The space of 1055 years, however, by no means represents the duration of the life of the glacier of Aosta. It may have existed for immense periods both anterior and posterior to the journeys of the Mont Blanc boulders. The frontal terminal moraines, which stretch from Caluso to Viverone (a distance of more than ten miles), are evidence that the snout of the glacier remained stationary, or nearly so, for a length of time which must at least be estimated by centuries, and probably extended over thousands of years. These moraines constitute important chains of hills whose bases are several miles across, and which attain a height of more than a thousand feet; and, as they were formed by the gradual and slow spreading out of the medial and lateral moraines, it is evident that they were not built up in a day.
Moreover, when the glacier of Aosta shrank away from Ivrea, its retrogression may have been comparatively rapid, or it may have been conducted with extreme deliberation. But, under any circumstances, the extinction of such a tremendous body of ice must have extended over many years, and for a portion of that time a large part of the mass must have been advancing down the valley, although the snout of the glacier was retreating, and although the entire mass was diminishing in volume. If the time is
dropped in a crevasse, at a horizontal distance of 4300 feet from the place at which it had been lost, showing an average annual motion of 430 feet. In the latter work there is a relation of the recovery of the remains of a ladder used by De Saussure, which had travelled about 13,000 feet in 44 years, or 295 feet per annum. Forbes says that the first of these two examples is better ascertained in all its particulars than the other. It should be observed that the knapsack in question made the descent of the well-known "ice-fall" of the Glacier de Talèfre, and that there was a difference of level between the place at which it was lost and that at which it was found of 1145 feet; that is to say, it descended one foot in every four that it advanced. This rapid descent undoubtedly accelerates the motion of the Glacier de Talèfre. The town of Ivrea, on the other hand, is 768 feet (Ball) above the level of the sea, while Entrèves (at the foot of Mont Blanc) is 4216 feet (Mieulet). So that the glacier which once spread over the sites of these two places (which are about 65 miles apart) descended by an average gradient of almost exactly 1 in 100. This moderate rate of inclination would as certainly tend to retard the motion of the glacier.
