Thanks to Arthur Ancrenaz for sending a fascinating paper about the small ice cap of Aubrac, in the French Massif Central. I was in that region a couple of weeks ago, and was struck by the well-nigh perfect conditions for the formation of small ice caps on the high plateaux, given a substantial drop in temperatures and adequate precipitation.
Arthur Ancrenaz et al,
Quat Science reviews 285 (2022) 107500
ABSTRACT
The chronology of the late Pleistocene glaciation in the Massif Central, France, is not well documented and there are divergent hypotheses regarding the timing of glacial events in this area. This study aims at reconstructing the chronology of late Pleistocene glacial fluctuations in the Aubrac Mountains. We present a new set of Be-10 and Al-26 exposure ages from twenty erratic boulders embedded in six glacial landforms. Glacial landforms were sampled to determine the timing of three glacial stades and the deglaciation of the Aubrac Mountains. These new data allowed us to constrain the timing of: (i) the Local Last Glacial Maximum, which is coeval with the early Marine Isotopic Stage 2 (MIS 2) (28e24 ka), and (ii) the Grandvals and Bouquincan stades, which are both coeval with the late MIS 2 (24e16 ka). Deglaciation occurred during Heinrich Stadial 1 (18e15.6 ka). This new direct glacial chronology is supported by regional paleoenvironmental proxies and it updates the chronostratigraphic framework available for the southwestern Massif Central. Western European atmospheric circulation changes (i.e., a southward shift in North Atlantic storm tracks and Mediterranean influences) during MIS 2 were identified as significant factors controlling glacier growth in the Aubrac Mountains as well as other glaciated mountains within the southwestern Massif Central, such as the Cantal, Margeride and Lozere mountains.
The uplands extend over 1300 km2 and culminate at the Signal de Mailhebiau at 1467 m above sea level -- mostly there are rounded summits composed of Miocene volcanic rocks. To the east the rocks are mostly granitic basement rocks. The main topographic ridge extends along 25 km with an elevation decrease from 1400 m in the SSE to 1200 m in the NNW. The last ice cap (which covered about 500 km2) is believed to have been 200m - 300m thick, with an ice divide running approx along the highland ridge and with a "plateau ice cap" to to the east. The steeper slopes and river valleys to the west gave rise to at least 8 small outlet glaciers, between 6 km and 8 km long, but to the east ice movement was more sluggish on the granite plateau, giving rise to a scoured topography of rock basins and scattered glacial deposits. Glacier margins generally terminated where there were constraints such as reverse slopes. During deglaciation the thin plateau ice cap seems to have disintegrated in situ, giving rise to a number of morainic and fluvioglacial features, most of which have a surface expression of less than 20m. So interpretations are very subtle indeed.
The chronology of the late Pleistocene glaciation in the Massif Central, France, is not well documented and there are divergent hypotheses regarding the timing of glacial events in this area. This study aims at reconstructing the chronology of late Pleistocene glacial fluctuations in the Aubrac Mountains. We present a new set of Be-10 and Al-26 exposure ages from twenty erratic boulders embedded in six glacial landforms. Glacial landforms were sampled to determine the timing of three glacial stades and the deglaciation of the Aubrac Mountains. These new data allowed us to constrain the timing of: (i) the Local Last Glacial Maximum, which is coeval with the early Marine Isotopic Stage 2 (MIS 2) (28e24 ka), and (ii) the Grandvals and Bouquincan stades, which are both coeval with the late MIS 2 (24e16 ka). Deglaciation occurred during Heinrich Stadial 1 (18e15.6 ka). This new direct glacial chronology is supported by regional paleoenvironmental proxies and it updates the chronostratigraphic framework available for the southwestern Massif Central. Western European atmospheric circulation changes (i.e., a southward shift in North Atlantic storm tracks and Mediterranean influences) during MIS 2 were identified as significant factors controlling glacier growth in the Aubrac Mountains as well as other glaciated mountains within the southwestern Massif Central, such as the Cantal, Margeride and Lozere mountains.
Map of the five local ice cap areas in the Massif Central. The largest by far was the western plateau and upland area incorporating Monts Dore and Cantal. Aubrac is a much smaller plateau with few traces of pre-Eemian glaciation but with a substantial moraine around its northern edge dating from the LGM.
There are no spectacular mountain peaks in the Aubrac, but the plateau is impressive -- and the landforms of glaciation are very subtle.
The uplands extend over 1300 km2 and culminate at the Signal de Mailhebiau at 1467 m above sea level -- mostly there are rounded summits composed of Miocene volcanic rocks. To the east the rocks are mostly granitic basement rocks. The main topographic ridge extends along 25 km with an elevation decrease from 1400 m in the SSE to 1200 m in the NNW. The last ice cap (which covered about 500 km2) is believed to have been 200m - 300m thick, with an ice divide running approx along the highland ridge and with a "plateau ice cap" to to the east. The steeper slopes and river valleys to the west gave rise to at least 8 small outlet glaciers, between 6 km and 8 km long, but to the east ice movement was more sluggish on the granite plateau, giving rise to a scoured topography of rock basins and scattered glacial deposits. Glacier margins generally terminated where there were constraints such as reverse slopes. During deglaciation the thin plateau ice cap seems to have disintegrated in situ, giving rise to a number of morainic and fluvioglacial features, most of which have a surface expression of less than 20m. So interpretations are very subtle indeed.
Cosmogenic exposure dating -- mostly using boulders on small morainic ridges, suggests that the local LGM occurred around 25,000 years ago, and that during deglaciation there were two distinct stades or ice edge stillstands -- which may or may not correlate with stages elsewhere. By about 18,000 years ago most of the glacier ice had melted, although there is evidence of extensive snowfields and periglacial conditions after that date.
Some of the subtle landscape features associated with the LGM glacial episode. Also shown: the locations of three of the cosmogenic exposure samples.
I'm intrigued by the lessons that this small ice cap may have for West Wales and the SW of England. Think Mynydd Preseli, Dartmoor and Exmoor. It's interesting that the ice only appears to have been around 250m thick; that in any case there were recognizeable morainic deposits associated with it; and that scattered till and fluvioglacial deposits were thick enough and distinctive enough to be mapped. Also I'm intrigued by the manner in which the ice edge position -- on the plateau at least -- was determined by the presence of reverse slopes.
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