Mackinaw Interstade. Studies north of Lake Ontario have led to the proposal of a subglacial lake in the Ontario basin at that time, which likely also drained into the Mohawk Valley.

Glacier extent and climate in the Maritime Alps during the Younger Dryas

This study focuses on an Egesen-stadial moraine located at 1906–1920 m asl in the NE Maritime Alps, Europe. Three moraine boulders are dated, via cosmogenic isotope analyses, to 12,490 ± 1120, 12,260 ± 1220 and 13,840 ± 1240 yr, an age compatible with the Younger Dryas cooling event. The reconstructed glacier that deposited the moraine has an equilibrium line altitude of 2349 ± 5 m asl, calculated with an Accumulation Area Balance Ratio of 1.6. The result is very similar to the equilibrium line altitude of another reconstructed glacier that deposited a moraine also dated to Younger Dryas, in the SW Maritime Alps. The similarity suggests comparable climatic conditions across the region during the cooling event. The Younger Dryas palaeoprecipitation is 1549 ± 26 mm/yr, calculated using the empirical law that links precipitation and temperature at a glacier equilibrium line altitude, with palaeotemperatures obtained from nearby palynological and chironomids studies. The palaeoprecipitation is similar to the present, thus indicating non-arid conditions during the Younger Dryas. This is probably due to the Maritime Alps peculiar position, at the crossroads between air masses from the Mediterranean and the North Atlantic, the latter displaced by the southward migration of the polar front. The equilibrium line altitude interval defined by the two reconstructed glaciers, is used to model the extent of another 66 potential Younger Dryas glaciers in the region. Each modelled glacier is reconstructed by iteratively changing the position of its front until the reconstructed glacier has an ELA that falls within the interval. The result, which is checked against geomorphological evidence, shows that glaciers covered 83.74 km2 during the Younger Dryas, with a volume of 5.39 km3. All valley heads were occupied by ice, except for the Maddalena/Larche Pass (1999 m asl), an ideal site for future archaeological, palaeoecological and palaeozoological studies.

Cirques in the Sierra de Guadarrama and7 Somosierra Mountains (Iberian Central System): Shape, size and controlling factors

The Guadarrama and Somosierra mountain ranges form the eastern sector of the Iberian Central System and hosted numerous glaciers during the Late Pleistocene (MIS2). Glaciation was of low intensity with glaciers of small sizes, strongly controlled by the climatic context and the topography. This study analyses the shape, size, distribution and location of 96 cirques existing in these mountain ranges. In addition to the standard morphometric parameters and controlling factors (altitude, aspect and lithology) used in most studies, additional factors were considered here in relation to the pre-glacial relief and fracture network. The data were obtained and processed using ArcGIS 10.4 and relations between the parameters and controlling factors were evaluated using statistical methods. The results indicate that most are simple cirques, tending to isometry, with low vertical incision capacity, considerable variation in size and predominantly east-facing. In the context of the Iberian Peninsula and other European mountains, these cirques are among the most isometric, the lowest in height and present the least overdeepening. The development of these cirques has generally been the result of random combination of various factors. Thus: (i) the largest cirques are located at intermediate altitudes, with the headwall located on the main divides, at former torrential valley heads or at the headwalls of fracture corridor valleys, and are north-facing; (ii) the longest cirques are located at former torrential valley heads, on metamorphic bedrock (i.e. schists, slates) and on uniform slopes. Finally, the prevailing eastern aspects are explained by topoclimatic factors and are in agreement with previous studies, which have proposed a Circulation Weather Type (CWT) model throughout the Iberian Peninsula during the Last Glacial Period, similar to its current configuration.

Topographic Controls on the Extension and Retraction of Flowing Streams

AbstractFlowing stream networks extend and retract as their surrounding landscapes wet up and dry out, both seasonally and during rainstorms, with implications for aquatic ecosystems and greenhouse gas exchange. Some networks are much more dynamic than others, however, and the reasons for this difference are unknown. Here we show that the tendency of stream networks to extend and retract can be predicted from down‐valley changes in topographic attributes (slope, curvature, and contributing drainage area), without measuring subsurface hydrologic properties. Topography determines where water accumulates within valley networks, and we propose that it also modulates flow partitioning between the surface and subsurface. Measurements from 17 mountain stream networks support this hypothesis, showing that undissected valley heads have greater subsurface transport capacities than sharply incised valleys downstream. In catchments where broad valley heads rapidly transition to sharply incised valleys, subsurface transport capacity decreases abruptly, stabilizing stream length through wet and dry periods.

