Late-Holocene and Younger Dryas glaciers in the northern Cairngorm Mountains, Scotland

Abstract

We present 17 cosmogenic 10 Be ages of glacial deposits in Coire an Lochain (Cairngorm Mountains), which demonstrate that glacial and nival deposits cover a longer timescale than previously recognised. Five ages provide the first evidence of a late-Holocene glacier in the British Isles. A previously unidentified moraine ridge was deposited after c. 2.8 kyr and defines a small slab-like glacier with an equilibrium line altitude (ELA) at c. 1047 m. The late-Holocene glacier was characterised by rapid firnification and a dominance of sliding, enabling the glacier to construct moraine ridges in a relatively short period. Isotopic inheritance means that the glacier may have existed as recently as the 'Little Ice Age' (LIA) of the 17th or 18th century ad, a view supported by glacier-climate modelling. Nine 10 Be ages confirm a Younger Dryas Stadial (YDS) age for a cirque-floor boulder till, and date the glacier maximum to c. 12.3 kyr when the ELA was at c. 963 m altitude. Both glaciers existed because of enhanced accumulation from wind-blown snow, but the difference in ELA of only c. 84 m belies the YDS-LIA temperature difference of c. 7°C and emphasises the glacioclimatic contrast between the two periods. Three 10 Be ages from till boulders originally deposited in the YDS yield ages <5.5 kyr and indicate snow-avalanche disturbance of older debris since the mid-Holocene, as climate deteriorated towards marginal glaciation.

A Snow-Push Mechanism for Ridge Formation in the Cairngorm Mountains, Scotland

“…The observations in this paper are site-specific, and the cirque is unusual in Scotland because of the juxtaposition of a moraine ridge and a smooth rock slab at high altitude, the product of lateHolocene glaciation (Kirkbride et al 2014). Active snow push at this site suggests that the process may occur more widely in the Scottish Highlands, but with subtle and very localised landscape modification.…”

“…There seems to be no possibility that the whole moraine could have formed by nival processes. Indeed, the (Kirkbride et al 2014) Nevertheless, even rare events introduce reworked debris to moraine crests, and could contaminate exposure-age determinations of moraine age with "young" boulders. The right-lateral moraine was dated by three cosmogenic 10 Be samples to less than 1.6 ± 0.3, 2.8 ± 0.5 and 5.5 ± 0.5 k yr (Kirkbride et al 2014 et al 2010).…”

“…The proximal moraine slope is a steep 4 m-high bank of freshly-exposed sandy gravel which is annually disturbed by snow creep, sliding and avalanching. Removal of material from the base of the bank maintains a high-angle, and erosion is progressively removing the toe of the late Holocene moraine (Kirkbride et al 2014). Nival activity disperses debris downslope as splays rather than as ridge forms.…”

“…A small ridge of fresh debris was observed on 18 June 2014, located on proximal side of the crest of an existing larger moraine ridge (Figure 1), the right-lateral moraine of a 3.6 ha cirque glacier dated to the late Holocene by Kirkbride et al (2014). The ridge had not been present on a visit 11 days 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 Figure 2) nor when rock samples were collected for dating.…”

“…A fuller site description is given by Kirkbride et al (2014). The site is unusual because the snow-push ridge has been constructed on the crest of a larger, older moraine ridge which has been dated by cosmogenic 10 Be to the late Holocene.…”