Extension of Glacier de Saint-Sorlin, French Alps, and equilibrium-line altitude during the Little Ice Age

Glacier de Saint-Sorlin, French Alps, left terminal moraines at 1.3, 2.9 and 3.7 km ahead of the present terminus. According to proxy data and to historical maps, these were formed in the 19th,18th and 17th centuries, respectively. A plateau at 27002625 in was then surrounded by ice but never became an accumulation area. This fact shows that the equilibrium-line altitude (ELA) on the glacier never dropped below 2300 in. The following simple models apply sufficiently to yield reliable estimations of past ELA: (1) a uniform and constant vertical gradient of the mass balance, down to the terminus; and (2) a plane bed, with a slope of 8.5degrees and a uniform width. Then in a steady situation the accumulation-area ratio is 1/2. Compared to the mean for 1956-72, at the onset of the Little Ice Age the balances were higher by 3.75 m ice a(-1), and the ELA was 400 m lower. Correlations between 1956-72 balances and meteorological data suggest that during the melting season the 0degreesC isotherm was about 800 in lower, while the winter precipitation at low altitudes did not change. These correlations may have been different in the past, but an equal lowering of the ELA and of the 0degreesC isotherm., as assumed by several authors, seems excluded.