Processes of ‘hummocky moraine’ formation in the Gaick, Scotland: insights into the ice‐marginal dynamics of a Younger Dryas plateau icefield

Abstract

Younger Dryas ice‐marginal (‘hummocky’) moraines in Scotland represent valuable terrestrial archives that can be used to obtain important information on ice‐marginal dynamics and glacier thermal regimes during a period of rapid climatic change. In this paper, we present detailed sedimentological studies of Younger Dryas ice‐marginal moraines in the Gaick, central Scotland, the former site of a spatially restricted plateau icefield. Exposures demonstrate that moraines in the Gaick represent terrestrial ice‐contact fans, with evidence of proglacial and subglacial glaciotectonization, as reported elsewhere in Scotland. The exposures also reveal the influence of local hydrogeological conditions, with pressurization of the groundwater system leading to the formation of hydrofracture fills within some moraines. Clast shape analysis shows that all the moraines contain debris consistent with transport in the subglacial traction zone. The sedimentological data, and the planform arrangement of the moraines as nested arcs or chevrons, indicate that retreat of the Younger Dryas Gaick Icefield outlets was incremental and oscillatory. This evidence strongly suggests a mainly temperate thermal regime and short glacier response times, but with narrow cold‐ice zones near the margins facilitating the elevation of basal debris to the glacier surface. Analogous glaciodynamic regimes occur at modern ice‐cap and plateau icefield outlets in Iceland and Norway, although there are significant differences in the nature of ice‐marginal deposition. The glaciodynamic signature recorded by moraines in the Gaick has allowed us to shed new light on the ice‐marginal dynamics and thermal regime of one of the most easterly Younger Dryas icefields in Scotland.

The role of topography in landform development at an active temperate glacier in Arctic Norway

“…In other years, or in other parts of the ice margin, a saturated proglacial area with high porewater pressures led to till squeezing and bulldozing at the margin and around ice pecten (e.g. Chandler et al, 2016; Chandler, Evans, et al, 2020; Chandler, Lukas, & Boston, 2020; Evans et al, 2016, 2017). The two well‐defined sediment slabs in FIN‐M3 could imply that the sawtooth moraine formed over at least 2 years (e.g.…”

“…However, it has rarely been recognised as contributing to sawtooth moraine development, where till squeezing around the ice‐marginal pecten and within longitudinal crevasses is often advocated (e.g. Chandler, Evans, et al, 2020; Chandler, Lukas, & Boston, 2020; Evans et al, 2016).…”

“…Chandler et al, 2016; but are thought to occur in situations where squeeze-push mechanisms become decoupled over a seasonal cycle. In these cases, strong summer glacier retreat enables (multiple) squeeze moraines to be formed in between winter push moraines, rather than a single composite push-squeeze moraine being constructed Chandler, Lukas, & Boston, 2020). The time frame coincides with a period in the 1990s when glaciers in Norway readvanced (Andreassen et al, 2005;Chinn et al, 2005), which would have also contributed to a slower overall recession rate and reoccupation of moraines.…”

“…Documenting this geomorphological imprint, and investigating the associated sub‐glacial and ice‐marginal processes responsible, offers a detailed insight into changes in individual glacier dynamics over time (e.g. Chandler et al, 2016; Chandler, Evans, et al, 2020; Chandler, Lukas, & Boston, 2020; Evans et al, 2019; Hiemstra et al, 2022; Lukas, 2012; Weber et al, 2019), which can help to understand the role of topography in glacial landform generation.…”

Surging glaciers in Scotland

“…Closely spaced recessional moraines are widespread within the limits of the former outlet glaciers that drained the plateau icecaps in the Grampians, including Creag Meagaidh (Finlayson, 2006;Jones et al, 2017), the Monadhliath , West Drumochter (Benn & Ballantyne, 2005), the Gaick plateau (Chandler et al, 2019(Chandler et al, , 2020 and the Cairngorms (Bennett & Glasser, 1991;Standell, 2014). A few outlet glaciers in this region exceed 8 km in length (Figure 5D), but all appear to have undergone prolonged active retreat and evidence for surging appears to be absent.…”