Deglaciation chronology of the Donegal Ice Centre, north‐west Ireland

ABSTRACTDuring the Last Glacial Maximum, Donegal in north‐west Ireland functioned as an independent centre of ice dispersal that separated and fed into the Donegal Bay Ice Lobe (sourced in the Irish Midlands) to the south and the Hebrides/Malin Sea Ice Stream to the north. We report geochronological data that demonstrate marked contrasts in the timing and rate of deglaciation in northern and southern Donegal. In northern Donegal, which occupied an inter‐ice‐stream/lobe location, decoupling from the Hebrides/Malin Sea Ice Stream resulted in formation of a marine embayment along the north coast by ∼22–21 ka, and subsequent slow (∼4 ± 1 m a−1) climatically driven inland retreat of the ice margin to mountain source areas by ∼17 ka. By contrast, in southern Donegal, which lay near the axis of the Donegal Bay Ice Lobe, deglaciation was delayed until ∼18 ka following readvance of ice to a moraine in outer Donegal Bay. The ice margin subsequently underwent net retreat, apparently uninterrupted by readvances, at a net rate of ∼ 18 ± 6 m a−1. A mean terrestrial cosmogenic nuclide age of ∼15.0 ka obtained for samples from the foothills of the Blue Stack Mountains in south‐east Donegal indicates that ice persisted in valley heads and cirques at the beginning of the Lateglacial Interstadial, suggesting that these and nearby mountains supported the last remnants of the Irish Ice Sheet before complete deglaciation of Ireland, and that almost all the shrinkage of the ice sheet in this sector occurred under stadial conditions before the onset of interstadial warming at ∼14.7 ka.

Age constraints of rock glaciers in the Southern Alps/New Zealand – Exploring their palaeoclimatic potential

Two rock glaciers in the valley head of Irishman Stream in the central Ben Ohau Range, Southern Alps/New Zealand, have been investigated using the electronic Schmidt-hammer (SilverSchmidt). Longitudinal profiles on both features reveal a consistent trend of decreasing R(Rebound)-values and, hence, increasing weathering intensity and surface-exposure age on their numerous transverse surface ridges from rooting zone towards the front. Previously published numerical ages obtained by terrestrial cosmogenic nuclide dating (TCND) allowed the calculation of a local Schmidt-hammer exposure-age dating (SHD) age-calibration curve by serving as the required fixed points. Age estimates for the lowermost rock glacier surface ridges fall within the early Holocene between 12 and 10.5 ka and indicate a fast disappearance of the Late Glacial glacier formerly occupying the valley head, followed by the initiation of rock glacier formation around or shortly after the onset of the Holocene. Although it cannot be judged whether the rock glaciers investigated were active within the entire Holocene or only repeatedly during multiple episodes within, their location and intact morphology exclude any substantial glacial activity at Irishman Stream during the Holocene. This has considerable regional palaeoclimatic implications because it opens for the hypothesis that climatic conditions during early Holocene were possibly comparatively dry and favourable for rock glacier initiation, but less so for glaciers. It would also challenge the view that air temperature is the sole major climate driver of glacier variability in the Southern Alps. More work utilising the palaeoclimatic potential of rock glaciers in the Southern Alps is advised.

Incision of Licus Vallis, Mars, From Multiple Lake Overflow Floods

AbstractLicus Vallis is a large valley (>350 km long, >2 km wide, and >150 m deep) that heads at the outlet breach of an ~30 km diameter impact crater. We present observations of the geomorphology and topography of this paleolake outlet valley and associated tributary valleys to constrain the history of incision of the Licus Vallis system. Licus Vallis has an abrupt increase in gradient by a factor of approximately 4 along its longitudinal profile, and a knickpoint that drops ~200 m over a reach of ~2 km approximately 12 km downstream from the valley head. We also describe a set of paired terraces within Licus Vallis, which are continuous for tens of kilometers and define an interior valley >2 km in width. We interpret the geomorphology of Licus Vallis as recording at least two discrete, major episodes of valley incision, both driven by lake overflow floods. The main portion of Licus Vallis formed by overflow flooding from a large (~103–104 km2) lake contained in an intercrater basin. Subsequently, overflow flooding from a lake within the ~30 km diameter impact crater reactivated Licus Vallis, forming a major knickpoint at the valley head and establishing the upstream section of the valley at a lower slope. Farther down the valley, this flood event incised an interior valley bounded by paired terraces. Regional tributary valleys that feed Licus Vallis also have prominent knickpoints, which have retreated farthest for downstream valleys. We conclude that these knickpoints record successive waves of incision that swept up Licus Vallis during lake overflow flooding, with erosion in the main trunk of the valley (from overflow floods) significantly outpacing erosion in the tributary valleys (from regional surface runoff). These observations of Licus Vallis illustrate how lake overflow floods may have provided an important control on the pace of landscape evolution on Mars.

Probable Deep Erosion by Continental Ice Sheet Melt Water Floods: Chalk Buttes Area of Carter County, Montana, USA

Topographic maps are used to determine erosional landform origins along the Little Missouri River-Powder River drainage divide in the Chalk Buttes areas of Carter County, Montana. Asymmetric drainage divides, drainage divide gaps, and isolated erosional remnants are used to determine a sequence of erosion events beginning with headward erosion of northeast-oriented Little Missouri River tributary valleys and ending with headward erosion of the deeper north-northeast and north-northwest oriented Powder River valley. Gaps notched into present day drainage divides and orientations of valley heads on either side of those gaps suggest many closely spaced southeast-oriented streams flowed across the region immediately prior to being captured by headward erosion of deeper north-oriented valleys. Buttes capped by horizontal Miocene Arikaree sandstones stand 500 feet (152 meters) or more above surrounding Little Missouri River tributary drainage basin elevations while the Powder River valley floor elevation is as much as 800 feet (244 meters) below those surrounding elevations. A water source could not be determined from the map evidence, however the study area is located to the south and west of a known continental ice sheet margin and large southeast-oriented ice-marginal melt water floods should have logically crossed the region.